Crosstalk between cAMP and pheromone signalling pathways in Ustilago maydis

In the phytopathogenic basidiomycete Ustilago maydis mating and dikaryon fo rmation are controlled by a pheromone/receptor system and the multiallelic b locus. Recently, a gene encoding a G protein alpha subunit, gpa3, was iso lated and has subsequently been implicated in pheromone signal transduction . Mutants deleted for gpa3 are sterile and nonpathogenic, and exhibit a mor phology that is similar to that of mutants with defects in the adenylate cy clase gene uacl. We have found that the sterility and mutant morphology of gpa3 deletion strains can be rescued by exogenous cAMP. In these mutants an d in the corresponding wild-type strains, exogenous cAMP stimulates pheromo ne gene expression to a level comparable to that seen in the pheromone-stim ulated state. In addition, we demonstrate that uacl is epistatic to gpa3. W e conclude that Gpa3 controls the cAMP signalling pathway in U. maydis and discuss how this pathway feeds into the pheromone response.