Schizosaccharomyces pombe map1+ encodes a MADS-box-family protein required for cell-type-specific gene expression.

We cloned the Schizosaccharomyces pombe map1 gene by virtue of its ability to stimulate transcription of the sxa2 gene, which encodes a carboxypeptidase expressed specifically in h- cells in response to mating-pheromone signaling. The cloned gene had a coding capacity of 398 amino acids split by two introns, and the deduced product was a protein of the MADS box family. This gene was most similar to Saccharomyces cerevisiae MCM1, which regulates cell-type-specific gene expression in budding yeast cells. Disruption of the S. pombe gene did not affect vegetative cell growth but conferred sterility. It blocked the mating ability of h+ cells completely and that of h- cells partially. Genetic and sequencing analysis indicated that the cloned gene is map1], which was originally defined by a mutation that caused h+-speciftic sterility. Northern (RNA) blot analysis showed that the function of map1 is absolutely essential for the expression of h+-specific genes and is required for the full activation of h--specific gene expression. Overexpression of map1 resulted in enhanced transcription of cell-type-specilic genes, but the range of genes affected by Map1 was restricted by the mating type of the cell. Results of yeast two-hybrid analysis suggested that Map1 may physically interact with Mat1-Pc, the product of the h(+)-specific mating-type gene mat1-Pc. On the basis of these observations, we speculate that Map1 may be a transcriptional regulator of cell-type-specific genes similar to S. cerevisiae MCM1, whose activity is modulated by the oil and alpha2 mating-type gene products.

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. The deduced ste7 gene product (Ste7p) was a protein of 569 amino acids with no significant similarity to other proteins. Transcription of ste7 was induced by nutrient starvation via the function of the transcription factor Ste11p. Disruption of the ste7 gene blocked both conjugation and meiosis, showing that Ste7p plays a positive role in these two processes, probably activating the pheromone signal pathway. Unexpectedly, overexpression of ste7(+) promoted conjugation but inhibited meiosis in wild-type cells. The temperature-sensitive pat1-114 mutant underwent ectopic conjugation at the semirestrictive temperature when its genetic background was ste7(+), whereas the same mutant initiated haploid meiosis when its genetic background was ste7Delta. Two-hybrid analysis suggested that Ste7p interacts physically with both 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 suggest that Ste7p may have a function to suppress the onset of meiosis until the conjugation process has been duly completed.

Transcriptional regulation of a Ras nucleotide-exchange factor gene by extracellular signals in fission yeast.

The ste6 gene of Schizosaccharomyces pombe encodes a putative GDP-GTP exchange factor for the ras1 gene product. Genetic analysis of the ste6 and ras1 genes has shown that they are required for mating and for the response to mating pheromones. In this study we show that expression of the ste6-encoded mRNA is induced by nitrogen starvation, the physiological signal that triggers mating and sexual differentiation. Exposure to mating pheromones enhances the induction of ste6 expression upon nitrogen starvation. Pheromone-induced expression requires not only the function of components of the pheromone-signalling pathway, but also ras1 function. Furthermore, mutants in which the Ras1 protein is activated have higher basal and induced levels of ste6 gene expression than wild-type cells. These observations indicate the existence of a positive-feedback loop through which Ras1 stimulates the expression of its own activator. Since Ste6 is likely to promote the exchange of guanine nucleotides on Ras1 protein, our results suggest an important role for GDP-GTP exchange in the regulation of Ras1 activity during the mating process in S. pombe.