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,
msa1, 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
msa1 RNA and
Msa1 protein diminished after several hours of
nitrogen starvation. Genetic analysis suggested that the function of
msa1 is independent of the cAMP pathway and stress-responsive pathway. Deletion of the ras1 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
msa1 reversed both phenotypes. Overexpression of
msa1 decreased activated Ras1(Val17)-induced expression of mam2. Phenotypic hypersporulation was similar between cells with deletion of only rad24 and both
msa1 and rad24, but simultaneous deletion of
msa1 and msa2/nrd1 additively increased hypersporulation. Therefore, we suggest that the primary function of
Msa1 is to negatively regulate sexual differentiation by controlling the expression of Ste11-regulated genes, possibly through the
pheromone-signaling pathway.