G-PROTEINS IN USTILAGO-MAYDIS - TRANSMISSION OF MULTIPLE SIGNALS

In the phytopathogenic fungus Ustilago maydis, cell fusion is governed by a pheromone signalling system, The pheromone receptors belong to t he seven transmembrane class that are coupled to heterotrimeric G prot eins, We have isolated four genes (gpa1 to gpa4) encoding alpha subuni ts of G proteins, Gpa1, Gpa2 and Gpa3 have homologues in other fungal species, while Gpa4 is novel. Null mutants in individual genes were vi able and only disruption of gpa3 caused a discernible phenotype. gpa3 mutant strains were unable to respond to pheromone and thus were matin g-deficient, A constitutively active allele of gpa3 (gpa3(Q206L)) was generated by site-directed mutagenesis, Haploid strains harbouring gpa 3(Q206L) were able to mate without pheromone stimulation, indicating t hat Gpa3 plays an active role in transmission of the pheromone signal, Surprisingly, Gpa3 is also required for pathogenic development, altho ugh pheromone signalling is not essential for this process.