Regulators of pseudohyphal differentiation in Saccharomyces cerevisiae identified through multicopy suppressor analysis in ammonium permease mutant strains

Nitrogen-starved diploid cells of the yeast Saccharomyces cerevisiae differ entiate into a filamentous, pseudohyphal growth form. Recognition of nitrog en starvation is mediated, at least in part, by the ammonium permease Mep2p and the G alpha subunit Gpa2p. Genetic activation of the pheromone-respons ive MAP kinase cascade, which is also required for filamentous growth, only weakly suppresses the filamentation defect of Delta mep2/Delta mep2 and De lta gpa2/Delta gpa2 strain. Surprisingly, deletion of Mep1p, an ammonium pe rmease not previously thought to regulate differentiation, significantly en hances the potency of MAP kinase activation, such that the STE11-4 allele i nduces filamentation to near wild-type levels in Delta mep1/Delta mep1 Delt a mep2/Delta mep2 and Delta mep1/Delta mep1 Delta gpa2/Delta gpa2 strains. To identify additional regulatory components, we isolated high-copy suppres sors of the filamentation defect of the Delta mep1/Delta mep1 Delta mep2/De lta mep2 mutant. Multicopy expression of TEC1, PHD1, PHD2 (MSS10/MSN1/FUP4) , MSN5, CDC6, MSS11, MGA1, SKN7, DOT6, HMS1, HMS2, or MEP2 each restored fi lamentation in a Delta mep1/Delta mep1 Delta mep2/Delta mep2 strain. Overex pression of SRK1 (SSD1), URE2, DAL80, MEP1, or MEP3 suppressed only the gro wth defect of the Delta mep1/Delta mep1 Delta mep2/Delta mep2 mutant strain . Characterization of these genes through deletion analysis and epistasis u nderscores the complexity of this developmental pathway and suggests that s tress conditions other than nitrogen deprivation may also promote filamento us growth.