From the Department of Pharmacology, University of Minnesota, Minneapolis, Minnesota 55455-0347
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ABSTRACT |
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A 34-nucleotide A-U sequence located immediately
upstream of the editing sites of the Leishmania tarentolae
cytochrome b mRNA induces a mitochondrial extract to
insert U nucleotides independent of guide RNA. Insertions are localized
to positions immediately 5' and 3' of the A-U sequence. When placed
within an unedited mammalian transcript, the A-U sequence is sufficient
to induce U-insertions. The sequence has a high degree of similarity
with the templating nucleotides of a cytochrome b guide RNA
and with a sequence adjacent to the editing sites in ND7 mRNA, the
other characterized kinetoplastid mRNA supporting guide
RNA-independent U-insertions. At least one protein specifically
interacts with the A-U sequence. The reaction is consistent with a
mechanism proposed for guide RNA-directed editing.
Several mitochondrial mRNAs of the kinetoplastid protozoa must
be edited through the precise insertion and deletion of U nucleotides (reviewed by Refs. 1-3). The editing has been proposed to proceed through a reaction that is initiated at an editing site by an endonucleolytic cleavage producing a 5'-fragment with a 3'-OH and a
3'-fragment with a 5'-monophosphate (4). U-addition by a terminal
uridylyltransferase activity (TUTase) or U-deletion by an exonuclease
activity at the 3'-OH followed by re-ligation of the 5'- and
3'-fragments would complete one editing cycle which could be reiterated
at subsequent sites. Experimental evidence in support of the proposed
mechanism has been obtained in vitro (5-8).
Genetic information specifying the location of the U-insertions and
deletions is carried on guide RNAs
(gRNAs).1 A specific gRNA is
able to bind to its cognate mRNA immediately 3' of a block of
editing sites and also contains a template complementary to a block of
edited sequence (4). It has been demonstrated in vitro for
certain Trypanosoma brucei and Leishmania
tarentolae mRNAs that the appropriate gRNA directs the
U-insertions and deletions at the editing sites; changing the
templating sequence of the gRNA results in the corresponding change to
the number of U-insertions and deletions (5-8).
U-insertions within some L. tarentolae mRNAs have been
reported to be catalyzed by a mitochondrial lysate independent of added gRNA. Initially it was suggested that gRNAs endogenous to the mitochondrial lysate were directing the insertions (9). However, it was
subsequently demonstrated that the U-insertions were unaffected by
mutations to the mRNA that should have inhibited interactions with
endogenous gRNA (10). Characterization of the reaction was complicated
by difficulty in solubilizing the majority of the U-insertion activity
from the crude mitochondrial lysate, making subsequent enrichment
inefficient and leaving the assays vulnerable to artifacts caused by
contaminating activities. The low level of U-insertions also hindered
any significant biochemical characterization and prevented the
localization of the U-insertions within individual RNAs. As a result,
the biological significance of the gRNA-independent reaction observed
in vitro was unclear.
We describe here a novel assay that has facilitated the enrichment and
characterization of the gRNA-independent U-insertion activity. The
majority of the U-insertions occurring independent of gRNA are adjacent
to a 34-nucleotide A-U sequence element that is immediately 5' of the
editing sites on the cytochrome b transcript. This element
is both necessary for the cytochrome b gRNA-independent editing and also sufficient to induce gRNA-independent insertions within a mammalian transcript. The sequence of the A-U element is
highly similar to the templating sequence of cytochrome b
gRNA I and a sequence immediately upstream of editing sites of the ND7
mRNA, which is the other characterized mRNA that supports gRNA-independent U-insertions (8). The intermediates produced during
the reaction are consistent with a mechanism proposed for gRNA-directed
editing (4). Our results suggest that gRNA-independent editing may
result from association of the editing components with the
34-nucleotide A-U element present on the cytochrome b mRNA.
Mitochondrial Extract Preparation--
An old laboratory strain
(UC strain) of L. tarentolae was grown to a density of 1 to
2 × 108 cells/ml in 2-liter flasks containing 500 ml
of BHI media (DIFCO) supplemented with 10 µg/ml hemin. The flasks
were loosely covered with foil and shaken at 100 rpm at 27 °C in an
Innova 4230 incubator (New Brunswick Scientific). After washing
three times with SHE (2 mM EDTA, 20 mM Hepes
7.5 and 0.25 M sorbitol), cells were lysed in a Stansted
Disrupter (Energy Service Co.) at 1200 p.s.i. as described
previously (10). Mitochondria were purified by flotation in a
Renograffin density gradient (11), and after washing three times with
SHE, resuspended in 950 µl of solubilization buffer (25 mM Hepes 7.5, 10 mM MgCl2, 1 mM EDTA, 20 mM KCl, 0.1 mM ATP, 1 mg/ml Pefabloc, and 10 µg/ml leupeptin) for every 500 ml of starting
culture. Triton X-100 was added to a final concentration of 0.5%, and
the solution was gently mixed by inversion and left on ice for 5 min.
The mitochondrial lysate was centrifuged at 11,000 × g
at 4 °C for 5 min and the supernatant frozen on dry ice for storage
at RNA Transcripts--
RNAs were transcribed either from PCR
products or directly from synthetic oligodeoxynucleotides using T7 RNA
polymerase (12). The template for the parental transcript (wild type)
containing the 5' 178 nucleotides of pre-edited cytochrome b
and a mutated gRNA-binding site was amplified from plasmid pNB2-S1 (9)
using oligodeoxynucleotide primers 5692 and 10211. The same plasmid was
used both for the amplification of the
Transcription reactions (50 to 100 µl) were carried out at 37 °C
with 1 mM each NTP, 0.5 mM dithiothreitol, 100 mM Tris, pH 7.7, 100 mM spermidine (basic), 1%
Triton X-100, 13 mM MgCl2, and 50 units of T7
RNA polymerase (Life Technologies, Inc.). A 5-fold molar excess of GMP
over GTP was included in the reactions for RNAs used in subsequent
ligation reactions. RNAs for the gel-shift assay were synthesized to a
specific activity of 2.4 × 106 cpm/µg by the
addition of [
Intramolecular circular RNAs were formed by incubating 2 to 5 µM linear RNAs for 2 h at 37 °C in 50 mM Tris, pH 7.8, 10 mM MgCl2, 10 mM dithiothreitol, 0.3 mg/ml bovine serum albumin, 1 mM ATP, 15% dimethyl sulfoxide, and 10 units/µl T4 RNA
ligase (New England Biolabs). The intramolecular circular ligation
products were purified on 6% denaturing polyacrylamide gels, eluted,
and precipitated with ethanol. Several criteria were used to identify and purify the circular RNAs from intermolecular ligation products potentially complicating the U-insertion assay. First, primer extension
analysis was used to extend through the ligation junction. Extension of
a circular RNA yields a cDNA product that is close to full-length
whereas extension of an intermolecular dimer yields both this cDNA
and, providing the RT primer-binding site is not at the extreme ends of
the RNA, a second shorter product. Second, when radiolabeled GTP primed
and unlabeled GMP primed RNAs are incubated together in a ligation
reaction, the linear ligation products, identified by the primer
extension, are the only predominant radiolabeled bands. Third, the
electrophoretic mobility of circular RNAs relative to linear RNAs
varies significantly with the percentage of acrylamide. Small
quantities of linear contaminants co-migrating with circular RNAs
on the 6% gel are readily detected when re-electrophoresed on higher
percentage gels.
