Reconstitution of transcription from the human U6 small nuclear RNA promoter with eight recombinant polypeptides and a partially purified RNA polymerase III complex

A structural perspective of human RNA polymerase III

RNA polymerase III (Pol III) is a large multisubunit complex conserved in all eukaryotes that plays an essential role in producing a variety of short non-coding RNAs, such as tRNA, 5S rRNA and U6 snRNA transcripts. Pol III comprises of 17 subunits in both yeast and human with a 10-subunit core and seven peripheral subunits. Because of its size and complexity, Pol III has posed a formidable challenge to structural biologists. The first atomic cryogenic electron microscopy structure of yeast Pol III leading to the canonical view was reported in 2015. Within the last few years, the optimization of endogenous extract and purification procedure and the technical and methodological advances in cryogenic electron microscopy, together allow us to have a first look at the unprecedented details of human Pol III organization. Here, we look back on the structural studies of human Pol III and discuss them in the light of our current understanding of its role in eukaryotic transcription.

MAF1 is a chronic repressor of RNA polymerase III transcription in the mouse

Maf1^−/− mice are lean, obesity-resistant and metabolically inefficient. Their increased energy expenditure is thought to be driven by a futile RNA cycle that reprograms metabolism to meet an increased demand for nucleotides stemming from the deregulation of RNA polymerase (pol) III transcription. Metabolic changes consistent with this model have been reported in both fasted and refed mice, however the impact of the fasting-refeeding-cycle on pol III function has not been examined. Here we show that changes in pol III occupancy in the liver of fasted versus refed wild-type mice are largely confined to low and intermediate occupancy genes; high occupancy genes are unchanged. However, in Maf1^−/− mice, pol III occupancy of the vast majority of active loci in liver and the levels of specific precursor tRNAs in this tissue and other organs are higher than wild-type in both fasted and refed conditions. Thus, MAF1 functions as a chronic repressor of active pol III loci and can modulate transcription under different conditions. Our findings support the futile RNA cycle hypothesis, elaborate the mechanism of pol III repression by MAF1 and demonstrate a modest effect of MAF1 on global translation via reduced mRNA levels and translation efficiencies for several ribosomal proteins.

Hiwi knockdown inhibits the growth of lung cancer in nude mice

Hiwi, a human homologue of the Piwi family, plays an important role in stem cell self-renewal and is overexpressed in various human tumors. This study aimed to determine whether an RNA interference-based strategy to suppress Hiwi expression could inhibit tumor growth in a xenograft mouse model. A rare population of SSCloAldebr cells was isolated and identified as lung cancer stem cells in our previous study. Plasmids containing U6 promoter-driven shRNAs against Hiwi or control plasmids were successfully established. The xenograft tumor model was generated by subcutaneously inoculating with lung cancer stem cell SSCloAldebr cells. After the tumor size reached about 8 mm in diameter, shRNA plasmids were injected into the mice via the tail vein three times a week for two weeks, then xenograft tumor growth was assessed. In nude mice, intravenously delivery of Hiwi shRNA plasmids significantly inhibited tumor growth compared to treatment with control scrambled shRNA plasmids or the vehicle PBS. No mice died during the experiment and no adverse events were observed in mice administered the plasmids. Moreover, delivery of Hiwi shRNA plasmids resulted in a significant suppressed expression of Hiwi and ALDH-1 in xenograft tumor samples, based on immunohistochemical analysis. Thus, shRNA-mediated Hiwi gene silencing in lung cancer stem cells by an effective in vivo gene delivery strategy appeared to be an effective therapeutic approach for lung cancer, and may provide some useful clues for RNAi gene therapy in solid cancers.

Quantifying ChIP-seq data: a spiking method providing an internal reference for sample-to-sample normalization

Chromatin immunoprecipitation followed by deep sequencing (ChIP-seq) experiments are widely used to determine, within entire genomes, the occupancy sites of any protein of interest, including, for example, transcription factors, RNA polymerases, or histones with or without various modifications. In addition to allowing the determination of occupancy sites within one cell type and under one condition, this method allows, in principle, the establishment and comparison of occupancy maps in various cell types, tissues, and conditions. Such comparisons require, however, that samples be normalized. Widely used normalization methods that include a quantile normalization step perform well when factor occupancy varies at a subset of sites, but may miss uniform genome-wide increases or decreases in site occupancy. We describe a spike adjustment procedure (SAP) that, unlike commonly used normalization methods intervening at the analysis stage, entails an experimental step prior to immunoprecipitation. A constant, low amount from a single batch of chromatin of a foreign genome is added to the experimental chromatin. This "spike" chromatin then serves as an internal control to which the experimental signals can be adjusted. We show that the method improves similarity between replicates and reveals biological differences including global and largely uniform changes.

