(Received for publication, November 20, 1996)
From the Department of Biochemistry and Molecular
Biology, Colorado State University, Fort Collins, Colorado 80523-1870 and the § Department of Medical Biochemistry, Southern
Illinois University, Carbondale, Illinois 62901-4409
A new ribosomal RNA promoter element with a functional role similar to the RNA polymerase II initiator (Inr) was identified. This sequence, which we dub the ribosomal Inr (rInr) is unusually conserved, even in normally divergent RNA polymerase I promoters. It functions in the recruitment of the fundamental, TATA-binding protein (TBP)-containing transcription factor, TIF-IB. All upstream elements of the exceptionally strong Acanthamoeba castellanii ribosomal RNA core promoter, to within 6 base pairs of the transcription initiation site (tis), can be deleted without loss of specific transcription initiation. Thus, the A. castellanii promoter can function in a manner similar to RNA polymerase II TATA-less promoters. Sequence-specific photo-cross-linking localizes a 96-kDa subunit of TIF-IB and the second largest RNA polymerase I subunit (A133) to the rInr sequence. A185 also photo-cross-links when polymerase is stalled at +7.
Because promoters for some small nuclear RNA genes switch between polymerase II and III as a result of simple sequence/spacing alterations, promoters for eukaryotic RNA polymerases II and III are considered more similar to each other than to the RNA polymerase I promoter (reviewed in Ref. 1). Most polymerase II promoters contain a TATA box, or an initiator element (Inr),1 or both (2). The Inr surrounds the transcription initiation site (tis) and the TATA box is upstream about 30 base pairs. The TATA box is the specific binding site for TATA-binding protein (TBP), the subunit common to the fundamental transcription factors (3) for all polymerases (4). On polymerase II genes with TATA boxes, TBP alone can nucleate the assembly of an initiation complex. In contrast, TATA-less promoters require additional factors or TFIID subunits, called TBP-associated factors (TAFs) (5). Drosophila TAFII150 (dTAFII150) specifically interacts with sequences including the Inr, tethering TBP to the promoter (6), and nucleating the assembly of the initiation complex. In humans, the functional dTAFII150 homolog is CIF and is not tightly associated with TFIID (7). Promoters with both sequence elements are unusually strong, because they bind TFIID very efficiently.
The Acanthamoeba castellanii rRNA core promoter is also unusually strong, binding TIF-IB with a dissociation constant of approximately 30 pM.2 We noted that this promoter, along with the promoters for other rRNA genes, contains a conserved sequence element which surrounds the tis (8). This is the only well-conserved sequence in eukaryotic rRNA promoters. In addition, Windle and Sollner-Webb (9) observed that in Xenopus laevis oocytes injected with huge quantities (approximately 1 × 107 copies) of plasmid ribosomal DNA, the minimal rRNA promoter only encompassed this conserved sequence, although this seemed to be unique to this system and unusual assay condition. In Arabidopsis thaliana, mutations in the conserved sequence in the context of the full-length rRNA promoter decrease transcription in transient expression assays (10). However, the conserved element alone was incapable of directing transcription initiation. Similarly, point mutations of this sequence in A. castellanii alter transcriptional activity in an in vitro transcription system (11). The advent of the extremely sensitive phosphorimaging technology prompted us to reexamine whether this conserved sequence element could independently direct specific transcription initiation in the A. castellanii highly purified transcription system, and, if so, to begin an investigation of its mechanism.
A. castellanii were grown in Neff's optimal growth medium in a gyratory shaker as described (12).
Transcription Factor and RNA Polymerase ITIF-IB was purified through the second promoter-DNA affinity column chromatography step as described (13). RNA polymerase I was purified from whole cell extracts either using the standard ammonium sulfate method (14) or a modified KCl procedure (12). Both RNA polymerase I preparations gave identical results.
In Vitro Transcription AssaysAvaI-SalI DNA fragments from the
plasmids pSBX60i, 12 and
6 deletions (15), were used as the
templates for runoff transcription. The reaction conditions were as
described in Ref. 16 with the following modifications: the KCl
concentration was reduced to 100 mM, the DNA template was
reduced to the amounts indicated on the figures, and the reactions were
processed and analyzed as described in Ref. 13.
The runoff transcription reactions
were scaled up 4-fold, and the transcripts were used as templates for
primer extension reactions as described (17). The primer was the same
as positions 891 (5) to 908 of pBR322, which should yield a
66-nucleotide extension product.
The photo-cross-linking reactions were carried out as described (18) with the following modifications. The indicated nucleotides were added to the reactions, and the reactions were incubated at 25 °C for 10 min, followed by the addition of heparin to 20 µg/ml. After an additional 10 min at 25 °C, the reactions were exposed to UV light.
