The post-transcriptional regulation of gene expression underlies several critical developmental phenomena. In metazoa, gene products that are expressed, silenced and packaged during oogenesis govern early developmental processes prior to nascent transcription activation. Furthermore, tissue-specific alternative splicing of several transcription factors controls pattern formation and organ development. A highly conserved family of proteins containing a STAR/GSG RNA-binding domain is essential to both processes. Here, we identify the consensus STAR-binding element (SBE) required for specific mRNA recognition by GLD-1, a key regulator of Caenorhabditis elegans germline development. We have identified and verified new GLD-1 repression targets containing this sequence. The results suggest additional functions of GLD-1 in X-chromosome silencing and early embryogenesis. The SBE is present in Quaking and How mRNA targets, suggesting that STAR protein specificity is highly conserved. Similarities between the SBE and the branch-site signal indicate a possible competition mechanism for STAR/GSG regulation of splicing variants.
The binding of the STAR protein GLD-1 to an element in the tra-2 3' untranslated region (3'UTR), called the TGE (tra GLI element), represses tra-2 translation, allowing for hermaphrodite spermatogenesis in Caenorhabditis elegans. GLD-1 is a member of the STAR family that includes the mammalian quaking (Qk) proteins. Here, we show that the 3'UTR of the nematode homologue of GLI1, called tra-1, also contains a TGE, through which translation is regulated by GLD-1. We find that GLD-1 activity is required for proper TRA-1 protein expression in hermaphrodites. RNA gel shift assays show that GLD-1 binds the predicted sites. Using reporter transgenes, we show that the human GLI1 (hGLI1) 3'UTR controls translation in the mouse embryo. We demonstrate that the addition of the mouse QK isoform-6 (QKI-6) protein to a mammalian cell line that lacks QKI-6 can confer regulation on reporter and GLI1 mRNAs in a TGE-specific manner, and reduction of QKI-6 activity with siRNA disrupts translational control. Further, siRNA knockdown of QKI-6 increases the activity of a reporter transgene that monitors the transcriptional activity of mouse Gli1 (mGli1) and increases mouse Gli1 protein. We show by immunoprecipitation that QKI-6 antibody specifically co-purifies TGE-containing mRNAs in ribonucleoproteins. Thus, we find that the mouse QKI-6 protein acts through the mGli1 and hGLI1 RNAs to repress translation. Our results suggest that STAR family-dependent translational control of GLI mRNAs is ancient, and that it existed before the division of nematodes and mammals.
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