Department of Biochemistry and Biophysics and Medicine, University of North Carolina-Chapel Hill, Chapel Hill, North Carolina 27514
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
There are several independent metabolic steps that determine the level of a protein in eukaryotic cells. The steady-state level of the mRNA encoding the specific protein is determined by rate of transcription, percentage of transcripts that are ultimately processed and transported to the cytoplasm, and half-life of the mRNA in cytoplasm. The amount of protein that accumulates from a particular transcript is influenced not only by the amount of mRNA present in the cytoplasm but also by the rate of translation of the mRNA and stability of the protein product. There is compelling evidence that the steady-state level of many proteins is regulated at multiple steps, and when there is a large change in the amount of either mRNA or protein it is likely that multiple steps in the metabolism of the mRNA and protein have been altered. In the case of type I collagen production in the fibrotic liver, recent work has shown that there is regulation of multiple steps resulting in an ~70-fold increase in collagen production by the hepatic stellate cells. In addition to the well-documented relatively small effect on transcription, there are effects on processing/transport of the mRNA, translation of the mRNA, and stability of the mRNA. Large changes of protein levels are produced by altering the rates or efficiency of multiple steps. The molecular details of some of these posttranscriptional regulatory events are currently being elucidated. Here we review the various potential steps for regulation in the synthesis of a protein and discuss how the synthesis of type I collagen may be regulated in the fibrotic liver.
translation; transcription; mRNA stability; collagen 1(I); gene
regulation
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
ONE ASPECT OF REGULATION OF protein synthesis that is often overlooked is the rate of translation. The context of the initiation codon is important for efficient recruitment of the 40S ribosomal subunit. mRNAs that contain a strong "Kozak sequence," the preferred consensus sequence, are translated more efficiently than mRNAs that have a poor context for the translation initiation codon (14). Other potential modulators of the rate of translation of a particular mRNA include the codon utilization. mRNAs that utilize the common codons for a particular amino acid for which there are abundant tRNAs are translated more efficiently than those that utilize rare codons.
There is normally competition among the many mRNAs for the
translation initiation factors, which clearly limit the translation process. Regulation of translation can then be accomplished by altering
the efficiency of assembly of the initiation complex on the mRNA. The
initiation of translation requires the recruitment of a number of
translation initiation factors to the mRNA that in turn result in the
recruitment of the 40S ribosomal subunit, followed by the initiator
methionine tRNA. The formation of an initiation complex requires the
recruitment of eIF4G to the mRNA, which in turn binds to eIF3
that recruits the 40S ribosomal subunit to the translation initiation
codon. Efficient translation of an mRNA requires that it contain both a
5' 7-methylguanine (7mG) cap and a poly(A) tail, both of which are
critical for efficient translation initiation (13, 20). A
model of a translation complex is shown in Fig.
1 and described below.
|
eIF4G is an ~200-kDa protein that has multiple regions that aid in assembly of the initiation complex. It binds with eIF4e, a ~25-kDa polypeptide that binds the 7mG cap found at the 5' end of mRNAs. Surprisingly, it also interacts with the poly(A) binding protein (PABP)-poly(A) complex found at the 3' end of all mRNAs except histone mRNAs (26, 12). This results in a "circular" mRNA, and a complex containing eIF4G bound to eIF4e at the 5' cap and PABP bound to the poly(A) tail may be the physiological translation initiation complex in mammalian cells. eIF4A, a member of the RNA helicase superfamily, also binds with eIF4G, and the complex then can translocate down the mRNA to the initiation codon.
At least three different types of sequences can affect regulation of translation initiation on an mRNA. Proteins that bind specific sequences in the 5' untranslated region (UTR) can block the formation of the initiation complex containing the 40S ribosomal subunit, completely inhibiting translation (8). The best-studied example is the regulation of iron metabolism, in which the iron response element binding protein binds the iron response element in the 5' UTR of a group of mRNAs, including ferritin mRNAs. As a result, the ferritin mRNA is found in the cytoplasm not associated with the ribosomes.