Direct Assay for U-insertion--
The circular cytochrome
b transcripts were heated at 65 °C for 3 min in a 10-µl
volume containing 5 mM Tris, pH 8.3 (27 °C), and 0.2 mM EDTA, pH 8.0. After the addition of 25 µl of reaction mixture (40 mM KCl, 20 mM MgCl2, 6 mM potassium phosphate, pH 7.6, 40 mM
dithiothreitol, 2 mM ATP, 2 mM GTP, 0.1 mM UTP, 5 µCi [ Enrichment of RNAs with gRNA-independent
U-insertions--
Circular cytochrome b transcripts were
extract treated under the direct assay conditions with 1 mM
4-thio-UTP (Amersham) substituted for both labeled and unlabeled UTP.
Circular RNAs were purified on 6% denaturing polyacrylamide gels,
eluted, and ethanol precipitated. RNAs were resuspended in a total
volume of 50 µl containing 0.5% SDS and 6 mM EDTA. After
heating at 65 °C for 3 min, the RNA solution was added to a 1.5-ml
plastic tube containing 10 µl of organomercury-derivatized agarose
(Bio-Rad) that had been washed three times in 1 × buffer (400 mM NaCl, 40 mM Hepes pH 7.5, 3 mM
EDTA, and 0.5% SDS) and resuspended in 50 µl of 2 × buffer
(800 mM NaCl and 80 mM Hepes pH 7.5). The
solution was incubated at room temperature for 30 min on a rocker. The
beads were pelleted for 5 s in a microcentrifuge and after
removing the supernatant, resuspended in 600 µl of 1 × buffer
and heated at 65 °C for 5 min. The beads were re-pelleted, the
supernatant discarded and the washing procedure repeated an additional
5 times. After the last wash, RNA was eluted from the beads by heating
at 65 °C in 1 × buffer containing 100 mM
dithiothreitol and 30 pmol of the DNA primer (7534 or 10040) used for
subsequent cDNA synthesis. The RNA was ethanol precipitated using 5 µg of glycogen (Boehringer-Mannheim) as a carrier, and the
organomercury column enrichment procedure repeated an additional two
times. The RNA from the 3rd cycle of enrichment was reverse transcribed into cDNA and PCR amplified for cloning using primers 7534 and 8494 or 10040 and 10041. Bio-X-ACT (Intermountain Scientific) was used to
increase the fidelity during the amplification.
Other Assays--
U-insertions within extract-treated linear
RNAs were detected using an RNase H reaction (9). Linear RNAs were
extract-treated as described for the circular substrates with the
exception that the UTP concentration was at 0.5 µM and
the specific activity increased to 2.5 × 105 Ci/mol.
Heparin (6 µg/ml) was included within the reactions to reduce the
amount of 3'-end labeling relative to the internal U-additions. The
extract reactions were terminated by the addition of EDTA to a final
concentration of 20 mM and incubation at 27 °C for 3 min
followed by the addition of SDS to 0.5%, proteinase K (50 µg/ml),
and a further 30-min incubation. The RNA was then phenol/chloroform
extracted, gel purified, eluted, and ethanol precipitated.
Oligodeoxynucleotide 10951 was annealed to the ferritin transcripts,
7622 to the 5'-extended cytochrome b RNAs, and 7534 to the
wt cytochrome b RNAs for subsequent RNase H digestion.
The gel-shift assays were performed similar to a previously described
procedure (13). After denaturation, 8 pmol of the RNAs were incubated
for 50 min with extract under U-insertion conditions but in the absence
of UTP (so that complications from TUTase activity would be avoided)
and GTP. The concentration of dithiothreitol was reduced to 3 mM. Where indicated, the competitor ferritin transcript was
added to the binding reactions at the same time as the A-U containing
RNA. The binding reactions were resolved on a 6% nondenaturing
polyacrylamide gel (acrylamide:bis-acrylamide weight ratio of 80:1).
The assay for ligase activity through the gradient was performed as for
U-insertion activity using 0.5 pmol of radiolabeled linear wild type
cytochrome b transcript that had been GMP primed.
Analysis of Cytochrome b Cleavage Products--
Circular
cytochrome b RNA was reacted with the fractionated extract
under U-insertion conditions either in the absence of NTP or presence
of 1 mM each of 4-thio-UTP, GTP, and ATP. Full-length linear RNAs formed during the reactions were gel purified, eluted, and
precipitated in ethanol. Linear RNAs isolated from the reaction containing 4-thio-UTP were enriched for those with insertions by one
passage through the organomercury column matrix. RNAs were incubated
with 4 units of poly(A) polymerase (Life Technologies, Inc.) at
37 °C for 30 min in a reaction containing 50 mM Hepes pH
8.0, 250 mM NaCl, 10 mM MgCl2, 2.5 mM MnCl2, 1 mM CTP, and 40 units of
RNasin (Promega). The poly(C) tailed RNAs were amplified for cloning
and sequencing with primers 9209 and 9295. Clones with the same mapped
cleavage site were included only if the length of each poly(C)
extension was unique indicating that they had originated from different
RNAs. There is a 1 nucleotide uncertainty in mapping the cleavage sites
adjacent to genomically encoded C nucleotides, and when UTP is included
within the editing reaction, the mapping is also complicated by
genomically encoded U nucleotides.
Northern Blot Analysis--
RNA isolated from the sucrose
fractions was electrophoresed in a 1.2% agarose/formaldehyde gel and
transferred to Hybond N+ membrane (Amersham). Hybridization
in Church-Gilbert buffer (0.5 M sodium phosphate, pH 7.2, 1 mM EDTA, 7% SDS, 1% bovine serum albumin) was performed
at 65 °C for the 12 S probe (oligodeoxynucleotide 10855) and at
55 °C for the gRNA probe (oligodeoxynucleotide 10854). Oligodeoxynucleotide probes were 5'-end labeled and were added to the
hybridizations at a final concentration of 1 pmol/ml and a specific
activity of 1 × 106 cpm/pmol.
Oligodeoxynucleotides--
Oligodeoxynucleotides shown below
were used in this study. The locations of the underlined sequences
within either the L. tarentolae maxicircle sequence (GenBank
entry LEIKPMAX) or the sequence of an EST encoding the human ferritin
light chain (DNA Sequence accession numbers AA323712 and EST76262) are
indicated. 5690, TAATACGACTCACTATA. 5692, TAATACGACTCACTATAGGGATAAATTTAATTTAAATTTTAATAATTATAAAAGCGG (nt 5371-5408 in LEIKPMAX). 7534, TAAGAATTCCATGCTAAGCAAACACCAC (nt 5503-5520 in LEIKPMAX).
