CYCLIN-SPECIFIC START EVENTS AND THE G1-PHASE SPECIFICITY OF ARREST BY MATING FACTOR IN BUDDING YEAST

The START cell cycle transition in the budding yeast Saccharomyces cer evisiae is catalyzed by the Cdc28 cyclin-dependent kinase associated w ith Gin-type cyclins. Since ectopic expression of the B-type cyclin CL B5 can efficiently rescue the inviability that results from CLN deplet ion, we tested the specificity of the CLN and CLB classes of cyclins f or promoting START-associated events. Several aspects of the regulatio n of the mating factor response were compared for cells in which START activity was provided by either Cln-cyclins or Clb5. Unlike Cln1 and Cln2, high level expression of Clb5 was unable to repress the activity of the mating factor response pathway at START. Downregulation of Far l protein at START is normal in cln- GAL1::CLB5 cells. Even though the Clb5-Cdc28 kinase activity in cln- GAL1::CLB5 cells is not downregula ted in response to mating factor, cells arrest in the first cycle afte r addition of mating factor with a similar sensitivity as wildtype cel ls. However, whereas wild-type cells treated with mating factor arrest specifically in G1 phase as unbudded cells with unreplicated DNA (pre -START), most cln- GAL1::CLB5 cells arrest as budded post-START cells with replicated DNA. Our findings demonstrate the ability of post-STAR T cells to arrest in response to mating factor and provide novel evide nce for mechanisms that contribute to restrict mating factor-induced a rrest in wild-type cells to the G1 phase of the cell cycle.