Gene duplication and neofunctionalization: POLR3G and POLR3GL

RNA polymerase III (Pol III) occurs in two versions, one containing the POLR3G subunit and the other the closely related POLR3GL subunit. It is not clear whether these two Pol III forms have the same function, in particular whether they recognize the same target genes. We show that the POLR3G and POLR3GL genes arose from a DNA-based gene duplication, probably in a common ancestor of vertebrates. POLR3G- as well as POLR3GL-containing Pol III are present in cultured cell lines and in normal mouse liver, although the relative amounts of the two forms vary, with the POLR3G-containing Pol III relatively more abundant in dividing cells. Genome-wide chromatin immunoprecipitations followed by high-throughput sequencing (ChIP-seq) reveal that both forms of Pol III occupy the same target genes, in very constant proportions within one cell line, suggesting that the two forms of Pol III have a similar function with regard to specificity for target genes. In contrast, the POLR3G promoter—not the POLR3GL promoter—binds the transcription factor MYC, as do all other promoters of genes encoding Pol III subunits. Thus, the POLR3G/POLR3GL duplication did not lead to neo-functionalization of the gene product (at least with regard to target gene specificity) but rather to neo-functionalization of the transcription units, which acquired different mechanisms of regulation, thus likely affording greater regulation potential to the cell.

A multiplicity of factors contributes to selective RNA polymerase III occupancy of a subset of RNA polymerase III genes in mouse liver

The genomic loci occupied by RNA polymerase (RNAP) III have been characterized in human culture cells by genome-wide chromatin immunoprecipitations, followed by deep sequencing (ChIP-seq). These studies have shown that only ∼40% of the annotated 622 human tRNA genes and pseudogenes are occupied by RNAP-III, and that these genes are often in open chromatin regions rich in active RNAP-II transcription units. We have used ChIP-seq to characterize RNAP-III-occupied loci in a differentiated tissue, the mouse liver. Our studies define the mouse liver RNAP-III-occupied loci including a conserved mammalian interspersed repeat (MIR) as a potential regulator of an RNAP-III subunit-encoding gene. They reveal that synteny relationships can be established between a number of human and mouse RNAP-III genes, and that the expression levels of these genes are significantly linked. They establish that variations within the A and B promoter boxes, as well as the strength of the terminator sequence, can strongly affect RNAP-III occupancy of tRNA genes. They reveal correlations with various genomic features that explain the observed variation of 81% of tRNA scores. In mouse liver, loci represented in the NCBI37/mm9 genome assembly that are clearly occupied by RNAP-III comprise 50 Rn5s (5S RNA) genes, 14 known non-tRNA RNAP-III genes, nine Rn4.5s (4.5S RNA) genes, and 29 SINEs. Moreover, out of the 433 annotated tRNA genes, half are occupied by RNAP-III. Transfer RNA gene expression levels reflect both an underlying genomic organization conserved in dividing human culture cells and resting mouse liver cells, and the particular promoter and terminator strengths of individual genes.

A role for Yin Yang-1 (YY1) in the assembly of snRNA transcription complexes

The RNA polymerase (pol) II and III human small nuclear RNA (snRNA) genes have very similar promoters and recruit a number of common factors. In particular, both types of promoters utilize the small nuclear RNA activating protein complex (SNAP(c)) and the TATA box binding protein (TBP) for basal transcription, and are activated by Oct-1. We find that SNAP(c) purified from cell lines expressing tagged SNAP(c) subunits is associated with Yin Yang-1 (YY1), a factor implicated in both activation and repression of transcription. Recombinant YY1 accelerates the binding of SNAP(c) to the proximal sequence element, its target within snRNA promoters. Moreover, it enhances the formation of a complex on the pol III U6 snRNA promoter containing all the factors (SNAP(c), TBP, TFIIB-related factor 2 (Brf2), and B double prime 1 (Bdp1)) that are sufficient to direct in vitro U6 transcription when complemented with purified pol III, as well as that of a subcomplex containing TBP, Brf2, and Bdp1. YY1 is found on both the RNA polymerase II U1 and the RNA polymerase III U6 promoters as determined by chromatin immunoprecipitations. Thus, YY1 represents a new factor that participates in transcription complexes formed on both pol II and III promoters.

