The SPN-4 Rbfox RNA-binding protein selects maternal mRNAs for CCR4-NOT-dependent clearance in early Caenorhabditis elegans embryos.

The elimination of maternal mRNAs is an essential feature of the maternal-to-zygotic transition. We report an essential pathway that clears many maternal transcripts from early C. elegans embryos using the Rbfox-related SPN-4 RNA-binding protein as a specificity factor and the CCR4-NOT deadenylase complex as an effector. We biochemically identified SPN-4-associated mRNAs from late-stage oocytes and found that the set of SPN-4-associated transcripts is enriched for maternal mRNAs that undergo early decay. Single-molecule fluorescence in situ hybridization experiments established that many SPN-4-associated mRNAs fail to be eliminated in the absence of SPN-4. In the 3'UTRs of two target mRNAs, we identified Rbfox motifs that are required for SPN-4-dependent clearance in vivo and bind SPN-4 in vitro. In a genetic screen to identify factors that work with SPN-4, we isolated mutant alleles of CCR4-NOT components. Auxin-induced degradation of the LET-711/NOT1 scaffold and the CCF-1 deadenylase disrupted clearance of two SPN-4-associated transcripts. Our results support a model in which SPN-4 initiates expression in late-stage oocytes, associates with maternal mRNA targets through RNA sequences in their 3'UTRs and promotes CCR4-NOT-mediated decay during early embryogenesis.