CHARACTERIZATION OF FUS1 OF SCHIZOSACCHAROMYCES-POMBE - A DEVELOPMENTALLY CONTROLLED FUNCTION NEEDED FOR CONJUGATION

In Schizosaccharomyces pombe, the fus1 mutation blocks conjugation at a point after cell contact and agglutination. The cell walls separatin g the mating partners are not degraded, which prevents cytoplasmic fus ion. In order to investigate the molecular mechanism of conjugation, w e cloned the fus1 gene and found that it is capable of encoding a 1,37 2-amino-acid protein with no significant similarities to other known p roteins. Expression of the fus1 gene is regulated by the developmental state of the cells. Transcription is induced by nitrogen starvation a nd requires a pheromone signal in both P and M cell types. Consequentl y, mutants defective in the pheromone response pathway fail to induce fus1 expression. The ste11 gene, which encodes a transcription factor controlling expression of many genes involved in sexual differentiatio n, is also required for transcription of fus1. Furthermore, deletion o f two potential Ste11 recognition sites in the fus1 promoter region ab olished transcription, and expression could be restored when we insert ed a different Ste11 site from the mat1-P promoter. Since this element was inverted relative to the fus1 element, we conclude that activatio n of transcription by Ste11 is independent of orientation. Although th e fus1 mutant has a phenotype very similar to that of Saccharomyces ce revisiae fus1 mutants, the two proteins appear to have different roles in the process of cell fusion. Budding yeast Fus1 is a typical membra ne protein and contains an SH3 domain. Fission yeast Fus1 has no featu res of a membrane protein, yet it appears to localize to the projectio n tip. A characteristic proline-rich potential SH3 binding site may me diate interaction with other proteins.