Sequences in the 3' UTR of the mRNA can affect the rate of translation initiation of an mRNA. Although the biochemical details of the function of these sequences have not been determined, these sequences likely function by binding specific proteins (2, 31). The resulting protein-RNA complex may affect the interaction of PABP either with the poly(A) tail or with eIF4G. Alternatively, they could affect the interaction of eIF4e with the cap. The result is to modulate the rate of translation of specific subsets of mRNAs. An mRNA that is being efficiently translated has multiple ribosomes bound to the mRNA at any one time, with the ribosomes located 80-100 nt apart. Translation occurs at ~5 amino acids/s (15 nt/s), so that the time between ribosomes is ~6 s. An mRNA that is being translated inefficiently due to a low rate of translation initiation will have the ribosomes widely spaced and will have fewer ribosomes bound at any one time. The distribution of mRNAs on polyribosomes can be determined by sucrose gradient analysis of cytoplasmic polyribosomes. As the translation rate of an mRNA increases, the mRNA becomes associated with larger polyribosomes.
A third mechanism of regulation of translation utilizes small open reading frames (ORFs) in the 5' UTR to control the rate of translation initiation. About 10% of mRNAs, many of which are involved in regulation of cell growth, contain short ORFs in the 5' UTR. These upstream ORFs are often translated, and regulation of the termination of their translation affects the translation of the downstream ORF (7). In the extreme cases, translation of the upstream ORF in the mRNA is arrested, with the mRNA associated with a single ribosome. Only when the ribosome completes translation is initiation of the downstream AUG activated. The regulation of translation of these mRNAs is determined by the termination and release of the upstream ORF-bound ribosome.
It may take up to 30 min to transcribe, process, and transport the average-sized mRNA and several hours to accumulate high levels of an mRNA. Translational regulation allows the cell to rapidly respond to changes in the environment by altering the rate of synthesis of specific proteins. It is probable that there is a large number of mRNAs whose translation is regulated. For example, a broad screen in yeast comparing the changes in mRNAs with the changes in proteins demonstrated that at least 20% of the yeast mRNAs did not show parallel changes with protein levels (9). Since this screen only detected the most abundant proteins, it likely underestimates the number of yeast mRNAs that are translationally controlled. In the future, we expect to see many more examples of posttranscriptional regulation being quantitatively important in the determination of the rate of production of a protein.
![]() |
REGULATION OF MRNA DEGRADATION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Despite the fact that most mRNAs decay via a common pathway
(21), there is a wide range of half-lives for different
mRNAs, ranging from minutes to hours. The stability of a particular
mRNA is determined by cis elements, which may be present in
the 5' UTR, the coding region, or in the 3' UTR (18).
Regulation of the half-life of mRNAs is probably accomplished by
alteration of factors that interact with specific sequences in the
mRNA. A model for the regulation of mRNA degradation is presented in Fig. 2 and described below.
|
The major strategies for degrading mRNAs are conserved from yeast to human. The most common decay pathway occurs as a result of shortening of the poly(A) tail. When the poly(A) tail reaches a critical minimal length, the PABP presumably is no longer bound, and the loss of PABP disrupts the circular mRNA structure described above. The mRNA is then decapped and becomes accessible to exonucleases that can degrade the mRNA 5' to 3'. There are also enzymes that can degrade the mRNA 3' to 5'. The rates of the initial steps in degradation of specific mRNAs can be modulated by cis and trans elements that specifically bind to regions in the mRNA and cause degradation of specific transcripts in response to certain stimuli. For example, AU-rich elements located in the 3' UTR of many mRNAs generally destabilize the mRNA by binding specific proteins (10, 16).
Since the normal location of an mRNA is on polyribosomes on which it is being actively translated, it is not surprising that the initial steps in mRNA decay occur while the mRNA is associated with polyribosomes. As a result, many mRNAs are stabilized by inhibitors of protein synthesis. Degradation of an mRNA requires several enzymatic processes: deadenylation, decapping, and exonuclease activity. Recent results have indicated that many of these activities may be contained in a single complex, termed the exosome, or degradosome (1). This complex may function in a manner similar to the proteosome complex for protein degradation.
![]() |
REGULATION OF MRNA DEADENYLATION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
In eukaryotes, PABP prevents deadenylation by preventing the
degradation machinery from interacting with the RNA. Thus any factors
that affect the rate of deadenylation will alter the half-life of the
mRNA. The stability of many mRNAs is regulated by sequences in their 3'
UTRs and determined by proteins interacting with these sequences.
