MUTATIONS IN THE MYP1 GENE OF USTILAGO-MAYDIS ATTENUATE MYCELIAL GROWTH AND VIRULENCE

Mating between haploid, budding cells of the dimorphic fungus Ustilago maydis results in the formation of a dikaryotic, filamentous cell typ e. Mating compatibility is governed by two mating-type loci called a a nd b; transformation of genes from these loci (e.g., a1 and b1) into a haploid strain of different mating type (e.g., a2 b2) allows filament ous growth and establishes a pathogenic cell type. Several mutants wit h a nonmycelial colony morphology were isolated after insertional muta genesis of a filamentous, pathogenic haploid strain. The mutagenized r egion in one such mutant was recovered by plasmid rescue and employed to isolate a gene involved in conditioning the mycelial phenotype (myp 1). An 1150 amino acid open reading frame is present at the myp1 locus ; the predicted polypeptide is rich in serine residues and contains sh ort regions with similarity to SH3 domain ligands. Construction of myp 1 disruption and deletion mutants in haploid strains confirmed that th is gene plays a role in mycelial growth and virulence.