The limits of the minimal A. castellanii rRNA promoter
were reevaluated in vitro using highly sensitive
phosphorimage technology to detect transcripts. Extensively purified
TIF-IB and RNA polymerase I were used to test the 5 boundary of the
promoter. Previous studies (15, 19) identified the region from
31 to
+8 as the minimal sequence necessary to obtain specific transcription
initiation. Deletion beyond
31 resulted in a loss of transcription
runoff RNA detectable by autoradiography. However, phosphorimage
technology offers an approach which is capable of detecting
isotopically labeled products at a significantly lower level. When the
same deleted templates used previously were assayed using storage
phosphor screens, transcription of genes deleted from their 5
end to
12 or even to
6 was detected (Fig. 1A,
lanes 2, 3, 5, and 6). The wild type sequence in these templates ends at +19, with the pBR322 sequence beyond. The efficiency of these deleted templates is significantly lower than the wild type template; for comparison, the
amount of wild type template was decreased to 5% that used in the
deleted template reactions (lane 1). Such weak transcripts would not have been detected using autoradiography and the correct exposure for the template containing the full promoter.
These transcripts are correctly initiated. 1) They are dependent upon
TIF-IB (lanes 4 and 7), which is required for RNA
polymerase I to recognize the promoter (19, 20). 2) Other restriction enzyme fragment templates produce correct runoff transcripts of predicted lengths (data not shown). 3) Both wild type and deletion mutant genes exhibit identical 5 ends in primer extension assays (Fig.
1B, lanes 2, 4, and 6). The
primer used in these assays is complementary to the pBR322 vector
sequence, so the assay is not detecting any contaminating rRNA which
might be present in the protein preparations. The latter is extremely
unlikely because these proteins have each been purified through
multiple chromatographic columns. Multiple primer extension products
have been observed before for in vitro rRNA transcripts in
this and other systems and result either from artifacts of the method
or heterogeneity of tis selection (20). They do not arise as
artifacts because of secondary structure in a readthrough transcript
originating upstream of tis. To test this notion, a T3 RNA
polymerase transcript which originates upstream of the normal
tis, thus having the same sequence as an RNA polymerase I
readthrough product, was analyzed by primer extension. The T3
readthrough product does not produce any primer extension products in
the region of the tis (lane 1), but does produce
a strong full-length extension product (data not shown).
The minimal promoter detected in these assays is much smaller than in
our previous study (15). Remarkably, both 12 and
6 are outside the
region footprinted by the fundamental transcription initiation factor,
TIF-IB, from A. castellanii (21, 22). However, this region
does exhibit enhanced bands in some footprinting experiments, and point
mutations in this region affect transcription efficiency (11).
To map the TIF-IB and RNA polymerase I subunits along the template, we
subjected template-committed complexes to site-specific photo-cross-linking by incorporation of
5-[N-(p-azidobenzoyl)-3-aminoallyl]-deoxyuridine monophosphate into the template (18).
5-[N-(p-azidobenzoyl)-3-aminoallyl]-dUMP was
incorporated into positions 1,
3,
5, and
7 in place of dTMP. In
9 separate experiments with several preparations of TIF-IB at various
stages of purity, a specific 98-kDa protein photo-cross-linked weakly
to the putative rInr sequence (Fig. 2A,
lane 3). This protein was identified as TAFI96
of TIF-IB based upon previously published criteria: TAFI145
yields cross-linked bands of 153-154 kDa, TAFI99 yields
cross-linked bands of 109-110 kDa, and TAFI96 yields
96.8-98-kDa, TAFI91 yields 92.5-93-kDa, and TBP yields
39-kDa bands because of the short covalently linked DNA tag. (See also
Ref. 18 for a complete discussion of assignments.) The experiment shown
in Fig. 2A used TIF-IB in which no proteins other than TBP
and the four TAFIs could be identified in silver-stained
polyacrylamide gels. However, even in impure preparations of TIF-IB in
which the TAFIs are not easily identified in the mass of
contaminating proteins, no other proteins photo-cross-link to this
position, indicating the specificity of this photo-cross-linking
technique.
When RNA polymerase I is added to the committed complex, a protein of approximately 135 kDa photo-cross-links (Fig. 2A, lane 4). To verify that this is a component of RNA polymerase I, photo-cross-linking was performed across the peak of RNA polymerase I from a glycerol gradient using the same affinity probe (Fig. 2, B and C). The amount of photo-cross-linked 135-kDa product (Fig. 2C) correlates closely with RNA polymerase I activity determined in a specific runoff assay (Fig. 2B). The homogeneous RNA polymerase I used in this experiment only contains the two large subunits, 185 and 133 kDa, in this size range (14, 23). On this basis, the photo-cross-linked subunit was identified as the second largest subunit, A133, of RNA polymerase I.