Factors that bind the 3' UTR are logical candidates for also
interacting with the PABP-poly(A) complex and affecting the rate of
deadenylation. For example, the HuR family of proteins binds
AU-rich elements in the 3' UTR, resulting in stabilization of the
poly(A) tail and the mRNA, whereas hnRNPD (Auf1) binds the
same sequences and destabilizes the mRNA. This may occur by a
disruption of the PABP interaction with either eIF4G or the poly(A)
tail of the transcript. The -globin and collagen
1(I) mRNAs are
stabilized by complexes bound to the 3' UTR that contain
CP2. These complexes may interact with PABP and
stabilize the binding of PABP to the poly(A) tail or eIF4G (5,
11, 17), resulting in an increased stability of the mRNAs.
![]() |
DECAPPING |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Decapping usually follows the loss of the poly(A) tail and is mediated by an enzyme found in the degradosome (1, 4). The decapping is thought to allow a 5' to 3' exonuclease (Xrn1 in yeast) to degrade the mRNA. An mRNA that has been deadenylated and decapped may be degraded by either or both 5' to 3' or 3' to 5' mechanisms. There is also evidence that degradation of some mRNAs is initiated with an endonucleolytic cleavage (4). Cleavage in the 3' UTR would effectively deadenylate the mRNA, whereas cleavage in the 5' UTR would effectively decap the mRNA. Once the initial cleavage occurs, then the exonucleases would rapidly complete the degradation of the mRNA. Factors binding specific sequences could either protect against or enhance specific cleavages, effectively regulating the mRNA half-life.
![]() |
POSTTRANSCRIPTIONAL REGULATION OF COLLAGEN MRNA |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Cirrhosis of all etiologies is characterized by increased type I
collagen mRNA levels, resulting in overproduction of collagen and the
development of fibrosis. Expression of reporter genes driven by the
collagen 1(I) promoter in transgenic mice showed that, to achieve
expression similar to the endogenous gene in collagen-producing
tissues, a transgene driven by the collagen promoter had to be present
in multiple copies. These mice also overexpressed the transgene in
tissues that normally produce a low level of collagen, suggesting that
transcription alone is not sufficient for regulated expression
(22).
Activated hepatic stellate cells (HSCs) are the predominant
source of collagen in fibrotic liver (6). The
transcriptional rate of collagen 1(I) gene was increased only
threefold in activated HSCs compared with quiescent HSCs, demonstrating
that most of the regulation leading to the 60- to 70-fold increase in
collagen mRNA was posttranscriptional (24). The half-life
of the
1(I) mRNA was increased ~16-fold in activated HSCs compared
with quiescent HSCs, and it is likely that other steps in collagen
expression are also regulated. A model for the regulation of collagen
1(I) is presented in Fig. 3 and
discussed below.
|
The three collagen mRNAs, 1(I),
2(I), and
1(III), which are
coordinately upregulated in fibrosis, contain a stem-loop structure in
the 5' UTR (Fig. 4) that encompasses the translation initiation codon
(start) (30). The 5' stem loop folds into a higher-order structure with a bulged A nucleotide
(23). The 5' stem loop has been well conserved in evolution, differing by only 2 nt among Xenopus, chicken, and human collagen mRNAs. The evolutionary
conservation of this sequence suggests that it must have an important
function.
|
Sequences in the 3' UTR are also critical in the regulation of collagen
mRNA half-life. We have shown that the protein CP2, implicated in globin mRNA stabilization, binds to the C-rich sequence located 23 nt 3' to the stop codon of
1(I) mRNA (24).
Binding of
CP2 can be demonstrated only in cytoplasmic
extracts of activated HSCs, in which the collagen mRNA is stable, and
not in extracts of quiescent HSCs, although both cell types contain the
protein as judged by Northwestern blot analysis. Mutation of the
CP2 binding site in a reporter mRNA that contains the 5'
stem loop also decreased mRNA stability in HSCs (23).
CP2 belongs to the K-homology domain family of
RNA binding proteins and shuttles between the nucleus and cytoplasm.
