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In Saccharomyces cerevisiae, mating pheromones activate two MAP kinases (MAPKs), Fus3p and Kss1p, to induce G1 arrest prior to mating. Fus3p is known to promote G1 arrest by activating Far1p, which inhibits three Clnp/Cdc28p kinases. To analyze the contribution of Fus3p and Kss1p to G1 arrest that is independent of Far1p, we constructed far1 CLN strains that undergo G1 arrest from increased activation of the mating MAP kinase pathway. We find that Fus3p and Kss1p both control G1 arrest through multiple functions that operate in parallel with Far1p. Fus3p and Kss1p together promote G1 arrest by repressing transcription of G1/S cyclin genes (CLN1, CLN2, CLB5) by a mechanism that blocks their activation by Cln3p/Cdc28p kinase. In addition, Fus3p and Kss1p counteract G1 arrest through overlapping and distinct functions. Fus3p and Kss1p together increase the expression of CLN3 and PCL2 genes that promote budding, and Kss1p inhibits the MAP kinase cascade. Strikingly, Fus3p promotes proliferation by a novel function that is not linked to reduced Ste12p activity or increased levels of Cln2p/Cdc28p kinase. Genetic analysis suggests that Fus3p promotes proliferation through activation of Mcm1p transcription factor that upregulates numerous genes in G1 phase. Thus, Fus3p and Kss1p control G1 arrest through a balance of arrest functions that inhibit the Cdc28p machinery and proliferative functions that bypass this inhibition. Show less

In the absence of a successful mating, pheromone-arrested Saccharomyces cerevisiae cells reenter the mitotic cycle through a recovery process that involves downregulation of the mating mitogen-activated protein kinase (MAPK) cascade. We have isolated a novel gene, POG1, whose promotion of recovery parallels that of the MAPK phosphatase Msg5. POG1 confers alpha-factor resistance when overexpressed and enhances alpha-factor sensitivity when deleted in the background of an msg5 mutant. Overexpression of POG1 inhibits alpha-factor-induced G1 arrest and transcriptional repression of the CLN1 and CLN2 genes. The block in transcriptional repression occurs at SCB/MCB promoter elements by a mechanism that requires Bck1 but not Cln3. Genetic tests strongly argue that POG1 promotes recovery through upregulation of the CLN2 gene and that the resulting Cln2 protein promotes recovery primarily through an effect on Ste20, an activator of the mating MAPK cascade. A pog1 cln3 double mutant displays synthetic mutant phenotypes shared by cell-wall integrity and actin cytoskeleton mutants, with no synthetic defect in the expression of CLN1 or CLN2. These and other results suggest that POG1 may regulate additional genes during vegetative growth and recovery. Show less

FUS3 is functionally redundant with KSS1, a homologous yeast protein kinase, for a step(s) in signal transduction between the beta subunit of the guanine nucleotide binding protein (G protein), STE4, and the mating type-specific transcriptional activator, STE12. Either FUS3 or KSS1 can execute this function; when neither gene encoding these protein kinases is present, signal transduction is blocked, causing sterility. This functional redundancy is strain dependent; some standard laboratory strains (S288C) are kss1-. FUS3 has additional functions required for cell cycle arrest and vegetative growth that do not overlap with KSS1 functions. FUS3 mediates cell cycle arrest during mating through transcriptional repression of two G1 cyclins (CLN1 and CLN2) and through posttranscriptional inhibition of a third G1 cyclin (CLN3). FUS3 is also required for vegetative growth in haploid strains dependent upon CLN3 for cell cycle progression but is not required in strains dependent upon either CLN1 or CLN2, suggesting a functional divergence among the three G1 cyclins. The diverse roles for FUS3 suggest that the FUS3 protein kinase has multiple substrates, some of which may be shared with KSS1. Show less

FUS3 is required for both the arrest of cells in G1 and mating. Upon exposure to mating pheromone, fus3-1 and fus3-2 mutants fail to arrest in G1 and continue to divide while undergoing the transcription induction and morphological changes typical of mating cells. The G1 arrest defect of these fus3 mutants is suppressed by a daf1/whi1 null mutation (also called cln3, a putative cyclin). FUS3 has a positive role in conjugation, because overexpression of FUS3 increases the pheromone sensitivity of wild-type cells, while the absence of FUS3 causes sterility. The suppression of a gpa1 null (G alpha subunit) by a fus3 null also suggests FUS3 is in the signal transduction pathway. The predicted FUS3 protein is 35% identical to the cdc2+/CDC28 kinases and 52% identical to the KSS1 predicted kinase. Show less