When RNA polymerase I is bound to the promoter, the efficiency of photo-cross-linking of TAFI96 is significantly increased (lane 4). Thus, the binding of polymerase results in closer proximity of TAFI96 to the template. Based on this and other data,3 we propose that polymerase stabilizes TIF-IB binding and suggests an interaction between TAFI96 and polymerase I. Perhaps this interaction induces a conformational change in TIF-IB making it more accessible to the photo-cross-linking reagent. Following initiation and stalling of the polymerase at +7 by addition of ATP and GTP, but omission of CTP, both the second largest (A133) and the largest (A185) subunits of polymerase I photo-cross-linked (lane 5). Under single-round transcription conditions, once polymerase has cleared the promoter following addition of all four ribonucleoside triphosphates, photo-cross-linking of the polymerase subunits is lost. This underscores the correct identification of the two polymerase subunits. Following promoter clearance, TAFI96 photo-cross-linking to the probe reverts to the weak efficiency seen before polymerase was added (Fig. 2A, lane 6), consistent with the loss of a polymerase-induced conformational change in TIF-IB. TIF-IB is left behind to recruit successive RNA polymerase I molecules to the template, as previously shown (24).
The efficiency of cross-linking is very reproducible and extremely
dependent upon the structure of the photo-cross-linking probe. We have
tested initiation (Fig. 3, lanes 1-5) and
stalled (Fig. 3, lanes 6-10) complexes using a range of
derivatives with arm lengths from 8.0-20.9 Å. All of the derivatives
cross-link the same subunits, but with different efficiencies. In the
initiation complex, the 12.3-Å probe is most efficient at
cross-linking TAFI96, and the 12.8-Å probe cross-links
A133 best. There is a strikingly different dependence in
the stalled complex, suggesting changes in probe conformation or
subunit positional differences between the complexes. The 8.0-Å probe
cross-links TAFI96 and A133 far more strongly
than any of the other probes. In contrast, A185 is
cross-linked about equally by the 8.0- and 12.8-Å probes, but the
12.3-Å probe and probes longer than 12.8 Å are less efficient at
photo-cross-linking this subunit. These probes also differ in the
hydrophobicity of the linker arm, which may account for the apparent
anomalous length dependence for the A185
subunit.4
We propose ribosomal RNA promoters contain a sequence which is
functionally similar to the Inr element found in RNA polymerase II
promoters, the rInr, and defines yet another class of Inr (2). Importantly, a similar small minimal promoter was found when large amounts of truncated (to 9) X. laevis rRNA genes were
injected into oocytes (9), also demonstrating functionality of an
Inr-like element in a non-polymerase II promoter. In further support of this notion, we note that all eukaryotic rRNA promoters have an element
near the tis whose mutation alters promoter activity. We
previously described a remarkably conserved sequence which is present
in a large number of rRNA promoters (8):
n(g/r)(g/r)Gt(T/A)aTnT
gGG(a/g)gAn, where the underlined A is the tis. The conservation of this
sequence in rRNA genes is significantly stronger than the consensus
sequence for the polymerase II Inr (25). Point mutations of the
A. castellanii rInr affect transcription (11). Similarly,
point mutants of the homologous rInr sequence in A. thaliana
affect transcriptional activity (10). However, even deletion of the
A. castellanii rInr in the context of the full-length
promoter does not affect subsequent RNA polymerase I binding (20).
Thus, as is the case for Inr-less polymerase II promoters,
TAFI96 can be tethered to the tis by upstream
promoter elements just as the Inr can tether TBP to TATA-less promoters
(6). This reveals a functional similarity between promoters for RNA
polymerases II and I which has previously gone unappreciated.
It has been argued that RNA polymerases II and III are more similar to each other because the sequences of their largest subunits are evolutionally more similar to each other than to polymerase I (26), and several snRNA gene promoters can be switched between polymerase II and III by rather simple deletions or promoter element spacing changes (reviewed in Refs. 1 and 27). However, this ignores the fact that polymerases I and III share more subunits in common than polymerases II and III (in yeast, the AC40 and AC19, in addition to the five subunits common to all three enzymes (28)). The demonstration here that the structure of the promoters for polymerases I and II might be more similar than previously recognized suggests this supposition should be reexamined.