7622, CATATACTCGTAATAATACTG. 7845, AAACCTAAACTAAAACC (nt
5462-5478 in LEIKPMAX). 8494, TCTGAATTCGGGA
TAAATTTAATTTAAATTT (nt 5371-5389 in LEIKPMAX). 9209, TAAGAATTCCGGGGGGGGGGGGGGG. 9295, TCTGAATTCAAATTGAAGTTCAGTATTAT. 9962, ATCTCCGCTTTTATAATTATTT (nt 5391-5411 in LEIKPMAX). 9964, TAATACGACTCACTATAGGAAAGAAAAGGCAAATTGAAG (nt 5413- 5423 in LEIKPMAX). 10040, TAAGAATTCCGTAATAATACTGAACTTC. 10041, TCTGAATTCGGTGTAGGTTTTAGTTTAGG (nt 5456-5475 in LEIKPMAX). 10076, TATAATTATTTAAAATTTAAATTAAATTTATCCC (nt 5371-5401 in
LEIKPMAX). 10132, TAATACGACTCAATAGGCAAATTGAAGTTCA. 10211, ACAAATAAAGCAACTAAAAAATAATCATGC (nt 5516-5545 in LEIKPMAX).
10513, TAATACGACTCACTATAGGTAATTTAAATTTTAAATAATTATAAAAGCGG (nt 5378-5408 in LEIKPMAX). 10703, TAATACGACTCACTATAGGGATAAATTTCCTTTCCCTTTTCCCTAATTATAAAAGCGGAGAG (nt 5394-5412 in LEIKPMAX). 10704, TAATACGACTCACTATAGGGACCATCTTCTCGGCCATC (nt 110-130 in
AA323712).10705, ATAGAAGCCCAGAGAGAGGTAGG (nt 279-301 in
AA323712). 10795, ATTATTTAAAATTTAAATTAAATTTATCCC (nt
5371-5397 in LEIKPMAX). 10796, TAATACGACTCACTATAGGAGAGAAAAGAAAAGGCAA (nt 5407-5424 in
LEIKPMAX). 10797, TAATACGACTCACTATAGGTATGCAAATTATATGTGG (nt
5487-5505 in LEIKPMAX). 10854, CTTTTAACTTCAAGTCATATG. 10855, AGGAGAGTAGGACTTGCC. 10899, CTTTTATAATTATTTAAAATTTAAATTAAATTTATCCTATAGTGAGTCGTATTA (nt
5371-5405 in LEIKPMAX). 10951, ACGGCTGCCTCCACGTCGGT (nt
221-240 in AA323712). 12121, ATAAATTTAATTTAAATTTTAAATAATTATAAAAGGCTCCCAGATTCGTCAGAAT (nt
195-214 in AA323712). 12122, GCCTTTTATAATTATTTAAAATTTAAATTAAATTTATAAACGGTGCTGGCAGGTCC (nt 142-160 in AA323712). 12309, AAGCCCAGAGAGAGGTAGGTGTAGGAGGCCTGCAGGTACCCTATAGTGAGTCGTATTA (nt 259-296 in AA323712). 12333, ATAAATTTCCTTTCCCTTTTCCCTAATTATAAAAGGCTCCCAGATTCGTCAGAA (nt
195-213 in AA323712). 12334, GCCTTTTATAATTAGGGAAAAGGGAAAGGAAATTTATAAACGGTGCTGGCAGGTCC (nt 142-160 in AA323712). 12525, CTTTTATAATTAGGGAAAAGGGAAAGGAAATTTATCCTATAGTGAGTCGTATTA.
An Assay for gRNA-independent U-insertions--
Cytochrome
b mRNA contains 15 sites near its 5'-end that are edited
in vivo through the insertion of a total of 39 U
nucleotides. In vivo editing of cytochrome b
mRNA has been proposed to be directed through the sequential action
of 2 different gRNAs (4, 10, 14). Cytochrome b gRNA I
contains the templating sequence to potentially guide the appropriate
number of U-insertions into the first (3'-most) seven editing sites.
Editing creates a gRNA-binding site for cytochrome b gRNA II
which contains the template to complete editing at the remaining sites.
An RNA transcript containing the first 178 nucleotides from the 5'-end
of the pre-edited cytochrome b mRNA of L. tarentolae was used as a substrate for an in vitro
U-insertion assay. The 178-nucleotide cytochrome b
transcript was circularized with T4 RNA ligase, and then incubated with
a solubilized L. tarentolae mitochondrial extract in the
presence of [
The gRNA binding sequence (anchor) on the cytochrome b
transcript was mutagenized (Fig. 1B) so as to inhibit
interaction with the small amount of endogenous cytochrome b
gRNA I. The mutagenized RNA is as good a substrate for U-insertions
indicating that the reaction does not require endogenous gRNA to
interact with the gRNA-binding site (Fig. 1A,
The solubilized extract supporting the gRNA independent U-insertion
activity can be fractionated over a linear 10-40% sucrose gradient
(Fig. 1A). Fractions were collected from the top of the gradient and assayed in the absence of added cognate-gRNA. The U-insertion activity is predominantly within fractions 6 and 7 and is
resolved from the endogenous ribosomal RNA (fractions 10 and 11),
identified by Northern analysis (Fig. 1A) and which also appears to be labeled by the TUTase activity (Fig. 1A, lanes
10 and 11, intensely labeled bands). TUTase activity
appears to be present through most of the gradient since the endogenous
RNA is labeled from fraction 3-11. The specific activity of the
gRNA-independent U-insertion activity in fractions 6 and 7 is increased
50-fold over the unfractionated extract. Fractions 6 and 7 each contain approximately 10% of the total protein loaded onto the gradient. The
remaining increase in specific activity is probably a result of
fractionating away RNAs or proteins that are inhibitory to the reaction.
The endogenous cytochrome b gRNA I is spread throughout the
gradient (Fig. 1A, Northern analysis). The wide distribution
is suggestive that the gRNA is in heterogeneously sized complexes with
other factors, in agreement with previous studies (18-21). The
fractions containing the peak U-insertion activity (6 and 7) each
contain approximately 10% of the endogenous gRNA.
The U-Insertions Are Not Random--
The low efficiency of the
gRNA-independent reaction had previously prevented the cloning and
sequencing of individual RNAs with insertions (9). We enriched for RNAs
containing inserted U nucleotides by exploiting the observation that
4-thiouridine could be incorporated by the mitochondrial extract into
the cytochrome b transcript with 85% of the efficiency of
uridine (data not shown). RNAs containing 4-thiouridine insertions form
mercaptides with an organomercurial column matrix which permit their
partitioning from the bulk non-modified RNA (Fig.
