Regulation of the Drosophila transcriptome by Pumilio and the CCR4-NOT deadenylase complex

The sequence-specific RNA-binding protein Pumilio (Pum) controls Drosophila development; however, the network of mRNAs that it regulates remains incompletely characterized. In this study, we use knockdown and knockout approaches coupled with RNA-seq to measure the impact of Pum on the transcriptome of Drosophila cells in culture. We also use an improved RNA coimmunoprecipitation method to identify Pum-bound mRNAs in Drosophila embryos. Integration of these data sets with the locations of Pum-binding motifs across the transcriptome reveals novel direct Pum target genes involved in neural, muscle, wing, and germ cell development and in cellular proliferation. These genes include components of Wnt, TGF-beta, MAPK/ERK, and Notch signaling pathways, DNA replication, and lipid metabolism. We identify the mRNAs regulated by the CCR4-NOT deadenylase complex, a key factor in Pum-mediated repression, and observe concordant regulation of Pum:CCR4-NOT target mRNAs. Computational modeling reveals that Pum binding, binding site number, clustering, and sequence context are important determinants of regulation. In contrast, we show that the responses of direct mRNA targets to Pum-mediated repression are not influenced by the content of optimal synonymous codons. Moreover, contrary to a prevailing model, we do not detect a role for CCR4-NOT in the degradation of mRNAs with low codon optimality. Together, the results of this work provide new insights into the Pum regulatory network and mechanisms and the parameters that influence the efficacy of Pum-mediated regulation.