In order to isolate genes that function downstream of the Ras-cAMP pathway in Saccharomyces cerevisiae, a YEp13-based genomic library was screened for clones that inhibit growth of cells with diminished A-kinase activity. One such gene, MKS1, was found to encode a hydrophilic 52 kDa protein that shares weak homology with the yeast SPT2/SIN1 gene product. Three lines of evidence suggest that the MKS1 gene product is a negative regulator downstream of the Ras-cAMP pathway: (i) overexpression of MKS1 inhibits growth of cyr1 disruptant cells on YPD medium containing a low concentration of cAMP; (ii) overexpression of MKS1 does not affect TPK1 expression; and (iii) the temperature-sensitive cyr1-230 mutation is partially suppressed by mks1 disruption. The mks1 mutant shows similar phenotypes to gal11/spt13, i.e., it cannot grow on YPGal containing ethidium bromide at 25 degrees C, or on YPGly or SGal at 37 degrees C. The mks1 gal11 double mutant shows more marked phenotypic changes than the single mutants. These results suggest that MKS1 is involved in transcriptional regulation of several genes by cAMP.