Purified recombinant
CP2 binds avidly to the C-rich
30-nt sequence derived from the collagen
1(I) mRNA 3' UTR. Change of
a single C nucleotide within the binding site dramatically decreases
the binding of recombinant protein (J. N. Lindquist, unpublished
observations). Interestingly,
CP2 binds a similar C-rich
sequence to stabilize the
-globin mRNA and binds the C-rich
sequences in the 3' UTRs of 15-lipoxygenase and tyrosine hydroxylase
mRNA (11). The reason why
CP2 does not bind
collagen
1(I) mRNA in quiescent HSCs is not clear and requires
further study. It most likely involves a posttranslation modification
of
CP2. Phosphorylation of
CP2 inhibits
its binding to RNA (15), and several kinases have been
recognized to phosphorylate the K-homologous domain type of proteins
(27).
CP2 and the 5' stem loop binding activities regulate
stability of collagen mRNA, but, since in eukaryotic cells translation and mRNA stability are coupled, they may also regulate its translation. A role of
CP2 in stimulating translation of some viral
mRNA has been documented, and
CP2 can interact with
PABP, a known stimulator of translation initiation (29).
Therefore, it is possible that
CP2 enhances translation
of collagen
1(I) mRNA as well as increasing its stability,
essentially by the same mechanism.
We have also analyzed a regulatory role of the 5' stem loop in two
experimental systems: in quiescent and culture-activated HSCs
(23) and in fibroblasts cultured in a three-dimensional matrix (25). The 5' stem loop prevented expression of the
reporter genes in quiescent HSCs but allowed for expression in
activated HSCs. This inhibitory effect of the stem loop was in part
mediated by a decreased half-life of the corresponding mRNAs. By
culturing mouse fibroblasts within a three-dimensional matrix composed
of type I collagen, the cells revert from an activated phenotype to a
more quiescent phenotype and downregulate collagen 1(I) mRNA
(25). Similarly, activated HSCs reverse to a more
quiescent phenotype when cultured in Matrigel or pure collagen type I
gels. In the context of full-size collagen
1(I) mRNA, the 5' stem
loop was also required for accelerated decay of the mRNA in
fibroblasts grown in the matrix. In activated HSCs, a cytosolic
protein factor(s) of 120 kDa binds to the stem loop and requires the
7mG cap on the RNA for binding, consistent with this protein playing a
role in promoting translation and/or stability of the mRNA. We do not know whether this complex directly interacts with the 7mG cap or with
the cap binding protein eIF4E. In quiescent HSCs we could not detect
any protein binding to the stem loop in vitro. The binding of this
complex is also greatly reduced if the cells are cultured in
three-dimensional matrix, in which there is little collagen mRNA and
collagen synthesis. Thus if these cytoplasmic proteins are absent, as
in quiescent HSCs, or reduced, as in fibroblasts grown in the gel,
1(I) mRNA may be inefficiently translated and targeted for
degradation. If they are present, as in activated HSCs and fibroblasts
grown in plastic, the
1(I) mRNA may be stabilized and directed for translation.
We have also detected a nuclear binding activity targeted to the
collagen 1(I) stem loop (25). Compared with the
cytoplasmic binding protein, this activity does not require the
presence of the 7mG cap for binding and it has a different
electrophoretic mobility in native gels. The nuclear binding is
inversely correlated with accumulation of collagen
1(I) mRNA,
suggesting that it may be a negative modulator of collagen
1(I) mRNA
expression acting at a nuclear step in mRNA metabolism. We have
cross-linked a protein of 110 kDa to the 5' stem loop in nuclear
extracts. Its relationship to the 120-kDa cytoplasmic cross-link is
unclear. So it is likely that collagen
1(I) mRNA is regulated by
complex interactions with sequence-specific RNA binding proteins in
both the nucleus, where the complexes act to block mRNA accumulation,
and the cytoplasm, where they promote translation and stabilize the mRNA.