Reconstitution of the yeast RNA polymerase III transcription system with all recombinant factors

Transcription factor TFIIIC is a multisubunit complex required for promoter recognition and transcriptional activation of class III genes. We describe here the reconstitution of complete recombinant yeast TFIIIC and the molecular characterization of its two DNA-binding domains, tauA and tauB, using the baculovirus expression system. The B block-binding module, rtauB, was reconstituted with rtau138, rtau91, and rtau60 subunits. rtau131, rtau95, and rtau55 formed also a stable complex, rtauA, that displayed nonspecific DNA binding activity. Recombinant rTFIIIC was functionally equivalent to purified yeast TFIIIC, suggesting that the six recombinant subunits are necessary and sufficient to reconstitute a transcriptionally active TFIIIC complex. The formation and the properties of rTFIIIC-DNA complexes were affected by dephosphorylation treatments. The combination of complete recombinant rTFIIIC and rTFIIIB directed a low level of basal transcription, much weaker than with the crude B'' fraction, suggesting the existence of auxiliary factors that could modulate the yeast RNA polymerase III transcription system.

Conditional inhibition of cancer cell proliferation by tetracycline-responsive, H1 promoter-driven silencing of PLK1

RNA interference (RNAi) is a powerful tool for studying gene function. We developed an inducible genetic element for short interfering RNA-mediated gene silencing. This system uses a tetracycline (Tet)-responsive derivative of the H1 promoter and the Tet repressor (TetR) for conditional expression of short hairpin RNA (shRNA) in HeLa cells. Promoter constructs were generated, which contain the Tet operator (TetO) derived from a prokaryotic Tet resistance transposon upstream and/or downstream of the TATA box. To quantify the response of controllable transcription units for shRNA expression, we examined the functional activity of polo-like kinase 1 (PLK1), a key component of mitotic progression, that is overexpressed in many human tumors. Cotransfection of plasmids for the expression of TetR and shRNA/PLK1 under the control of an H1 promoter-variant carrying TetO upstream of the TATA box did not alter PLK1 expression and proliferation properties of HeLa cells in the absence of doxycycline. Addition of the antibiotic led to marked downregulation of endogenous PLK1 accompanied by strong inhibition of cellular proliferation. Our data indicate that an inducible transcription system for shRNAs based on the human H1 promoter could be a versatile tool for controlled gene silencing in vitro.

A role for beta-actin in RNA polymerase III transcription

When transcription from the human U6 snRNA gene is reconstituted with recombinant factors and purified RNA polymerase III (pol III), pol III must be treated with CK2 to be active. We show that highly purified pol III contains associated beta-actin, and beta-actin localizes to an active U6 promoter in vivo. Pol III immunoprecipitated from IMR90 cells treated with a genotoxic agent lacks associated beta-actin and is inactive in the reconstituted assay. Transcription is regained upon treatment of pol III with CK2 and addition of beta-actin. This suggests that beta-actin associated with pol III is essential for basal pol III transcription.

Cancer inhibition in nude mice after systemic application of U6 promoter-driven short hairpin RNAs against PLK1

BACKGROUND

RNA interference initiated by small interfering RNAs effectively suppresses gene expression, but the suppression is transient, which limits the therapeutic use of this technique. Polo-like kinase 1 (PLK1) is a key cell cycle regulator that is overexpressed in various human tumors. We used a xenograft mouse model to determine whether an RNA interference-based strategy that used short hairpin RNAs (shRNAs) to suppress PLK1 expression could inhibit tumor growth in vivo.

METHODS

HeLa S3 cervical and A549 lung cancer cell lines were transfected with plasmids containing U6 promoter-driven shRNAs against human PLK1 or control (parental or scrambled) plasmids. Plasmids were treated with the nuclease inhibitor aurintricarboxylic acid (ATA) as protection against nucleases in murine blood. Nude mice carrying xenograft tumors were injected with shRNA plasmids, and their xenograft tumor growth was assessed. Northern and western blot analyses were used to measure PLK1 mRNA and protein expression, respectively, in transfected cultured cells and in xenograft tumors. All statistical tests were two-sided.