2A). Those RNAs containing thiouridine insertions were eluted from the organomercurial matrix with
dithiothreitol for RT-PCR, cloning, and sequencing.
After organomercury column enrichment, 28% of the sequenced clones
contained inserted U nucleotides (Fig. 2, B and
C). As a result of the assay design, only 108 nucleotides of
the substrate RNA could initially be screened for insertions
(arrows in Fig. 2C); the remaining 70 nucleotides
were used as primer-binding sites during the RT-PCR amplification and
any U-insertions within this region would not have been detected. In a
parallel experiment, however, primers binding to different regions of
the circular cytochrome b RNA were used for the RT-PCR. This
resulted in the generation of molecules with circularly permuted ends
and permitted U-insertions to also be detected within the original
primer-binding sites (circles in Fig. 2C). The
U-insertions are predominantly localized either within an 8-nucleotide
sequence corresponding to the 5'-end of the non-permuted cytochrome
b transcript or within an 18-nucleotide region that is
normally edited in vivo (Fig. 2C, boxed
sequence). It should be emphasized that there are no insertions
between nucleotides 9 and 34 and only rare insertions within the region
3' of the first editing site (nucleotides 58-178). Thus, mutation of
the gRNA-binding site still results in U-insertions within editing
sites, although with less accuracy than occurs in vivo (Fig.
2C, bottom panel). All clones contained the mutagenized gRNA-binding site eliminating any possibility that they had been derived from endogenous mRNA. Insertions that occurred adjacent to
genomically encoded U nucleotides could not be precisely localized (Fig. 2C, positions 8, 108, and 128).
The observed U-insertions are not the result of either a PCR or cloning
artifact based on the following observations. First, clones containing
inserted U nucleotides were only detected if the RNAs were enriched by
passage through the organomercury column prior to RT-PCR and cloning.
When the extract-treated RNAs were amplified and cloned without
sufficient enrichment, none of 60 sequenced clones had insertions.
Second, the insertions within all of the sequenced clones were
restricted to U nucleotides. If the U-insertions had resulted from
errors during the PCR amplification or cloning, the insertion of other
nucleotides would have been expected.
Nine of the cloned RNAs that were enriched for U-insertions also
contain deletions. These deletions are preferentially located either at
U-nucleotides or adjacent to inserted U-nucleotides (data not shown).
An A-U Sequence Element Immediately Adjacent to the 5'-Most Editing
Site Is Necessary for the gRNA-independent U-insertions--
Since the
editing sites of most pre-edited mRNAs are rich in purines, it was
initially assumed that the purines may serve as a recognition site for
the U-insertion machinery, accounting for the predominance of
gRNA-independent insertions within this region (Fig. 2C, boxed
sequence). However, deletion of the purine-rich pre-edited region
(Fig. 3,
The U-insertions, instead, are dependent on a 34-nucleotide A-U
sequence that is within the untranslated region immediately 5' of the
in vivo editing sites (Fig. 3A). Deletion of 7 nucleotides from the 5'-end of this sequence resulted in the loss of
82 ± 8% (mean from six reactions) of the gRNA-independent
insertions (Fig. 3B, lane 2), and deletion of 7 nucleotides
from the 3' end of the A-U sequence resulted in an 84 ± 4% (mean
from six reactions) reduction (Fig. 3B, lane 3).
Substitution of eight of the internal A nucleotides with C nucleotides
likewise abolished 86 ± 2% (mean from six reactions) of the
insertions (Fig. 3B, lane 1). The decreased number of
U-insertions within the mutated RNAs is not a result of a decreased
stability within the mitochondrial extract; incubation of radiolabeled
RNA with the mitochondrial extract in the absence of
[
The A-U sequence is sufficient to induce U-insertions when placed
within an unedited mammalian transcript. A 192-nucleotide linear
transcript containing part of the coding sequence of mammalian ferritin
was treated with the mitochondrial extract. After gel-purification, the
extract-treated transcript was annealed with an oligodeoxynucleotide complementary to a sequence at the 3'-end. Digestion with RNase H
followed by electrophoresis on a denaturing gel resolved the 5'-fragment containing internal U-insertions from the U nucleotides added at the 3'-OH by the TUTase (Fig.
4); the 3'-labeled fragments are too
small to be visible in these reactions. The ferritin transcript has a
low level of internal U-insertions as indicated by the labeling of the
5'-fragment (Fig. 4A, lanes 3 and 4). However,
when the 34-nucleotide A-U element was substituted in place of
nucleotides 51 to 85 of the ferritin transcript (Fig. 4A, lanes
5 and 6), the level of internal U-insertions increased
16 ± 1-fold (mean from six reactions). The majority of the
U-additions are internal since the labeling of the 5'-RNase H fragment
(Fig. 4A, lane 6) is 70 ± 10% that of the
non-digested transcript (Fig. 4A, lane 5). The difference
(
A sequence extension on the 5'-end of the A-U element significantly
enhances U-insertions into the linear cytochrome b
transcript but is not absolutely required. The efficiency of
U-insertions within the linear 178-nucleotide cytochrome b
transcript (Fig. 4B, lanes 3 and 4) is decreased
approximately 15-fold relative to the corresponding circular RNA (data
not shown). The 8A to 8C substitution within the linear transcript
(Fig. 4B, lanes 5 and 6) reduced the U-insertions
by a further factor of 2.0 ± 0.4 (mean from 8 reactions),
suggesting that approximately half the U-insertions within the linear
transcript are still dependent on the A-U element (Fig. 4A, lanes
3 and 4). If 60 nucleotides are added to the 5'-end of
linear cytochrome b transcript through circular permutation
(22), the level of insertions approximates that of the corresponding
circular RNA (Fig. 4A, lanes 7 and 8). Thus, the
greater efficiency of U-insertion detected with the circular RNA is
probably a consequence of adding nucleotides to the 5'-end of the A-U
element and not an indirect effect of the circularization. The 8A to 8C
substitution within the A-U element of the 5'-extended transcript
inhibits the U-insertions (Fig. 4B, lanes 9 and
10). This is consistent with the effect of the same mutation
on U-insertions within the circular RNA (Fig. 3B, lane
1).
There Is Specific Binding of the A-U Element to at Least One
Protein within the Fractionated Mitochondrial Extract--
Gel-shift
analysis was used to detect factors within the fractionated
mitochondrial extract that specifically interact with the A-U sequence.
Incubation of sucrose gradient fractions 6 and 7, the peak U-insertion
fractions (Fig. 1A), with a 37-nucleotide transcript
containing the A-U element results in formation of 6 complexes (Fig.
5, lane 2, A-F). Four of the
complexes (labeled A, B, D, and E) are not
detected when radiolabeled 8A to 8C substituted RNA is used in the
assay (lane 1). These four bands are sensitive to
pretreatment of the extract with proteinase K (data not shown). Since
the same 8A to 8C substitutions also inhibited U-insertions within the
cytochrome b transcripts (Fig. 3, lane 1), the
missing complexes (A, B, D, and E) could be
directly relevant to the U-insertion reaction. A greater amount of
complex C forms with the 8A to 8C substituted RNA than with the wild
type A-U transcript. This is suggestive that complex C could possibly
be a precursor to the formation of complex E which is inhibited by the
same mutation.