Subcellular targeting of collagen 1(I) and
2(I) mRNAs and
their coordinate translation has not been studied in detail. Collagens are secreted proteins, and their translation is coupled to the export
of the peptides into the endoplasmic reticulum. All collagen
1(I)
mRNA is associated with membrane-bound polysomes and is not found on
free polysomes or in postpolysomal supernatant (unpublished observations). All three peptides initiate folding into the
heterotrimer while still associated with polysomes on the endoplasmic
reticulum (3, 28). When folding is initiated, the collagen
trimer is released in the lumen of endoplasmic reticulum. Heat-shock
protein HSP 47 is a chaperone for folding of collagen triple helix and is present in activated HSCs. This protein coimmunoprecipitates with
the polysome-associated collagen
1(I) chains, so it also targeted to
the site of collagen translation (19). It is possible that
the translation machinery specific for synthesis and assembly of
collagen type I forms at the membrane of the endoplasmic reticulum and
the 5' stem loop of collagen mRNAs and its cognate binding proteins may
be required for formation of such synthetic machinery.
![]() |
FOOTNOTES |
---|
Article published online before print. See web site for date of publication(http://physiolgenomics.physiology.org).
Address for reprint requests and other correspondence: B. Stefanovic, Dept. of Biochemistry and Biophysics and Medicine, 154 Glaxo Bldg. CB# 7038, UNC-Chapel Hill, Chapel Hill, NC 27514 (E-mail: lindyx3{at}med.unc.edu).
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Allmang, C,
Petfalski E,
Podtelejnikov A,
Mann M,
Tollervey D,
and
Mitchell P.
The yeast exosome and human PM-Scl are related complexes of 3'5' exonucleases.
Genes Dev
13:
2148-2158,
1999
2.
Antic, D,
Lu N,
and
Keene JD.
ELAV tumor antigen, Hel-N1, increases translation of neurofilament M mRNA and induces formation of neurites in human teratocarcinoma cells.
Genes Dev
13:
449-461,
1999
3.
Beck, K,
Boswell BA,
Ridgway CC,
and
Bachinger HP.
Triple helix formation of procollagen type I can occur at the rough endoplasmic reticulum membrane.
J Biol Chem
271:
21566-21573,
1996
4.
Carpousis, AJ,
Vanzo NF,
and
Raynal LC.
mRNA degradation. A tale of poly(A) and multiprotein machines.
Trends Genet
15:
24-28,
1999[ISI][Medline].
5.
Chkheidze, AN,
Lyakhov DL,
Makeyev AV,
Morales J,
Kong J,
and
Liebhaber SA.
Assembly of the -globin mRNA stability complex reflects binary interaction between the pyrimidine-rich 3' untranslated region determinant and poly(C) binding protein
CP.
Mol Cell Biol
19:
4572-4581,
1999
6.
Friedman, SL.
Hepatic stellate cells.
Prog Liver Dis
14:
101-130,
1996[Medline].
7.
Geballe, AP,
and
Morris DR.
Initiation codons within 5'-leaders of mRNAs as regulators of translation.
Trends Biochem Sci
19:
159-164,
1994[ISI][Medline].
8.
Gray, NK,
and
Hentze MW.
Iron regulatory protein prevents binding of the 43S translation pre-initiation complex to ferritin and eALAS mRNAs.
EMBO J
13:
3882-3891,
1994[Abstract].
9.
Gygi, SP,
Rochon Y,
Franza BR,
and
Aebersold R.
Correlation between protein and mRNA abundance in yeast.
Mol Cell Biol
19:
1720-1730,
1999
10.
Hagedorn, CH,
Spivak-Kroizman T,
Friedland DE,
Goss DJ,
and
Xie Y.
Expression of functional eIF-4Ehuman: purification, detailed characterization, and its use in isolating eIF-4E binding proteins.
Protein Expr Purif
9:
53-60,
1997[ISI][Medline].
11.
Holcik, M,
and
Liebhaber SA.
Four highly stable eukaryotic mRNAs assemble 3' untranslated region RNA-protein complexes sharing cis and trans components.
Proc Natl Acad Sci USA
94:
2410-2414,
1997
12.
Imataka, H,
Gradi A,
and
Sonenberg N.
A newly identified N-terminal amino acid sequence of human eIF4G binds poly(A)-binding protein and functions in poly(A)-dependent translation.
EMBO J
17:
7480-7489,
1998
13.
Jacobson, A.
PolyA metabolism and translation: the closed loop model.
In: Translational Control, edited by Hershey JW,
Mathews MB,
and Sonenberg N.. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press, 1996, p. 451-480.
14.
