ANALYSIS OF THE STRUCTURAL GENES ENCODING M-FACTOR IN THE FISSION YEAST SCHIZOSACCHAROMYCES-POMBE - IDENTIFICATION OF A 3RD GENE, MFM3

We previously identified two genes, mfm1 and mfm2, with the potential to encode the M-factor mating pheromone of the fission yeast Schizosac charomyces pombe (J. Davey, EMBO J. 11:951-960, 1992), but further ana lysis revealed that a mutant strain lacking both genes still produced active M-factor. Here we describe the isolation and characterization o f a third M-factor gene, mfm3. A mutant lacking all three genes fails to produce M-factor, indicating that all functional M-factor genes now have been identified. The triple mutant exhibits an absolute mating d efect in M cells, a defect that is not rescued by addition of exogenou s M-factor. A mutational analysis reveals that all three mfm genes con tribute to the production of M-factor. Their transcription is limited to M cells and requires the mat1-Mc and ste11 gene products. Each gene is induced when the cells are starved of nitrogen and further induced by a pheromone signal. Additionally, the signal transduction machiner y associated with the pheromone response is required for transcription of the mfm genes in both stimulated and unstimulated cells.