Regulation of mRNA Stability by the RNA-binding Protein Pumilio during Early Drosophila Embryogenesis

The maternal-to-zygotic transition (MZT) is a characteristic phase of early metazoan development where control of embryogenesis transitions from products encoded by the mother to those encoded by the zygotic genome. Post-transcriptional regulation (PTR) plays a critical role in the MZT, particularly in the clearance of maternal mRNAs. Mechanisms of maternal transcript decay that rely exclusively on maternal protein factors and function early during the MZT, as well as mechanisms that require zygotic factors and function later during the MZT, have been characterized. The Drosophila embryo has long-served as a model for the MZT. The RNA-binding protein (RBP) SMAUG (SMG) has been shown to function during the early (maternal) phase of degradation. In contrast, computational methods suggest the RBP PUMILIO (PUM) functions in the late (zygotic) phase of maternal mRNA degradation. Such a role is curious as PUM is maternally-contributed and functional during the maternal (early) phase of embryogenesis. I show here that: 1) PUM is required for the degradation of approximately 500 maternal mRNAs during the late (zygotic) wave of degradation; 2) degradation of PUM target mRNAs is likely delayed to the late (zygotic) phase due to the presence of sub-optimal PUM binding sites within these target mRNAs, 3) degradation of PUM targets is dependent on additional factors such as the RBP BRAIN TUMOUR (BRAT) and a core component of the RNAi machinery, ARGONAUTE 1 (AGO1); and 4) a critical function of PUM appears to be clearance of smg mRNA, since in pum mutant embryos SMG protein persists post-MZT and is associated with an inappropriate down-regulation of SMG target transcripts. Taken together, these data support a multi-factorial view of RBP function, in which the activity of a given RBP is determined by other RBPs associated with a particular mRNA.Ph.D.2018-02-12 00:00:0