In sexually reproducing organisms, fertilization results in fusion of not only the paternal and maternal genomes but also cytoplasmic components including RNA and protein. In early embryonic development prior to the onset of zygotic transcription, post-transcriptional regulation of cytoplasmic mRNA contributes to cell fate specification and patterning. In the model organism Caenorhabditis elegans, there are several RNA-binding proteins (RBPs) that have been shown to regulate oogenesis, spermatogenesis, and embryogenesis through post-transcriptional regulation of mRNAs. MEX-3 is an RNA-binding protein that is essential for early embryogenesis. Loss of function
mex-3 mutants are maternal effect sterile with anterior cell fate specification defects. Although we know how MEX-3 controls cell fate, we know little about how
mex-3 mRNA itself is regulated to ensure its proper spatial and temporal expression. In this work, we investigate the post-transcriptional regulation of
mex-3 via its 3'UTR. To identify the RNA-binding proteins that contribute to localization of MEX-3, we performed a candidate RNAi screen using a strain where the endogenous MEX-3 is tagged with GFP. We found that reduction of GLD-1, GLD-2, OMA-1/2, LIN-41, or DAZ-1 altered the expression pattern of MEX-3. To test whether these effects are 3'UTR dependent, we repeated the RNAi experiments using a transgenic
mex-3 3'UTR reporter strain and observed similar effects. To assess the biological relevance of
mex-3 3'UTR, we used CRISPR/Cas9 to target the endogenous 3'UTR. Deletion of the majority of the 3'UTR resulted in worms that exhibit maternal effect sterility characterized by smaller than usual oocytes and delocalization of MEX-3. The pattern of GFP-MEX-3 in the mutant is similar to that observed in GLD-1 RNAi. GLD-1 has been shown to bind directly to the
mex-3 3'UTR suggesting that it directly regulates
mex-3 mRNA. Together, our results show that the
mex-3 3'UTR is required to control the pattern of MEX-3 expression and is essential to fertility. Future studies will investigate the mechanisms by which the
mex-3 3'UTR governs the MEX-3 expression, contributing to our understanding of gamete and early embryo development.