G(1) CYCLINS CLN1 AND CLN2 REPRESS THE MATING FACTOR RESPONSE PATHWAYAT START IN THE YEAST-CELL CYCLE

Transcriptional induction by the mating pheromone alpha-factor was mon itored at different stages of the yeast cell cycle. G(2)/M-phase and p re-Start cells showed strong FUS1 mRNA induction, whereas in post-Star t cells the signaling was reduced significantly. This reduction in sig naling activity in post-Start cells was correlated with the presence o f CLN1 or CLN2 transcripts and was not observed in synchronized cells lacking functional CLN1 and CLN2 genes. Activation of the Cln-Cdc28p k inase by overexpression of CLN2 from the GAL1 promoter strongly reduce d FUS1 mRNA induction. CLN1 overexpression had a similar effect when t he FAR1 gene, encoding a negative regulator of CLN1/2 function, was de leted. This reduction of pheromone signaling was specific for CLN1 and CLN2, as it was not observed when CLN3 was overexpressed. Inactivatio n of the Cln-Cdc28p kinase complex by thermal inactivation of temperat ure-sensitive Cdc28p prevented repression of FUS1 signaling. CLN2 over expression suppressed the constitutive signaling and division-arrest p henotypes of cells with a disrupted gpa1 gene, indicating that the sit e of action for repression is downstream of the alpha-subunit (Gpa1p) of the heterotrimeric G protein. The repression at Start of pheromone signaling by Cln1-Cdc28p or Cln2-Cdc28p kinase complexes may contribut e to the acquisition of pheromone resistance as cells execute Start.