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Volume 272, Number 20, Issue of May 16, 1997 pp. 13180-13188
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.

Overexpression of the G₁-cyclin Gene CLN2 Represses the Mating Pathway in Saccharomyces cerevisiae at the Level of the MEKK Ste11

(Received for publication, November 15, 1996, and in revised form, March 7, 1997)

From the Institute for Biochemistry and Molecular Cell Biology, Ludwig Boltzmann Forschungsstelle, University of Vienna, Dr. Bohr, Gasse 9, 1030 Vienna, Austria

Basal and induced transcription of pheromone-dependent genes is regulated in a cell cycle-dependent way. FUS1, a gene strongly induced after pheromone treatment, shows high mRNA levels in mitosis and early G₁ phase of the cell cycle, a decrease in G₁ after START and again an increase in S phase. Overexpression of CLN2 was shown to repress the transcript number of pheromone-dependent genes (1). We asked whether the activities of components of the mating pathway fluctuate during the cell cycle. We were also interested in determining at what level Cln2 represses the signal transduction machinery. Here we show that the activity of the mitogen-activated protein kinase Fus3 indeed fluctuates during the cell cycle, reflecting the oscillations of the gene transcripts. CLN2 overexpression represses Fus3 kinase activity, independently of the phosphatase Msg5. Additionally, we show that the activity of the MEK Ste7 also fluctuates during the cell cycle. Increased Cln2 levels repress the ability of hyperactive STE11 alleles to induce the pathway. G protein-independent activation of Ste11 caused by an rga1 pbs2 mutation is resistant to high levels of Cln2 kinase. Therefore our results suggest that Cln2-dependent repression of the mating pathway occurs at the level of Ste11.

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