Genetic analysis of pseudohyphal growth in the budding yeast Saccharomyces cerevisiae

The budding yeast Saccharomyces cerevisiae is a dimorphic organism that can assume either a yeast-like or pseudohyphal form. Nitrogen limitation induces pseudohyphal growth, which is characterized by branched chains of elongated cells. Pseudohyphal cells can grow invasively in agar medium, whereas yeast-like cells do not. To identify factors involved in morphologic differentiation, S. cerevisiae mutants exhibiting a constitutive cell elongation morphology were isolated. Genetic analysis identified 28 recessive and 2 semi-dominant mutations that cause abnormal morphology, and placed these in 14 distinct gene loci, termed ELM, for ELongated Morphology. Many elm mutations cause multiple aspects of pseudohyphal growth, and thus are proposed to allow execution of a specific morphologic differentiation pathway in inappropriate conditions. Supporting this hypothesis, gene dosage at several ELM loci affected pseudohyphal differentiation in response to nitrogen limitation. Genetic interactions were detected between many elm mutations, suggesting these genes function in the same, or functionally related biological pathways;Molecular cloning studies identified several proteins likely to be involved in morphologic differentiation in S. cerevisiae. ELM1 was isolated and shown to code for a protein similar in amino acid sequence to known serine/threonine protein kinases. Mutations in ELM1 showed genetic interactions with a mutation in CDC55 that affects a protein similar in amino acid sequence to the B regulatory subunit of mammalian protein phosphatase 2A. Thus, protein phosphorylation is likely to be involved in pseudohyphal differentiation. ELM13 was found to be identical to CDC12, which codes for a component of the 10 nm filament structure located at the bud neck. Loss of function of PPS1, a gene coding for phosphoribosylpyrophosphate synthase, a key rate limiting enzyme in the biosynthesis of histidine, tryptophan, and purine and pyrimidine nucleotides, interacts genetically with elm4-1 to potentiate the constitutive pseudohyphal growth phenotype;These results suggest an ordered signal transduction pathway recognizes conditions of nitrogen limitation and then causes pseudohyphal differentiation to occur. Loss of function mutations in ELM genes are proposed to alter this pathway, resulting in constitutive pseudohyphal growth