Three of the bands shifted by the A-U element can be competed by an
unlabeled 41-nucleotide ferritin transcript (Fig. 5, lanes 3-9). Whereas complexes A, B, and D are efficiently competed by the inclusion of low levels of the ferritin RNA during the binding reaction, bands C, E, and F are only competed approximately 50% by a
10-fold molar excess. The low level of U-insertion detected within the
192-nucleotide linear ferritin transcript (Fig. 4A, lanes 3 and 4) and in other transcripts not containing the A-U element (Fig. 3B, lanes 1-3) suggests that the U-insertion
machinery can also interact relatively nonspecifically with RNA. It
seems probable, however, that the induction of U-insertions when the A-U element is present involves complex E as it is both sensitive to
the same A to C substitutions that inhibit U-insertions and is also
relatively resistant to competition by the ferritin transcript. The
Kd for this complex is approximately 200 nM.
The gRNA-independent U-insertion Reaction Is Consistent with a
Mechanism Proposed for gRNA-directed Editing--
Previous studies
have suggested that gRNA-directed editing proceeds through a mechanism
involving sequential endonucleolytic cleavage at an editing site
followed by the addition of uridine nucleotides to the 3'-OH group of
the 5'-fragment and re-ligation of the fragments (23). For the in
vitro editing of the T. brucei ATPase 6 mRNA, the
best characterized in vitro kinetoplastid editing system,
the U-additions and deletions are dependent upon the addition of gRNA
(24, 25). If the U-insertion components can assemble on some L. tarentolae pre-edited mRNAs independent of gRNA, an analogous
mechanism could be mediating the gRNA-independent reaction. Such an
assembly of U-insertion components on the pre-edited mRNA is
supported by the gel-shift analysis (Fig. 5).
Consistent with a cleavage/U-addition/ligation mechanism, an RNA ligase
is present within the same sucrose gradient fractions as the
gRNA-independent U-insertion activity (Fig.
6A, lanes 1-11). Fractions 6 and 7, the peak U-insertion fractions (Fig. 1A), efficiently ligate radiolabeled linear cytochrome b RNAs containing a
3'-OH and 5'-monophosphate in a similar manner to T4 RNA ligase; linear dimers and intramolecular circles are formed in both reactions (Fig.
6A, lanes 6 and 7). The relative concentration of
dimer and intramolecular ligation products is dependent upon the RNA substrate concentration and the sequence of the RNA used in the reaction. The reactions were performed in the presence of ATP and UTP
under the same conditions in which the U-insertions occur. RNAs with a
3'-monophosphate and a 5'-OH are not substrates for the mitochondrial
ligase activity (Fig. 6B), suggesting that if the reaction
does proceed through a cleavage/ligation mechanism, it would require
that a 3'-OH be produced during the endonucleolytic cleavage. The peak
fraction (Fig. 6A, lane 6) has a low level of ligase
activity in the absence of added ATP which is probably the result of
ligases that had been adenylated prior to the fractionation (Fig.
6A, lane 12). Similar to some other RNA ligases (26), the
kinetoplast ligase can also utilize UTP (Fig. 6A, lane 13). The presence of both ATP and UTP stimulates the reaction 6-fold relative to UTP alone (Fig. 6A, compare lanes 6 and 13). Kinetoplastid RNA ligases have previously been
described (27-30).
Intermediates were isolated from the gRNA-independent reaction that are
consistent with a cleavage/U-addition/ligation reaction mechanism.
Endonucleolytic cleavage of the circular cytochrome b
transcript during the U-insertion reaction produces full-length linear
RNAs. The full-length linear molecules formed during the U-insertion
reaction were gel purified and a poly(C) extension was added by poly(A)
polymerase. The extension products were amplified for cloning by RT-PCR
using a poly(G) primer and a primer containing the sequence of the
mutated gRNA-binding site. The position at which the circular RNA was
cleaved within the extract could then be ascertained by determining the
sequence immediately adjacent to the poly(C) extension (Fig.
6C). Under the U-insertion reaction conditions, the sites of
RNA cleavage do not correspond well with the sites of U-insertion
(filled circles, Fig. 6C). Since the gRNA-independent U-insertion machinery appears to be part of a complex
(Fig. 1A), it was reasoned that those RNAs cleaved within the extract as part of the editing reaction would be rapidly re-ligated back to circular RNAs, whereas those that are cleaved by contaminating nucleases would accumulate at a faster rate. In fact, when NTPs are
excluded from the reaction so as to inhibit the ligase (Fig. 6A, A novel direct assay was developed that permitted the enrichment
and characterization of gRNA-independent U-insertions. We have found
that these insertions are dependent on a 34-nucleotide A-U element that
is located within the 5'-untranslated sequence immediately adjacent to
the editing sites. The U-insertions are preferentially localized both
5' and 3' of the A-U sequence. Previously, greater than 95% of the
U-insertions within a population of extract-treated linear cytochrome
b RNAs were indirectly mapped to the pre-edited region using
an RNase H-based assay (9). The localization of the U-insertions (Fig.
2C) combined with the results from the deletion and
substitution mutagenesis (Fig. 3), the induction of U-insertions within
a mammalian ferritin transcript (Fig. 4A), and the gel-shift assay
(Fig. 5) collectively point to the A-U element being a binding site for
at least one component of the U-insertion machinery.
The L. tarentolae maxicircle sequence was searched for
similarity to the A-U element. There is a 20-nucleotide match of the A-U element to a 22-nucleotide sequence of cytochrome b gRNA
I (Fig. 7A). The 8A to 8C
substitutions that reduce nearly 90% of the gRNA-independent
U-insertions (Fig. 3, lane 1) do not support the formation
of complexes A, B, D, and E (Fig. 5, lane 1) and inhibit the
induction of U-insertions within the ferritin mRNA (Fig.
4A) are entirely within this aligned sequence (Fig.
7A). Within this same sequence, there is also a 16 out of
18-nucleotide alignment with the sequence immediately upstream of the
5'-most editing site of the mRNA encoding ND7 (Fig. 7A),
the only other characterized L. tarentolae transcript that
undergoes gRNA-independent U-insertion (9). Deletion of the first 4 nucleotides of the aligned sequence within an ND7 transcript had
previously been shown to significantly reduce U-insertions (ND7.2X
transcript in Ref. 8). This was determined by a primer extension assay limited to the detection of U-insertions within the first editing site.
The result had been interpreted in terms of a structural model but
given the importance of the A-U sequence, the inhibition is probably
more likely a direct consequence of its deletion. These sequence
matches are too high to occur by chance. As a result, there must be an
evolutionary selective pressure to maintain the sequence and,
therefore, there is probably also an associated biological
function.