RESULTS

Levels of PLK1 mRNA and protein were lower in HeLa S3 and A549 cancer cells transfected with PLK1 shRNA plasmids than in corresponding cells transfected with control parental or scrambled PLK1S shRNA plasmids. Proliferation of cells transfected with PLK1 shRNA was lower than that of cells transfected with either control plasmid, and proliferation of cells transfected with ATA-treated PLK1 shRNA plasmids was even lower. In mice with human xenograft tumors, PLK1 shRNA expression from ATA-treated plasmids reduced tumor growth to 18% (95% confidence interval [CI] = 12% to 26%; P =.03) and from untreated plasmids reduced tumor growth to 45% (95% CI = 26% to 64%; P =.1) of that of tumors in mice treated with scrambled control PLK1S shRNA plasmids.

CONCLUSIONS

The combination of shRNA-mediated gene silencing with effective in vivo gene delivery strategies appears to generate a long-lasting silencing signal.

Structure of the La motif: a winged helix domain mediates RNA binding via a conserved aromatic patch

The La protein is a ubiquitous nuclear phosphoprotein that recognizes the 3' uridylates found in all newly synthesized RNA polymerase III transcripts. La binding stabilizes these transcripts from exonucleases and may also assist their folding. Here we present the first structural insights into how the La protein specifically interacts with its RNA substrates. The most conserved region of the La protein is the La motif, a domain also found in several other RNA-binding proteins. We have determined the structure of the La motif from the Trypanosoma brucei La protein to 1.6 A resolution (PDB code 1S29). The La motif adopts a winged helix-turn-helix architecture that has a highly conserved patch of mainly aromatic surface residues. Mutagenesis experiments support a critical role for this patch in RNA binding and show that it partly determines binding specificity for RNAs ending in 3' hydroxyl, a defining characteristic of the La protein. These findings reveal that the La motif is essential for high-affinity binding and also contributes to specificity.

Assembly and isolation of intermediate steps of transcription complexes formed on the human 5S rRNA gene

By employing purified transcription factors and RNA polymerase III (pol III), we generated active pol III transcription complexes on the human 5S rRNA gene. These large complexes were separated by size exclusion chromatography from non- incorporated proteins. In addition, we succeeded in isolating specific intermediate stages of complex formation. Such isolated partial complexes require complementation with the missing activities for full transcription activity. One central finding is that a 5S DNA-TFIIIA-TFIIIC2-TFIIIBbeta complex could be isolated which had been assembled in the absence of the general pol III transcription factor IIIC1. Thus TFIIIC1 is not an assembly factor for other transcription factors. Although pol III has the potential to bind unspecifically to DNA, such polymerase molecules cannot be rendered initiation competent by direct recruitment to a 5S DNA-TFIIIA-TFIIIC2- TFIIIBbeta complex, but this process strictly requires additional TFIIIC1 activity. This clearly demonstrates that in contrast to yeast cells, hTFIIIB(beta), although required, does not suffice for the functional recruitment of polymerase III. These data document that TFIIIC1 is the second transcription factor required for the recruitment of pol III in mammalian cells.

Multiple, dispersed human U6 small nuclear RNA genes with varied transcriptional efficiencies

Vertebrate U6 small nuclear RNA (snRNA) gene promoters are among the founding members of those recognized by RNA polymerase III in which all control elements for initiation are located in the 5'-flanking region. Previously, one human U6 gene (U6-1) has been studied extensively. We have identified a total of nine full-length U6 loci in the human genome. Unlike human U1 and U2 snRNA genes, most of the full-length U6 loci are dispersed throughout the genome. Of the nine full-length U6 loci, five are potentially active genes (U6-1, U6-2, U6-7, U6-8 and U6-9) since they are bound by TATA-binding protein and enriched in acetylated histone H4 in cultured human 293 cells. These five all contain OCT, SPH, PSE and TATA elements, although the sequences of these elements are variable. Furthermore, these five genes are transcribed to different extents in vitro or after transient transfection of human 293 cells. Of the nine full-length U6 loci, only U6-7 and U6-8 are closely linked and contain highly conserved 5'-flanking regions. However, due to a modest sequence difference in the proximal sequence elements for U6-7 and U6-8, these genes are transcribed at very different levels in transfected cells.

