1.8.28
THE G-PROTEIN ALPHA SUB-UNIT GPA3 AND ITS ROLE IN PATHOGENIC DEVELOPMENT OF USTILAGO MAYDIS

J KRUGER, G LOUBRADOU, E REGENFELDER and R KAHMANN

Institut für Genetik und Mikrobiologie der Universität München, Maria-Ward-Str. 1a, 80638 München, Germany

Background and objectives
The corn smut fungus Ustilago maydis exists in two distinct morphological forms: haploid cells grow yeast-like and upon mating form dikaryotic hyphae which are able to infect the maize host. Mating and dikaryon formation are controlled by a pheromone/receptor system and certain pairwise combinations of two homoeodomain proteins, bE and bW. Known components of the pheromone signalling cascade include the pheromones, their respective receptors and the transcription factor prf1 that is activated by this cascade. In addition, a G-protein alpha sub-unit has been identified, which when mutated leads to sterility [1]. This gene, gpa3, is also required for pathogenic development, although this process does not require pheromone signalling. We are interested in this second signalling cascade involving gpa3, and wish to identify additional genes from this pathway.

Results and conclusions
To isolate such genes we made use of a constitutively active mutant allele of gpa3, designated gpa3Q206L. This mutant forms glossy colonies on plates and causes abnormal tumour development. We generated UV mutants of a gpa3Q206L strain in which the characteristic phenotype of the gpa3Q206L allele was reverted to wild type. Two of these mutants have been complemented.

Log1 encodes a seven-transmembrane protein to which the closest homologue is a hypothetical S. pombe protein, followed by a receptor in M. musculus. Null mutants of this gene display a reduced pathogenicity in haploid solopathogenic strains as well as in gpa3Q206L mutant strains, but not when introduced in wild-type strains.

Log2 shows significant similarity to a carbamoylphosphate synthase; the phenotype of respective mutants is under investigation. In addition we have isolated a number of mutants which show the same phenotype as gpa3Q206L mutants. The affected genes were cloned by complementation. We will describe how these genes fit into a signalling pathway for pathogenic development.

References
1. Regenfelder E, Spellig T, Hartmann A et al., 1997. EMBO Journal 16, 1934-1942.