In early embryonic development prior to zygotic transcription, post-transcriptional regulation of mRNA is key to cell fate specification and patterning. In C. elegans, there are several RNA-binding proteins (RBPs) that coordinate oogenesis, spermatogenesis, and embryogenesis through 3'UTR-mediated post-transcriptional mechanisms. How these RBPs are themselves regulated is largely unknown. Here, we assess the role of the 3'UTR in both patterning the expression and coordinating the function of the highly conserved RBP MEX-3. Null
mex-3 mutants are maternal-effect embryonic lethal with anterior cell fate specification defects (Draper et al., 1996). Previous studies demonstrate that the 3'UTR of
mex-3 is sufficient to establish its germline expression pattern (Merritt et al., 2008, Kaymak et al., 2016). To test whether the UTR is necessary for patterning and reproductive viability, we used CRISPR/Cas9 to make an allelic series of
mex-3 3'UTR deletions in a strain where the endogenous MEX-3 is tagged with GFP (GFP::MEX-3). Among the 3'UTR deletion mutants, two variants displayed increased expression of GFP::MEX-3, with the biggest deletion showing ectopic de-repression throughout the germline indicating that the 3'UTR is required for MEX-3's expression pattern. Surprisingly, both variants were homozygous fertile but exhibited reduced fecundity. To identify the RBPs that mediate MEX-3 expression and activity through its 3'UTR, we used RNAi to knock down several candidate RBPs in the GFP::MEX-3 strain and a
mex-3 3'UTR transgenic reporter strain (GFP::
mex-3 3'UTR). We confirm that GLD-1, LIN-41, and OMA-1/2 regulate MEX-3 expression, and show that this repression is mediated through its 3'UTR. We also found that an additional RBP, DAZ-1, regulates MEX-3 expression but not through the 3'UTR. To determine the mechanisms involved in MEX-3 patterning, we used RNAi to systemically knock down components of the adenylation and de-adenylation complexes and translation initiation factors. Our results revealed that both poly(A) tail length and translational control orchestrate the unique expression pattern of MEX-3, with different mechanisms predominating in different regions of the germline. Taken together, our results demonstrate that the
mex-3 3'UTR is essential for patterning its expression, but not required for fertility. Our results describe the complex network of pathways required to coordinate the MEX-3 expression pattern, but also suggest that precise spatiotemporal control is not as critical as previously thought.