CONTROL OF FILAMENTOUS GROWTH BY MATING AND CYCLIC-AMP IN USTILAGO

Mating between compatible haploid cells of the corn smut fungus, Ustil ago maydis, results in a switch from budding to filamentous growth. We are analyzing the multiallelic b incompatibility locus that governs m aintenance of the infectious, filamentous dikaryon. Specificity region s have been identified in the N-terminal portions of the bE and bW gen es at the b locus. In addition, we have found that heterozygosity at t he b locus attenuates fusion. This result suggests that b gene product s may exert a negative influence on some processes, in addition to the ir generally recognized role in maintaining filamentous growth. The, b genes have also been characterized in Ustilago hordei. This species h as a bipolar mating system in which the b genes are linked to genes re quired for pheromone production and response to form one large mating- type region (MAT locus) with two allelic specificities. In a separate study, we have discovered that defects in adenylate cyclase result in constitutive filamentous growth and greatly reduce the virulence of U. maydis on corn seedlings. Mutations have been identified that suppres s the adenylate cyclase defect and restore budding growth. Sequence an alysis revealed that one of the suppressor mutations is in the gene en coding the regulatory subunit of cAMP-dependent protein kinase (PKA). Mutants altered in PKA activity have the interesting phenotype of mult iple budding and frequent mislocalization of the bud site.