Regulators of Pseudohyphal Differentiation in Saccharomyces cerevisiae Identified Through Multicopy Suppressor Analysis in Ammonium Permease Mutant Strains

Abstract

Abstract Nitrogen-starved diploid cells of the yeast Saccharomyces cerevisiae differentiate into a filamentous, pseudohyphal growth form. Recognition of nitrogen starvation is mediated, at least in part, by the ammonium permease Mep2p and the Gα subunit Gpa2p. Genetic activation of the pheromone-responsive MAP kinase cascade, which is also required for filamentous growth, only weakly suppresses the filamentation defect of Δmep2/Δmep2 and Δgpa2/Δgpa2 strain. Surprisingly, deletion of Mep1p, an ammonium permease not previously thought to regulate differentiation, significantly enhances the potency of MAP kinase activation, such that the STE11-4 allele induces filamentation to near wild-type levels in Δmep1/Δmep1 Δmep2/Δmep2 and Δmep1/Δmep1 Δgpa2/Δgpa2 strains. To identify additional regulatory components, we isolated high-copy suppressors of the filamentation defect of the Δmep1/Δmep1 Δmep2/Δmep2 mutant. Multicopy expression of TEC1, PHD1, PHD2 (MSS10/MSN1/FUP4), MSN5, CDC6, MSS11, MGA1, SKN7, DOT6, HMS1, HMS2, or MEP2 each restored filamentation in a Δmep1/Δmep1 Δmep2/Δmep2 strain. Overexpression of SRK1 (SSD1), URE2, DAL80, MEP1, or MEP3 suppressed only the growth defect of the Δmep1/Δmep1 Δmep2/Δmep2 mutant strain. Characterization of these genes through deletion analysis and epistasis underscores the complexity of this developmental pathway and suggests that stress conditions other than nitrogen deprivation may also promote filamentous growth.