A novel gene, msa1, inhibits sexual differentiation in Schizoscharomyces pombe

Sexual differentiation in the fission yeast Schizosaccharomyces pombe is triggered by nutrient starvation or by the presence of mating pheromones. We identified a novel gene, msal, which encodes a 533-aa putative RNA-binding protein that inhibits sexual differentiation. Disruption of the msa1 gene caused cells to hypersporulate. Intracellular levels of msal RNA and Msal protein diminished after several hours of nitrogen starvation. Genetic analysis suggested that the function of msal is independent of the cAMP pathway and stress-responsive pathway. Deletion of the rust gene in diploid cells inhibited sporulation and in haploid cells decreased expression of mating-pheromone-induced genes such as mei2, mam2, ste11, and rep1; simultaneous deletion of msal reversed both phenotypes. Overexpression of msal decreased activated Ras1(Val17)-induced expression of mam2. Phenotypic hypersporulation was similar between cells with deletion of only rad24 and both msal and rad24, but simultaneous deletion of msal and msa2/nrd1 additively increased hypersporulation. Therefore, we suggest that the primary function of Msal is to negatively regulate sexual differentiation by controlling the expression of Ste11-regulated genes, possibly through the pheromone-signaling pathway.