OVEREXPRESSION OF THE G(1)-CYCLIN GENE CLN2 REPRESSES THE MATING PATHWAY IN SACCHAROMYCES-CEREVISIAE AT THE LEVEL OF THE MEKK STE11

Basal and induced transcription of pheromone-dependent genes is regula ted in a cell cycle dependent way, FUS1, a gene strongly induced after pheromone treatment, shows high mRNA levels in mitosis and early G(1) phase of the cell cycle, a decrease in G(1) after START and again an increase in S phase, Overexpression of CLN2 was shown to repress the t ranscript number of pheromone dependent genes (1), We asked whether th e activities of components of the mating pathway fluctu ate during the cell cycle. We were also interested in determining at what level Cln2 represses the signal transduction machinery, Here we show that the ac tivity of the mitogen-activated protein kinase Fus3 indeed fluctuates during the cell cycle, reflecting the oscillations of the gene transcr ipts. CLN2 overexpression represses Fus3 kinase activity, independentl y of the phosphatase Msg5. Additionally, we show that the activity of the MEK Ste7 also fluctuates during the cell cycle, Increased Cln2 lev els repress the ability of hyperactive STE11 alleles to induce the pat hway, G protein-independent activation of Ste11 caused by an rga1 pbs2 mutation is resistant to high levels of Cln2 kinase, Therefore our re sults suggest that Cln2-dependent repression of the mating pathway occ urs at the level of Ste11.