The relationship between translation and mRNA turnover is critical to the regulation of gene expression. One major pathway for mRNA turnover occurs by deadenylation, which leads to decapping and subsequent 5-to-3 degradation of the body of the mRNA. Prior to mRNA decapping, a transcript exits translation and enters P bodies to become a potential decapping substrate. To understand the transition from translation to decapping, it is important to identify the factors involved in this process. In this work, we identify Sbp1p (formerly known as Ssb1p), an abundant RNA binding protein, as a high-copy-number suppressor of a conditional allele in the decapping enzyme. Sbp1p overexpression restores normal decay rates in decappingdefective strains and increases P-body size and number. In addition, Sbp1p promotes translational repression of mRNA during glucose deprivation. Moreover, P-body formation is reduced in strains lacking Sbp1p. Sbp1p acts in conjunction with Dhh1p, as it is required for translational repression and P-body formation in pat1⌬ strains under these conditions. These results identify Sbp1p as a new protein that functions in the transition of mRNAs from translation to an mRNP complex destined for decapping.Control of mRNA stability plays a crucial role in regulating gene expression. In eukaryotes, there are two major mRNA decay pathways that both require deadenylation before decay occurs. In yeast, the most common mechanism of mRNA turnover is where deadenylation is followed by decapping. Once the mRNA is deadenylated, the 5Ј m7G cap structure is removed by the Dcp1p/Dcp2p decapping complex and the mRNA is subsequently degraded 5Ј to 3Ј (12,20,33). In addition to the decapping enzyme complex, there are a set of factors that act as decapping activators, including Dhh1p, Pat1p, and the Lsm1-7p complex (4,10,18,44,45). Alternatively, once deadenylation occurs, the mRNA can be degraded by the slower 3Ј-to-5Ј pathway, which is mediated by the exosome (1, 5, 34; for a review, see reference 47).Decapping is an important step in 5Ј-to-3Ј decay, as it permits transcript degradation and is a site of regulation. For instance, short-lived mRNAs are decapped more rapidly than long-lived mRNAs (33, 34). In addition, nonsense transcripts, which are targeted for nonsense-mediated decay, can bypass the need for deadenylation and are rapidly decapped (35). Furthermore, in mammalian cells, the AU-rich element binding proteins TTP and BRF-1 have been proposed to recruit the decapping enzyme to their targets (31). Given these observations, to understand differential mRNA stability, it is important to determine the events that contribute to the control of mRNA decapping.Several lines of evidence indicate that a key contribution to the control of mRNA decapping is that translation and mRNA decapping are in competition. First, in yeast, mRNAs poorly translated because of secondary structures in their 5Ј untranslated region (UTR) or weak AUG context are decapped much faster than their wild-type counterparts (27, 34). Second, muta...
In C. elegans, tra-2 mRNA nuclear export is controlled by a 3'UTR element, the TRE. In the absence of TRA-1, the TRE retains tra-2 mRNA in the nucleus. The binding of TRA-1 to the 3'UTR overcomes this retention resulting in export of a TRA-1/tra-2 mRNA complex. Here, we find that, unlike most mRNAs, tra-2 mRNA exits the nucleus via an alternative pathway to NXF-1 that requires CRM1 activity. Inhibition of export by NXF-1 depends upon the TRE, CeNXF-2, CeREF-1, and CeREF-2. Removal of the TRE or any one of these factors results in export of tra-2 by NXF-1. NXF-2 and REF-1 specifically bind the TRE, suggesting that they directly control tra-2 mRNA export. Furthermore, choice of proper export pathway affects tra-2 translational control. Therefore, tra-2 mRNA export is highly regulated and plays an important role in development by regulating the activity of tra-2 mRNA in the cytoplasm.
TRA-1, a member of the GLI family of transcription factors, is required for C. elegans female development. We find that TRA-1 has a sex-specific distribution consistent with its role in female development: nuclear TRA-1 is higher in hermaphrodite intestines and in specific germline regions than in males. TRA-1 patterns rely on nuclear export since treatment with leptomycin B, a CRM1-dependent export inhibitor, increases nuclearTRA-1 in males. TRA-1 export requires TRA-1 binding to the tra-2 3' untranslated region (3' UTR), as disruption of binding increases nuclear TRA-1 and female development. Our data are consistent with coexport of a TRA-1/tra-2 mRNA complex reducing TRA-1 nuclear activity, and identify an interesting RNA-based mechanism for controlling transcriptional activity and cell fate determination.
Volume 26, no. 13, p. 5120-5130, 2006. In the course of examining the experiments in this paper, we detected an error in that some of the strains used in the experiments shown in Fig. 5 and 6 were incorrect. In addition, an error was made in the preparation of Fig. 5, where a polysome tract was inadvertently used twice. To correct these errors, we reconstructed the pat1⌬ sbp1⌬ double mutants (yRP2168 and yRP2169) and repeated the experiments with the correct strains.As presented in our now corrected Fig. 5 and 6, we observed two differences from our earlier report. First, we observed that the sbp1⌬ strain was able to repress translation essentially like the wild-type strain. This is in contrast to our earlier report that this single mutant was partially defective in translational repression during glucose deprivation. Second, we observed that the pat1⌬ sbp1⌬ double mutant is still defective at translation repression but to a more modest degree than previously reported. Specifically, while the wild type undergoes ϳ95% repression, the pat1⌬ sbp1⌬ strain undergoes approximately 40 to 50% repression (in contrast to the 11% repression that we previously reported). Thus, we still saw a two-to threefold decrease in translational repression in a pat1⌬ sbp1⌬ strain as compared to a pat1⌬ strain, whereas previously, we saw almost a complete loss of translational repression in the pat1⌬ sbp1⌬ strain.The differences in the degree of repression do not invalidate the conclusion of the paper that Sbp1p can affect the process of translation repression and targeting of mRNAs to P-bodies for decapping, but they reduce the magnitude of the effect that sbp1⌬ has on this process. Moreover, our reexamination of this work argues that all the other figures in this paper are still valid and accurate.We offer our sincere apologies for any inconvenience or wasted effort these errors may have caused for members of the scientific community.
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