ACTIVATED ALLELES OF YEAST SLN1 INCREASE MCM1-DEPENDENT REPORTER GENE-EXPRESSION AND DIMINISH SIGNALING THROUGH THE HOG1 OSMOSENSING PATHWAY

Two-component signal transduction systems involving histidine autophos phorylation and phosphotransfer to an aspartate residue on a receiver molecule have only recently been discovered in eukaryotes, although th ey are well studied in prokaryotes. The Sln1 protein of Saccharomyces cerevisiae is a two-component regulator involved in osmotolerance. Pho sphorylation of Sln1p leads to inhibition of the Hog1 mitogen-activate d protein kinase osmosensing pathway. We have discovered a second func tion of Sln1p by identifying recessive activated alleles (designated n rp2) that regulate the essential transcription factor Mcm1. nrp2 allel es cause a 5-fold increase in the activity of an Mcm1-dependent report er, whereas deletion of SLN1 causes a 10-fold decrease in reporter act ivity and a corresponding decrease in expression of Mcm1-dependent gen es. In addition to activating Mcm1p, nrp2 mutants exhibit reduced phos phorylation of Hog1p and increased osmosensitivity suggesting that nrp 2 mutations shift the Sln1p equilibrium toward the phosphorylated stat e, Two nrp2 mutations map to conserved residues in the receiver domain (P1148S and P1196L) and correspond to residues implicated in bacteria l receivers to control receiver phosphorylation state, Thus, it appear s that increased Sln1p phosphorylation both stimulates Mcm1p activity and diminishes signaling through the Hog1 osmosensing pathway.