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Schizosaccharomyces pombe contains a single gene, rasi, which is a homolog of the mammalian RAS genes.rasl is required for conjugation, sporulation, and normal cell shape. rasi has been previously identified as steS.We report here a gene we call byr2 that can encode a predicted protein kinase and can partially suppress defects in rasi mutants. rasi mutant strains expressing high levels of byr2 can sporulate competently but are still defective in conjugation and abnormally round. byr2 mutants are viable and have normal shape but are absolutely defective in conjugation and sporulation. byr2 is probably identical to ste8. In many respects, byr2 resembles the byrl gene, another suppressor of the rasi mutation, which has been identified previously as stel. Our data indicate that if rasi, byr2, and byri act along the same pathway, then the site of action for byr2 is between the sites for rasi and byrl.RAS proteins are ubiquitous in evolution. They are lowmolecular-weight guanine nucleotide-binding proteins that function in signal transduction pathways (1). Mutant activated RAS genes are found in a large number of mammalian tumors, but despite their importance, their function in mammals is unknown. We have studied RAS in the yeast Saccharomyces cerevisiae, in which two RAS proteins, RAS1 and RAS2, regulate the function of adenylyl cyclase (23). The latter does not appear to be the function of RAS in vertebrates or even in the fission yeast Schizosaccharomyces pombe (7,20). We have therefore begun to study RAS function in S. pombe in the hope of learning whether there are general principles which govern the functions of RAS proteins in cells.S. pombe contains a single RAS gene, rasl (6, 21). rasi is not an essential gene but functions in the sexual differentiation pathways of that yeast (7,20). rasl mutant cells fail to conjugate and to sporulate. Such cells are also round, unlike wild-type cells, which are elongated. S. pombe cells that contain the activated mutant rasJVal17 allele are also partially sterile. Such cells enter the early phase of conjugation and develop elongated conjugation tubes but fail to enter the subsequent phases.In S. pombe, there are two mating types, designated h+ and h-(4). Only opposite mating types conjugate, and only upon starvation. Homothallic (h90) haploid strains regularly switch mating type and therefore self-mate. Heterothallic (h+ and h-) strains do not switch mating type and do not self-mate. Conjugation can be divided into an early phase, marked by an increase in cell agglutination and the formation of a conjugation tube, and a later phase, marked by the fusion of cells and karyogamy. Immediately following conjugation, most cells undergo zygotic sporulation. Diploid cells, formed either by mating or by other means, can be propagated stably, but diploid strains containing both mating type loci will undergo azygotic sporulation upon starvation. The four-spored asci formed by zygotic sporulation look different from the four-spored asci formed by azygotic * Corresponding author. sporul...

We present genetic evidence that three presumptive protein kinases of Schizosaccharomyces pombe, byr2, byrl, and spkl that are structurally related to protein kinases of Saccharomyces cerevisiae, STE11, STE7, and FUS3, respectively, are also functionally related. In some cases, introduction of the heterologous protein kinase into a mutant was sufficient for complementation. In other cases (as in a stell-mutant of S. cerevisiae), expression of two S. pombe protein kinases (byr2 and byrl) was required to observe complementation, suggesting that byr2 and byrl act cooperatively. Complementation in S. pombe mutants is observed as restoration of sporulation and conjugation and in S. cerevisiae as restoration of conjugation, pheromone-induced cell cycle arrest, and pheromone-induced transcription of the FUSI gene. We also show that the S. pombe kinases bear a similar relationship to the mating pheromone receptor apparatus as do their S. cerevisiae counterparts. Our results indicate that pheromone-induced signal transduction employs a conserved set of kinases in these two evolutionarily distant yeasts despite an apparently significant difference in function of the heterotrimeric G proteins. We suggest that the STE11/byr2, STE7/byrl, and FUS3/spkl kinases comprise a signal transduction module that may be conserved in higher eukaryotes. Consistent with this hypothesis, we show that a mammalian mitogenactivated protein (MAP)

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The interaction between RAS proteins and adenylyl cyclase was studied by using dominant interfering mutations of adenylyl cyclase from the yeast Saccharomyces cerevisiae. RAS proteins activate adenylyl cyclase in this organism. A plasmid expressing a catalytically inactive adenylyl cyclase was found to interfere dominantly with this activation. The interfering region mapped to the leucine-rich repeat region of adenylyl cyclase, which is homologous to domains present in several other proteins and is thought to participate in protein-protein interactions.