The U5/U6 snRNA genomic repeat of Taenia solium

The U6 and U5 snRNA (small nuclear ribonucleic acid) genes were identified in Taenia solium with the aim of characterizing their sequence and genomic structures. They are contained within a shared 1,009-nt tandem genomic repeat and present at approximately 3 copies per haploid genome. The U6 snRNA gene shares 92 and 95% sequence similarity with the U6 homologs from humans and Schistosoma mansoni, respectively. The U5 snRNA gene of T. solium is 70% similar to the human U5 sequence in the 5' stem and loop 1 domains. The U6 and U5 snRNA genes are on complementary genomic strands and separated by 458 nt at their "heads" and 306 nt at their "tails." The nucleotides upstream of the U6 gene lack a recognizable TATA box and proximal sequence elements (PSEs), and the putative gene promoter for U5 snRNA does not resemble vertebrate examples. There are short blocks of similarity between the sequences upstream of the U5 and U6 snRNA genes, and these may be sites of shared transcription factor binding at the respective RNA polymerase II and III promoters. It is possible that this unusual allied U5/U6 snRNA genomic repeat may help mediate coordinated regulation of expression of the 2 snRNAs.

Characterization of human RNA polymerase III identifies orthologues for Saccharomyces cerevisiae RNA polymerase III subunits

Unlike Saccharomyces cerevisiae RNA polymerase III, human RNA polymerase III has not been entirely characterized. Orthologues of the yeast RNA polymerase III subunits C128 and C37 remain unidentified, and for many of the other subunits, the available information is limited to database sequences with various degrees of similarity to the yeast subunits. We have purified an RNA polymerase III complex and identified its components. We found that two RNA polymerase III subunits, referred to as RPC8 and RPC9, displayed sequence similarity to the RNA polymerase II RPB7 and RPB4 subunits, respectively. RPC8 and RPC9 associated with each other, paralleling the association of the RNA polymerase II subunits, and were thus paralogues of RPB7 and RPB4. Furthermore, the complex contained a prominent 80-kDa polypeptide, which we called RPC5 and which corresponded to the human orthologue of the yeast C37 subunit despite limited sequence similarity. RPC5 associated with RPC53, the human orthologue of S. cerevisiae C53, paralleling the association of the S. cerevisiae C37 and C53 subunits, and was required for transcription from the type 2 VAI and type 3 human U6 promoters. Our results provide a characterization of human RNA polymerase III and show that the RPC5 subunit is essential for transcription.

Assembly of human small nuclear RNA gene-specific transcription factor IIIB complex de novo on and off promoter

In humans, transcription factor IIIB (TFIIIB)-alpha governs basal transcription from small nuclear RNA genes by RNA polymerase III (pol III). One of the components of this complex, BRFU/TFIIIB50, is specific for these promoters, whereas TATA-binding protein (TBP) and hB" are required for pol III transcription from both gene external and internal promoters. We show that hB" is specifically recruited to a promoter-bound TBP.BRFU complex, which we have previously demonstrated as forming on TATA-containing templates. The N-terminal region of BRFU, containing a zinc ribbon domain, acts as a damper of hB" binding. TBP deactivates this negative mechanism through protein-protein contacts with both BRFU and hB", which may then promote their cooperative binding to form TFIIIB-alpha. In addition, we have identified a GC-rich sequence downstream from the TATA box (the BURE) which, depending on the strength of TATA box, can either enhance BRFU binding to the TBP.DNA complex or hB" association with the BRFU.TBP.DNA complex, and subsequently stimulate pol III transcription. Moreover, mutation of the BURE reduces pol III transcription and induces transcription by RNA polymerase II from the U2 gene promoter carrying a minimal TATA box.

RNA interference: the new somatic cell genetics?

RNAi is evolving into a powerful tool for manipulating gene expression in mammalian cells with potential utility for investigating gene function, for high-throughput, function-based genetic screens and potentially for development as a therapeutic tool.

