Characterization of α-factor pheromone and pheromone receptor genes of Ashbya gossypii

The genome of Ashbya gossypii contains homologs of most of the genes that are part of the Saccharomyces cerevisiae pheromone-signal transduction cascade. However, we currently lack understanding of a potential sexual cycle for this pre-whole genome duplication hemiascomycete. The sequenced strain bears three identical copies encoding MATa. We show that the syntenic A. gossypii homolog of MF alpha 1 (AFL062w) does not encode a mature alpha-factor peptide, but identified another gene, AAR163c, which encodes a candidate alpha-specific mating pheromone and is thus reannotated as AgMF alpha 2. The expression of the AgSTE2 alpha-factor receptor in an Scste2 S. cerevisiae MATa strain resulted in dosage-dependent growth arrest upon exposure to A. gossypii alpha-factor, which indicated that the pheromone response was effectively coupled to the S. cerevisiae signal transduction cascade. Comparison of alpha-pheromones and alpha-pheromone receptors showed greater conservation between Eremothecium cymbalariae and S. cerevisiae than between A. gossypii and E. cymbalariae. We constructed A. gossypii strains deleted for the STE2 and STE3 pheromone receptors. These strains showed no phenotypic abnormalities and an ste2, ste3 double mutant is still able to sporulate. The deletion of STE12 as the downstream target of pheromone signalling, however, led to a hypersporulation phenotype.