Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Zoology and Genetics

First Advisor

Alan Myers


Saccharomyces cerevisiae is an ascomycete that has several developmental options, with concomitant morphological changes, that depend on cell type and nutrient status. In response to these signals, vegetative Saccharomyces cerevisiae cells assume one of two distinct morphologies, the yeast form or the filamentous form. Molecules affecting this morphogenetic response were identified by analyzing transposon-induced or spontaneous suppressor mutations that restore yeast form growth to a strain that constitutively exhibits filamentous form characteristics owing to mutation of the protein kinase Elm1p. One transposon-tagged mutation was isolated and is allelic to the negative regulatory subunit Bcy1p, thus activation of cAMP dependent protein kinase (PKA) by deletion of Bcy1p suppressed the constitutive filamentous growth phenotype of elm1 strains. The suppression is cell-type dependent and occurs on minimal media. PKA, therefore, has differential effects on cell morphology depending on the presence or absence of the a1/[alpha]2 repressor coded for by the MAT locus, and on the nutritional environment;A second screen for spontaneous extragenic suppressors resulted in the isolation of ten mutant strains that define three gene loci, termed SEL1, SEL2, SEL3. SEL2 has been cloned and sequence analysis indicates that the gene product encodes an S. cerevisiae homolog of the serine/threonine protein kinase nim1+ of S. pombe. Evidence from this study suggests a characterized protein kinase cascade that impacts upon activity of the cyclin-dependent kinase Cdc28p, participates in regulation of filamentous growth. Deletion of SWE1, a homologue of S. pombe wee1+, also restored yeast form growth to elm1 strains. Swe1p is functionally analogous to the wee1+ kinase in that it phosphorylates Cdc28p. Sel2p, based on its similarity to the nim1+ kinase, is predicted to inhibit Swe1p. Together, these data suggest regulation of Cdc28p by phosphorylation determines whether the pseudohyphal growth characteristics induced by elm1 mutations can occur. Se12p is localized to the bud neck during the budded phase of the cell cycle. Overexpression of Se12p results in a growth arrest late in mitosis that is suppressed by mutations in Clb2p, and an allele of CDC28, suggesting that Se12p degradation may be required for the telophase to G1 cell cycle transition.



Digital Repository @ Iowa State University,

Copyright Owner

Nicholas Patrick Edgington



Proquest ID


File Format


File Size

215 pages