Epistasis analysis provides evidence that Mep2 and Ste12 are functionally related. After growth on galactose low-ammonium sulfate (50 μM) plates for 6 d at 30°C, pseudohyphal growth was assayed in diploid mutants lacking Tpk2, Gpa2, Gpr1, Ras2 and Ste12 that expressed MEP2 from a galactose-inducible promoter (A) and diploid ste12Δ/ste12Δ cells that contained either a galactose-inducible STE12 gene or the RAS2Val19 allele (B). (C) Cells lacking Mep2 and containing plasmids expressing MCM1, the RAS2Val19 allele, the STE11-4 allele, GAL-MEP2, or GAL-STE12.