REGULATION OF DIMORPHISM IN SACCHAROMYCES-CEREVISIAE - INVOLVEMENT OFTHE NOVEL PROTEIN-KINASE HOMOLOG ELM1P AND PROTEIN PHOSPHATASE 2A

The Saccharomyces cerevisiae genes ELM1, ELM2, and ELM3 were identifie d on the basis of the phenotype of constitutive cell elongation. Mutat ions in any of these genes cause a dimorphic transition to a pseudohyp hal growth state characterized by formation of expanded, branched chai ns of elongated cells. Furthermore, elm1, elm2, and elm3 mutations cau se cells to grow invasively under the surface of agar medium. S. cerev isiae is known to be a dimorphic organism that grows either as a unice llular yeast or as filamentous cells termed pseudohyphae; although the yeast-like form usually prevails, pseudohyphal growth may occur durin g conditions of nitrogen starvation. The morphologic and physiological properties caused by elm1, elm2, and elm3 mutations closely mimic pse udohyphal growth occurring in conditions of nitrogen starvation. There fore, we propose that absence of ELM1, ELM2, or ELM3 function causes c onstitutive execution of the pseudohyphal differentiation pathway that occurs normally in conditions of nitrogen starvation. Supporting this hypothesis, heterozygosity at the ELM2 or ELM3 locus significantly st imulated the ability to form pseudohyphae in response to nitrogen star vation. ELM1 was isolated and shown to code for a novel protein kinase homolog. Gene dosage experiments also showed that pseudohyphal differ entiation in response to nitrogen starvation is dependent on the produ ct of CDC55, a putative B regulatory subunit of protein phosphatase 2A , and a synthetic phenotype was observed in elm1 cdc55 double mutants. Thus, protein phosphorylation is likely to regulate differentiation i nto the pseudohyphal state.