INTRODUCTION
Top
Abstract
Introduction
References
EXPERIMENTAL PROCEDURES
80 °C. The protein concentration of the unfractionated extract
was 2.3 ± 0.1 µg/µl. A 250-µl aliquot of the extract was thawed on ice and loaded onto a linear 5-ml 10-40% sucrose gradient and centrifuged in a Ti55 rotor at 100,000 × g for
12 h at 4 °C. Fractions (450 µl) were collected from the top
of the gradient.
(3-10) template with primers
10513 and 10211, and the S(8A to 8C) template with 10703 and 10211. The
(35-54) template was synthesized through reverse transcription of
circular wild type cytochrome b RNA with primer 10076 followed by PCR amplification of the extension products using 10076 and
10132. The template for
(31-39) was similarly synthesized by RT-PCR
using 10795 and 10796. The templates for the 5'-extended wild type and
S(8A to 8C) cytochrome b RNAs were synthesized by RT-PCR of
circular wild type cytochrome b and circular S(8A to 8C)
RNAs, respectively, using primers 7845 and 10797. The template for the
RNA transcript used to demonstrate the importance of a 5'-phosphate and
3'-OH to the ligation reaction was synthesized by RT-PCR of circular
wild type cytochrome b using primers 9962 and 9964. This RNA
has 5' and 3' termini that would be produced by endonucleolytic
cleavage 3' of base 44, a location where several U insertions were
detected (Fig. 2C). The template for the wild type
192-nucleotide human light chain ferritin transcript was amplified from
ATCC clone 124280 using primers 10704 and 10705. The template for the
ferritin transcript in which 34 internal nucleotides were substituted
with the A-U element, was constructed in 3 steps. First, the
5'-fragment of the template was amplified with the forward PCR primer
10704 and reverse primer 12122 containing a 3'-extension complementary
to the AU element. Second, the 3'-fragment was amplified with forward
primer 12121, containing a 5'-extension with the sequence of the A-U
element, and reverse primer 10705. Third, the products from the first
two PCR amplifications were gel purified, mixed together, and PCR
amplified using primers 10704 and 10705. The product was cloned into
pBluescript II KS(+) (Stratagene) and verified by sequencing. The
template containing the 8A to 8C mutated A-U element was synthesized
similarly except that primers 12333 and 12334 were used in place of
12121 and 12122. The RNAs for the gel shift assay were transcribed
directly from synthetic oligodeoxynucleotides: 10899 for the wild type
RNA, 12309 for the ferritin competitor transcript, and 12525 for the 8A
to 8C mutant. The promoter portion of the oligonucleotide templates was
annealed to oligonucleotide 5690 prior to initiating the reactions.
-32P]UTP under the conditions described
above with the exception that the total MgCl2 concentration
was increased to 22 mM and the GTP, ATP, and CTP
concentrations to 4 mM each. The transcript used to
demonstrate the importance of 5'-phosphate and 3'-OH groups to the
ligation reaction was primed with 1 mM ApG. ITP was
substituted for GTP during this transcription to ensure that all
transcripts were primed with a 5'-OH (12). The RNA was then either
radiolabeled through kination of the 5'-OH with
[
-32P]ATP or through ligation of
[5'-32P]Cp to the 3'-OH. All cytochrome b
transcripts were purified on 6% polyacrylamide
(acrylamide:bis-acrylamide weight ratio of 19:1), 8 M urea;
the shorter RNAs used in the gel-shift assays were purified on 8%
polyacrylamide, 8 M urea gels. RNAs were eluted and
precipitated with ethanol.
-32P]UTP, 1 mg/ml
Pefabloc SC, and 10 µg/ml leupeptin), the RNAs were incubated at
27 °C for 10 min. Either 15 µl of the fractionated or
non-fractionated mitochondrial extracts were added to the RNAs and
incubated at 27 °C for an additional 50 min. Reactions with non-fractionated extract contained 1 pmol of substrate RNA, whereas those with the fractionated extract contained 0.5 pmol. Reactions were
terminated by phenol/chloroform extraction followed by ethanol precipitation. Products were resolved on 9% denaturing polyacrylamide gels and quantified after PhosphorImager scanning (Molecular Dynamics).
RESULTS
-32P]UTP, salts, and the appropriate
cofactors (Fig. 1A).
Approximately 1% of the input circular cytochrome b
transcripts are modified with U-insertions (assuming an average of 2 insertions per molecule). A major advantage of a circular substrate
over a linear one is that the circularization prevents a TUTase
activity from catalyzing U-addition to the 3'-end. Thus, all the
U-additions to a circle, by definition, have to be internal. Circular
cytochrome b RNA has a significantly different mobility on a
denaturing gel than the corresponding linear molecules, and as a result
the circular RNA is resolved from both the endogenous molecules that
become intensely labeled during the reaction (Fig. 1A,
RNA) and from cytochrome b RNA degradation products
that are also substrates for the TUTase activity (Fig. 1A,
marker). In addition, circular RNAs have been demonstrated
to be more stable to nucleolytic degradation than their linear
counterparts, and the functional properties of many RNAs are not
significantly altered as a result of the ligation (15-17).
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Fig. 1.
An assay for gRNA-independent
U-insertions. A, a circular cytochrome b
transcript containing a guide RNA-binding site (+anchor) was
incubated with solubilized mitochondrial extract in the presence of
[ -32P]UTP and the appropriate salts and cofactors. The
32P-labeled RNA is resolved from the endogenous molecules
(
RNA) that become intensely labeled during the reaction.
Non-extract treated radiolabeled circular and linear cytochrome
b transcripts were used as markers. Mutation of the
gRNA-binding site (
anchor) has little effect on the level
of U-insertions within the circular transcript. The solubilized
mitochondrial extract was fractionated through a 10-40% sucrose
gradient and fractions were individually assayed for U-insertion
activity. The mitochondrial ribosomal RNA and cytochrome b
gRNA I within individual fractions were identified by Northern analysis
(bottom panels). B, mutation to the gRNA-binding
site (nucleotides 58-73) of the pre-edited cytochrome b
transcript used in the assay.
anchor).
This transcript is used in all subsequent assays and will be referred
to as "wild type."
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Fig. 2.
The U-insertions are preferentially localized
either 5' or 3' of an A-U sequence. A, the enrichment
of RNAs with 4-thiouridine insertions. Extract-treated RNAs were
incubated with organomercury-derivatized agarose and after washing,
eluted with dithiothreitol (DTT) for RT-PCR and cloning.
B, the sequence of the pre-edited cytochrome b
transcript prior to treatment with the editing extract (non-treated),
and the single lane U-ladder for 10 of the RNAs cloned after extract
treatment and organomercury column enrichment. The in vivo
editing sites are bracketed. C, the location and
number of the in vitro gRNA-independent U-insertions within
the individually sequenced clones. The results are from two independent
experiments that permitted the entire sequence to be screened for
insertions. Either nucleotides 1-26 and 144-178 were used as
primer-binding sites during the RT-PCR (arrows) or
alternatively nucleotides 73-91 and 99-118 (circles). The
boxed sequence corresponds to the region that is edited
in vivo; the correctly edited sequence is indicated
below.
