The ORB2 RNA-binding protein represses translation of its target transcripts during the Drosophila maternal-to-zygotic transition via its functionally conserved Zinc-binding 'ZZ' domain.

RNA-binding proteins (RBPs) are key components of the post-transcriptional regulatory machinery. We show that the ORB2 RBP, the Drosophila ortholog of human Cytoplasmic Polyadenylation Element Binding Protein (hCPEB) 2-4 protein family, binds to hundreds of maternally provided mRNAs in early embryos, identify a U-rich motif enriched in ORB2's targets, and show that this motif confers ORB2 binding and repression to a luciferase reporter mRNA in S2 tissue culture cells. ORB2's target transcripts are translationally repressed and unstable during the maternal-to-zygotic transition (MZT), a developmental phase during which a large proportion of maternally provided mRNAs are repressed and cleared. We show that, when tethered to a luciferase reporter, ORB2 and hCPEB2 (but not ORB and hCPEB1) repress translation and that the C-terminal Zinc-binding ('ZZ') domain of ORB2 is necessary and sufficient for repression. ORB2 interacts with a suite of post-transcriptional regulators in early embryos; a subset of these interactions is lost upon deletion of the ZZ domain, notably with the Cup repressive complex. Analysis of the early embryo's translatome in the presence or absence of the endogenous ZZ domain, shows that ORB2's targets move onto polysomes upon ZZ domain deletion, indicating that this domain mediates translational repression of ORB2's targets during the MZT. Together, our results assign a function to the ZZ domain and support a significant role for ORB2 in post-transcriptional regulation of maternal mRNAs during the Drosophila MZT.