Kozak, M.
Initiation of translation in prokaryotes and eukaryotes.
Gene
234:
187-208,
1999[ISI][Medline].
15.
Leffers, H,
Dejgaard K,
and
Celis JE.
Characterisation of two major cellular poly(rC)-binding human proteins, each containing three K-homologous (KH) domains.
Eur J Biochem
230:
447-453,
1995[Abstract].
16.
Loflin, P,
Chen CYA,
and
Shyu AB.
Unraveling a cytoplasmic role for hnRNP D in the in vivo mRNA destabilization by the AU-rich element.
Genes Dev
13:
1884-1897,
1999
17.
Mangus, DA,
Amrani N,
and
Jacobson A.
Pbp1, a factor interacting with Saccharomyces cerevisiae poly(A)-binding protein, regulates polyadenylation.
Mol Cell Biol
18:
7383-7396,
1998
18.
McCarthy, JE,
and
Kollmus H.
Cytoplasmic mRNA-protein interactions in eukaryotic gene expression.
Trends Biochem Sci
20:
191-197,
1995[ISI][Medline].
19.
Nagata, K.
Hsp47: a collagen-specific molecular chaperone.
Trends Biochem Sci
21:
22-26,
1996[Medline].
20.
Sachs, AB,
Sarnow P,
and
Hentze MW.
Starting at the beginning, middle, and end: translation initiation in eukaryotes.
Cell
89:
831-838,
1997[ISI][Medline].
21.
Schwartz, DC,
and
Parker R.
Mutations in translation initiation factors lead to increased rates of deadenylation and decapping of mRNAs in Saccharomyces cerevisiae.
Mol Cell Biol
19:
5274-5256,
1999.
22.
Slack, JL,
Liska DJ,
and
Bornstein P.
An upstream regulatory region mediates high-level, tissue-specific expression of the human alpha 1(I) collagen gene in transgenic mice.
Mol Cell Biol
11:
2066-2074,
1991[ISI][Medline].
23.
Stefanovic, B,
Hellerbrand C,
and
Brenner DA.
Regulatory role of the conserved stem-loop structure at the 5'end of collagen 1(I) mRNA.
Mol Cell Biol
19:
4334-4342,
1999
24.
Stefanovic, B,
Hellerbrand C,
Holcik M,
Briendl M,
Aliebhaber S,
and
Brenner DA.
Posttranscriptional regulation of collagen 1(I) mRNA in hepatic stellate cells.
Mol Cell Biol
17:
5201-5209,
1997[Abstract].
25.
Stefanovic, B,
Lindquist J,
and
Brenner DA.
The 5' stem-loop regulates expression of collagen 1(I) mRNA in mouse fibroblasts cultured in a three-dimensional matrix.
Nucleic Acids Res
28:
641-647,
2000
26.
Tarun, SZ, Jr,
and
Sachs AB.
Association of the yeast poly(A) tail binding protein with translation initiation factor eIF-4G.
EMBO J
15:
7168-7177,
1996[Abstract].
27.
Van Seuningen, I,
Ostrowski J,
and
Bomsztyk K.
Description of an IL-1-responsive kinase that phosphorylates the K protein. Enhancement of phosphorylation by selective DNA and RNA motifs.
Biochemistry
34:
5644-5650,
1995[ISI][Medline].
28.
Veis, A,
and
Kirk TZ.
The coordinate synthesis and cotranslational assembly of type I procollagen.
J Biol Chem
264:
3884-3889,
1989
29.
Wang, Z,
Day N,
Trifillis P,
and
Kiledjian M.
An mRNA stability complex functions with poly(A)-binding protein to stabilize mRNA in vitro.
Mol Cell Biol
19:
4552-4560,
1999
30.
Yamada, Y,
Mudryj M,
and
de Crombrugghe B.
A uniquely conserved regulatory signal is found around the translation initiation site in three different collagen genes.
J Biol Chem
258:
14914-14919,
1983
31.
Zhang, BL,
Gallegos M,
Puoti A,
Durkin E,
Fields S,
Kimble J,
and
Wickens MP.
A conserved RNA-binding protein that regulates sexual fates in the C. elegans hermaphrodite germ line.
Nature
390:
477-484,
1997[ISI][Medline].