(35-54)), rather than
inhibiting U-insertions, stimulated the reaction by a factor of
1.7 ± 0.3 (mean from six reactions). As already indicated (Fig.
1), modification of the gRNA-binding site has no significant effect on
the reaction efficiency. Deletions made 3' of the gRNA-binding site
also have no effect (data not shown).
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Fig. 3.
A 34-nucleotide A-U sequence is necessary for
the U-insertions. A, description of the mutated
cytochrome b transcripts. The deletions ( ) are
bracketed. The mutant S(8A to 8C) contains substitutions at
the indicated positions (dashed lines). All RNAs have a
mutated gRNA-binding site and are circular. The boxed
sequence corresponds to the region that is edited in vivo.
The location of the putative AUG translation initiation codon, created
through editing, is indicated by a star (43). B,
effect of the mutations on the gRNA independent U-insertions
(extract labeled, lanes 1-6). To assess the
relative stability of the RNAs within the extract, radiolabeled
transcripts were incubated with the extract under U-insertion
conditions in the absence of [
-32P]UTP (markers,
lanes 7 and 8). Minus denotes no added RNA.
-32P]UTP indicated that the S(8A to 8C) mutated RNA
is as stable as the wild type (Fig. 3B, lanes 7 and
8).
30%) represents labeling at the 3'-end which also appears to be
stimulated approximately 2-fold relative to the ferritin transcript.
Substitution of 8A with 8C nucleotides within the 34 nucleotide element
(S(8A to 8C), see also Fig. 3A) of the chimeric transcript
inhibited the induction of internal U-insertions (Fig. 4A, lane
8) but did not inhibit the additions to the 3'-end (Fig. 4A,
lane 7). This is consistent with the cytochrome b RNA
mutagenesis (Fig. 3B, lane 1) and suggests that the
U-insertions are induced directly by the A-U element rather than being
an indirect consequence of the 34-nucleotide substitution. It is
possible, however, that the stimulation of 3'-end labeling observed
with both the wild type A-U and the 8A to 8C substitutions are the result of such an indirect effect.
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Fig. 4.
Induction of U-insertions.
Extract-labeled linear RNA was annealed at its 3'-end to a
oligodeoxynucleotide and incubated in the presence (+) or absence ( )
of RNase H. Non-extract treated radiolabeled transcripts were used as
markers for the RNase H reaction. A, U-insertions are
induced within a 192-nucleotide ferritin transcript (FER, lanes
3 and 4) when 34 nucleotides of internal ferritin
sequence are replaced with the A-U element (plus A-U, lanes
5 and 6). The 8A to 8C substitution within the A-U
element of the chimeric transcript inhibits this induction (S(8A
to 8C), lanes 7 and 8). B, nucleotides 5' of
the A-U element influence the U-insertions. The RNase H assay was also
used to detect U-insertions within the linear wild type cytochrome
b RNA containing the mutated gRNA-binding site (lanes
3 and 4). The insertions are induced 15-fold when 60 nucleotides are added to the 5'-end of the A-U element (5'-extension,
lanes 7 and 8). The U-insertions within both the
linear and 5'-extended RNAs are inhibited by the 8A to 8C substitution
(lanes 5, 6, 9, and 10).
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Fig. 5.
The A-U element specifically binds at least
one protein within the fractionated mitochondrial extract. Several
RNA-protein complexes form upon incubating the fractionated
mitochondrial extract (fractions 6 and 7 pooled) with a 37-nucleotide
transcript containing the 34-nucleotide A-U element (lane 2)
or the 8A to 8C substitution (lane 1). An unlabeled ferritin
transcript was added as competitor with the wild type RNA in
lanes 3-8. As a control, the wild type sequence was also
incubated in the absence of extract (lane 9).
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Fig. 6.
The gRNA-independent U-insertion reaction is
consistent with the mechanism proposed for gRNA-directed editing.
A, an RNA ligase co-fractionates with the U-insertion
activity. Linear 32P-labeled cytochrome b
transcript containing a 5'-monophosphate and a 3'-OH group was
incubated with the sucrose gradient fractions. There is little ligation
in the absence of added NTP ( NTP, lane 12). UTP can
support the reaction (+UTP, lane 13). B, although
linear cytochrome b RNAs containing a 3'-OH and a
5'-monophosphate are chased into circular products by the fractionated
mitochondria extract, RNAs with a 3'-monophosphate and a 5'-OH are not.
C, under the U-insertion reaction conditions, the sites of
endonucleolytic cleavage (filled circles) are not within the
in vivo edited region (boxed) and do not
correspond well with the sites of gRNA-independent U-insertion (Fig.
2C). If the RNA ligase is inhibited through the exclusion of
NTP from the reaction, the sites of cleavage (open circles)
in 50% of the sequenced molecules now correspond to the sites of
U-addition.
NTP), the sites of gRNA-independent
cleavage in 50% of the sequenced RNAs now correspond with the sites of
the gRNA-independent U-insertions (compare Fig. 2C with the
open circles in Fig. 6C). In the presence of UTP,
41% of the linearized RNAs also had 1 to 3 U nucleotides added at the
3'-end (data not shown). Thus, the intermediates of the
gRNA-independent reaction are suggestive of an
endonuclease/U-addition/ligase-mediated reaction.
DISCUSSION
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Fig. 7.
A biological role for the A-U element.
A, the L. tarentolae maxicircle was searched for
sequences containing the A-U element. There is a 20-nucleotide match of
the A-U element within a 22-nucleotide sequence of cytochrome
b gRNA I (nucleotides 16760-16781 in LEIKPMAX) and a
16-nucleotide match within an 18-nucleotide sequence immediately 5' of
the editing sites in the mRNA encoding ND7 (nucleotides 3282-3299
in LEIKPMAX). Vertical lines indicate identical bases. The
location of the 8A to 8C substitutions that inhibit over 80% of the
U-insertions are also indicated. Cytochrome b gRNA I is
shown below base pairing with the pre-edited mRNA
(bottom). Lowercase letters represent the
templating nucleotides. The sequence of the gRNA that matches that of
the A-U element is shaded and the first 9 editing sites are
boxed. B, A model for the function of the A-U
element. Editing factors assemble on the A-U element of the pre-edited
mRNA independent of gRNA. Accurate editing would ensue after the
binding of gRNA and the transfer of the editing factors to its A-U
element.