Short hairpin RNAs (shRNAs) induce sequence-specific silencing in mammalian cells

RNA interference (RNAi) was first recognized in Caenorhabditis elegans as a biological response to exogenous double-stranded RNA (dsRNA), which induces sequence-specific gene silencing. RNAi represents a conserved regulatory motif, which is present in a wide range of eukaryotic organisms. Recently, we and others have shown that endogenously encoded triggers of gene silencing act through elements of the RNAi machinery to regulate the expression of protein-coding genes. These small temporal RNAs (stRNAs) are transcribed as short hairpin precursors (approximately 70 nt), processed into active, 21-nt RNAs by Dicer, and recognize target mRNAs via base-pairing interactions. Here, we show that short hairpin RNAs (shRNAs) can be engineered to suppress the expression of desired genes in cultured Drosophila and mammalian cells. shRNAs can be synthesized exogenously or can be transcribed from RNA polymerase III promoters in vivo, thus permitting the construction of continuous cell lines or transgenic animals in which RNAi enforces stable and heritable gene silencing.

The activity of transcription factor IIIC1 is impaired during differentiation of F9 cells

Differentiation in vitro of mouse F9 embryonal carcinoma (EC) cells to the parietal endoderm (PE) mimics processes of development of the early mouse embryo. This differentiation is accompanied by a dramatic down-regulation of all genes transcribed by RNA polymerase III (pol III). Complementation of extracts from cells, differentiated for various time periods with purified pol III transcription factors show for the first time that TFIIIC1 can substantially restore this impaired transcription, particularly in the early stages of differentiation. At later stages (day 7) the TBP (TATA-binding protein )-TAF complex, TFIIIBbeta, may also become limiting, which can contribute to but cannot account for the reduced transcription of type 2 promoters in PE cells. Because TFIIIBbeta is not required for the expression of type 3 promoters, other components must necessarily be involved, and our results show that U6 transcription can significantly be reactivated by TFIIIC1. By employing a variant type 3 promoter construct, which essentially requires a mutant form of TBP (TBP-DR2), we show that TBP is not limiting in PE extracts. The partial purification of pol III transcription factors from PE and EC cells revealed that TFIIIC2 activity could be purified from both cell types, whereas TFIIIC1 activity was dramatically reduced in extracts from PE cells.

BRFU, a TFIIB-like factor, is directly recruited to the TATA-box of polymerase III small nuclear RNA gene promoters through its interaction with TATA-binding protein

The human snRNA genes transcribed by RNA polymerase II (pol II) and III (pol III) have different core promoter elements. Both gene types contain similar proximal sequence elements (PSEs) but differ in the absence (pol II) or presence (pol III) of a TATA-box, which, together with the PSE, determines the assembly of a pol III-specific pre-initiation complex. BRFU is a factor exclusively required for transcription of the pol III-type snRNA genes. We report that recruitment of BRFU to the TATA-box of these promoters is TATA-binding protein (TBP)-dependent. BRFU in turn stabilizes TBP on TATA-containing template and extends the TBP footprint both upstream and downstream of the TATA element. The core domain of TBP is sufficient for BRFU.TBP.DNA complex formation and for interaction with BRFU off the template. We have mapped amino acid residues within TBP and domains of BRFU that mediate this interaction. BRFU has no specificity for sequences flanking the TATA-box and also forms a stable complex on the TATA-box of the pol II-specific adenovirus major late promoter (AdMLP). Furthermore, pol III-type transcription can initiate from an snRNA gene promoter containing an AdMLP TATA-box and flanking sequences. Therefore, the polymerase recruitment is not simply determined by the sequence of the TATA-box and immediate flanking sequences.

The La protein

Ubiquitous in eukaryotic cells, the La protein associates with the 3' termini of many newly synthesized small RNAs. RNAs bound by the La protein include all nascent transcripts made by RNA polymerase III as well as certain small RNAs synthesized by other RNA polymerases. Recent genetic and biochemical analyses have revealed that binding by the La protein protects the 3' ends of these RNAs from exonucleases. This La-mediated stabilization is required for the normal pathway of pre-tRNA maturation, facilitates assembly of small RNAs into functional RNA-protein complexes, and contributes to nuclear retention of certain small RNAs. Studies of mutant La proteins have given some insights into how the La protein specifically recognizes its RNA targets. However, many questions remain regarding the molecular mechanisms by which La protein binding influences multiple steps in small RNA biogenesis. This review focuses on the roles of the La protein in small RNA biogenesis and also discusses data that implicate the La protein in the translation of specific mRNAs.