Characterization and Molecular Genetic Complementation of Mutants Affecting Dimorphism in the FungusUstilago maydis

Ustilago maydis,the causal agent of corn smut disease, displays dimorphic growth in which it alternates between a unicellular, nonpathogenic yeast-like form and a dikaryotic, pathogenic filamentous form. Previously, a constitutively filamentous haploid mutant was obtained. Complementation of this mutant led to the isolation of the gene encoding adenylate cyclase,uac1.Secondary mutagenesis of auac1disruption strain allowed the isolation of a large number of suppressor mutants, termedubc,forUstilagobypass of cyclase, lacking the filamentous phenotype. Analysis of one of these suppressor mutants previously led to the identification of theubc1gene, encoding the regulatory subunit of cAMP-dependent protein kinase. In this report we describe the isolation of cosmids containing three newubcgenes, termedubc2, ubc3,andubc4.We also describe the morphology of theubc2, ubc3,andubc4mutants in auac1⁻background as well as in a background with a functionaluac1gene. In addition, we describe several mutant strains not complemented with any of the genes currently in hand and that are thus presumed to possess mutations in additionalubcgenes.