Schizosaccharomyces pombe Ste7p is required for both promotion and withholding of the entry to meiosis

The fission yeast ste7 mutant cannot mate and undergo meiosis, but shows no defect in vegetative growth. We cloned and characterized the ste7 gene. Th e deduced ste7 gene product (Ste7p) was a protein of 569 amino acids with n o significant similarity to other proteins. Transcription of ste7 was induc ed by nutrient starvation via the function of the transcription factor Ste1 1p. Disruption of the ste7 gene blocked both conjugation and meiosis, showi ng that Ste7p plays a positive role in these two processes, probably activa ting the pheromone signal pathway. Unexpectedly, overexpression of ste7(+) promoted conjugation but inhibited meiosis in wild-type cells. The temperat ure-sensitive pat1-114 mutant underwent ectopic conjugation at the semirest rictive temperature when its genetic background was ste7(+), whereas the sa me mutant initiated haploid. meiosis when its genetic background was ste7 D elta. Two-hybrid analysis suggested that Ste7p interacts physically with bo th Pat1p and Mei2p, which together constitute the major switch to initiate meiosis. Ste7p tagged with green fluorescent protein accumulated in haploid cells under nutrient starvation until they completed conjugation, but this protein disappeared when they were to enter meiosis. These observations su ggest that Ste7p may have a function to suppress the onset of meiosis until the conjugation process has been duly completed.