There are deletions to the A-U element outside the region of sequence
overlap with cytochrome b gRNA I and the ND7 mRNA that also inhibit U-insertions (Fig. 3B, lanes 2 and
3). Both cytochrome b gRNA I and the A-U element
of the mRNA are predicted by MFOLD (31) to form imperfect hairpin
loops of low thermodynamic stability (
3 kcal/mol) analogous to
that described for several T. brucei gRNAs (32). It is
possible that the sequence flanking the alignment of the 3 RNAs
contributes to the formation of a common secondary structure. The
post-transcriptionally added poly(U) tail of the gRNA, for example,
could have a structural role analogous to that of the extended A-U
sequence on the cytochrome b mRNA.
The A-U element was demonstrated to be important for the gRNA-independent reaction, and the high degree of similarity with the templating sequence of cytochrome b gRNA I implies that this part of the gRNA may interact with some of the same components of the U-insertion machinery. This is further supported by the observation that, with the exception of complex B, the same complexes that form in the editing extract with the A-U element (Fig. 5) also form with cytochrome b gRNA I.2 In addition, we have found that cytochrome b gRNA I is able to compete with the A-U element for binding of the six protein complexes. The cytochrome b gRNA I competes 20-80-fold better than the ferritin competitor (Fig. 5) for binding to the proteins comprising complexes C, E, and F.2 Thus, changes to the gRNA templating sequence that disrupt interactions with editing components would be predicted to be detrimental even if the potential to guide a functional coding sequence within the pre-edited mRNA was maintained. As a result, there could be two different evolutionary forces conserving this sequence and limiting the opportunity for genetic drift.
The intermediates produced during the gRNA-independent reaction are consistent with the endonuclease/U-addition/ligation mechanism previously proposed for gRNA-directed editing based on the following observations. First, there is a ligase activity within the fractionated mitochondrial extract that co-fractionates with the gRNA-independent U-insertion activity. Linear molecules containing 3'-OH and 5'-phosphate groups, the same groups produced during gRNA-directed editing (24, 25), are chased into products by the co-fractionating ligase activity whereas RNAs containing a 3'-phosphate and 5'-OH are not. Second, there is significant accumulation of linear RNAs corresponding to cleavage of the circular cytochrome b RNA at sites of U-insertion when the ligase activity is inhibited through the exclusion of NTPs from the reaction. Third, in the presence of UTP, U nucleotides are added to the 3'-end of the linear molecules. These apparent similarities in mechanism are consistent with the possibility that the gRNA-independent reaction exploits some of the same editing components used during gRNA-directed editing.
The A-U element could, thus, be serving as an assembly point for the editing machinery on those pre-edited mRNAs for which it is especially important to be efficiently edited (Fig. 7B). The original model for gRNA-directed editing, in fact, postulated that editing components may interact with the pre-edited mRNA independent of gRNA (4). The pre-assembled editing factors would be able to insert U nucleotides within both the 5' and 3' adjacent sequence (Fig. 7B, top). This would explain the localization of the majority of the in vitro gRNA-independent U-insertions (Fig. 2C). U-insertion components could be transferred to the A-U element of the gRNA upon its binding to the pre-edited mRNA (Fig. 7B, middle). Cytochrome b gRNA I was previously shown to specifically inhibit the independent U-insertion reaction (9, 10) and this, in part, could be a result of the machinery being transferred from the A-U element of the mRNA to that of the gRNA. Accurate editing would occur after the transfer of editing factors to the gRNA (Fig. 7B, bottom). Additional experiments are being performed to futher test this model.
Although the A-U element has only been identified within cytochrome b gRNA I, there are several other kinetoplastid gRNAs that are very A-U rich. Selection-amplification (33, 34) is being used to identify the sequence permutations of the A-U element that are able to support the internal U-insertions. Knowing the permissible permutations will be informative in determining whether related elements are found on other gRNAs. It is also possible that there are gRNA and/or mRNA-specific factors required for editing. The latter possibility is likely since there are life cycle stages in T. brucei during which specific mRNAs are not edited, even though both the pre-edited mRNA and appropriate gRNA are present (35-37).
Although the A-U element is able to support the U-insertion reaction and form specific complexes with the mitochondrial extract, the reaction is significantly enhanced by the addition of 5'-nucleotides either as a result of the intramolecular ligation or an artificial extension (Fig. 4B). This additional requirement for efficient U-insertion could also limit inappropriate U-insertions by the editing components assembled on the mRNA in vivo. The enhancement could be occurring in vivo, however, if the cytochrome b pre-edited mRNA, like the mitochondrial genome of some eucaryotes (see Ref. 38; reviewed in Ref. 39) is transcribed as part of a polycistronic precursor. Alternatively, an enhancement could possibly result from interactions between the 5'- and 3'-ends of the mRNA during processing and translation (40, 41). There are examples of in vivo misediting of cytochrome b mRNA (42), and it is possible that some of these events are a consequence of having editing components assembled on the pre-edited mRNA independent of gRNA.
The enhancement of the gRNA-independent U-insertions by an artificial 5'-extension on the cytochrome b mRNA is in agreement with previous results from a primer extension-based assay (10). However, the results from the primer extension assay also suggested that the 5'-extension was required to be base paired, and no such requirement was detected in this study. The discrepancy probably is a result of the primer extension assay being limited to the detection of U-insertions within the first 2 editing sites. Subtle changes in the structure of the RNA could be influencing the sites at which the U-insertions are occurring. Although the A-U element is a major determinant of the reaction, we cannot eliminate the possibility that the reaction is also influenced by other sequence and/or structural elements. Changes in structure may account for the increased level of U-insertion observed with the deletion of the pre-edited region (Fig. 3B, lane 4).
A-U rich sequences are found within the 5'-untranslated region of
several mitochondrial mRNAs. The L. tarentolae A-U
element is not completely conserved within the cytochrome b
mRNA of T. brucei and Crithidia fasciculata,
but both of these related genera of trypanosomes have similar sequences
immediately upstream of the editing sites (43). An A-U-rich sequence
present within the 5'-untranslated region of the Saccharomyces
cerevisiae cytochrome b mRNA has been implicated in
an interaction with the translation machinery (44). This raises the
intriguing possibility that the editing components interacting with the
L. tarentolae A-U element could either be associated with
the translation machinery or perhaps evolutionarily related.
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ACKNOWLEDGEMENTS |
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We are grateful to B. Bass, H. Cameron, E. Christian, H. Hiasa, J. Maher, K. Musier-Forsyth, T. Pan, and P. Siliciano for helpful suggestions during the course of this work.
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FOOTNOTES |
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* This work was supported by the University of Minnesota Graduate School, American Cancer Society Grant IRG IN-13, and National Institutes of Health Grant R29-AI41138.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
To whom correspondence should be addressed. Tel.: 612-624-3132;
Fax: 612-625-8408; E-mail: gconnell{at}lenti.med.umn.edu.
2 L. Brown and G. Connell, unpublished results.
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ABBREVIATIONS |
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The abbreviations used are: gRNA, guide RNA; PCR, polymerase chain reaction; RT, reverse transcriptase; nt, nucleotide; TUTase, terminal uridylyltransferase.
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REFERENCES |
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