SUPPRESSORS OF A SACCHAROMYCES-CEREVISIAE PKC1 MUTATION IDENTIFY ALLELES OF THE PHOSPHATASE GENE PTC1 AND OF A NOVEL GENE ENCODING A PUTATIVE BASIC LEUCINE-ZIPPER PROTEIN

The PKC1 gene product, protein kinase C, regulates a mitogen-activated protein kinase (MAPK) cascade, which is implicated in cell wall metab olism. Previously, we identified the pkc1-4 allele in a screen for mut ants with increased rates of recombination, indicating that PKC1 may a lso regulate DNA metabolism. The pkc1-4 allele also conferred a temper ature-sensitive (ts) growth defect. Extragenic suppressors were isolat ed that suppress both the ts and hyperrecombination phenotypes conferr ed by the pkc1-4 mutation. Eight of these suppressors fell into two co mplementation groups, designated KCS1 and KCS2. KCS1 was cloned and fo und to encode a novel protein with homology to the basic leucine zippe r family of transcription factors. KCS2 is allelic with PTC1, a previo usly identified type 2C serine/ threonine protein phosphatase. Althoug h mutation of either KCS1 or PTC1 causes little apparent phenotype, th e kcs1 Delta ptc1 Delta double mutant fails to grow at 30 degrees. Fur thermore, the ptc1 deletion mutation is synthetically lethal in combin ation with a mutation in MPK1, which encodes a MAPK homologue proposed to act in the PKC1 pathway. Because PTC1 was initially isolated as a component of the Hog1p MAPK pathway, it appears that these two MAPK ca scades share a common regulatory feature.