The interplay between Zfh1 and Nau orchestrates the myogenic stage-dependent expression of Rbfox1 and its target Stat92E via a mir-9a -mediated negative feedback loop in Drosophila melanogaster

RNA-binding Fox protein 1 (Rbfox1) controls gene expression at various levels: as a transcription co-factor, a splicing factor, and a regulator of mRNA stability and translation. Rbfox1 and its vertebrate orthologues have critical functions during development. Rbfox1 mRNA, during adult myogenesis in Drosophila, exhibits bimodal expression, suggestive of the presence of a negative feedback loop. However, there are no known negative regulators of Rbfox1 expression for this context. In this study, we show that the microRNA mir-9a functions as a repressor of Rbfox1 expression. Furthermore, the expression of mir-9a is regulated by the expression of the identity transcription factor Nau. Nau and its target mir-9a are expressed at mid myogenesis stage, and serve to restrict the expression of Rbfox1 , and its targets Stat92E and zfh1 , to the early and the late myogenesis stages, as their ectopic expression is detrimental to early myofibrillogenesis. Zfh1, in turn, represses mir-9a expression, completing the feedback loop. Therefore, our findings identify the mechanism by which the temporal expression of the developmental gene Rbfox1 and its targets are tightly regulated, essential for their functions during myogenesis.