The RNA binding protein Csx1 promotes sexual differentiation in Schizosaccharomyces pombe.

Sexual differentiation is a highly regulated process in the fission yeast Schizosaccharomyces pombe and is triggered by nutrient depletion, mainly nitrogen source. One of the key regulatory proteins in fission yeast sexual differentiation is the transcription factor Ste11. Ste11 regulates the transcription of many genes required for the initial steps of conjugation and meiosis, and its deficiency leads to sterility. Ste11 activity is mainly regulated at two levels: phosphorylation and abundance of its mRNA. Csx1 is an RNA binding protein that we have previously described to bind and regulate the turnover rate of the mRNA encoding the transcription factor Atf1 in the presence of oxidative stress. We have observed that Csx1-deficient cells have defects in sexual differentiation and are partially sterile. We investigated how Csx1 is regulating this process in S. pombe. Csx1 associates with ste11+ mRNA and cells lacking Csx1 are sterile with a reduced amount of ste11+ mRNA. Overexpression of ste11+ mRNA completely rescues the mating deficiencies of csx1Δ cells. Here, we present a novel mechanism of ste11+ mRNA positive regulation through the activity of Csx1, an RNA binding protein that also have key functions in the response to oxidative stress in fission yeast. This finding opens interesting question about the possible coordination of sexual differentiation and oxidative stress response in eukaryotes and the role of RNA binding proteins in the adaptation to environmental signals.