Isolation of genes involved in signal transduction in Candida albicans
The Saccharomyces cerevisiae STE4 gene encodes the beta subunit of the heterotrimeric G-protein involved in the pheromone response pathway. Two different strategies were used in attempts to isolate a STE4 homologue from Candida albicans (HST4, for Homologue of ST E4). Several C. albicans DNA libraries were screened using the S. cerevisiae STE4 gene as a probe, but sequencing of a DNA fragment hybridizing to STE4 proved not to carry a HST4 gene. Therefore, a S. cerevisiae strain (MATa ade2, ura3, leu2, trpl, ste4::TRP1, leu2::FUSI-lacZ, far1::ura3) was constructed to allow the selection of a C. albicans STE4 homologue by complementation. This S. cerevisiae strain was transformed with a C. albicans genomic DNA library. DNA sequencing of the region that restored the pheromone response pathway revealed a C. albicans gene (named HST7) with significant homology to the S. cerevisiae STE7 gene and the byr 1 gene of S. pombe. Therefore, HST7 (like STE7 and byr 1) is expected to be a member of the MAP kinase kinase family, which have also been described in higher eucaryotes and are involved in signal transducing protein kinase cascades. HST7 complements a S. cerevisiae ste7 mutant, and moreover functions like a hyperactive allele of STE7 as it elevates the basal transcription levels of reporter genes driven by the pheromone-inducible FUSI promoter and suppresses the mating defects of ste5, ste20 and ste11 mutants. However, HST7 does not suppress a ste12 deletion. HST7 did not complement the phenotype generated by mutations in PBS2 and MKK1/MKK2, two other closely related S. cerevisiae protein kinases involved in pathways controlling osmotolerance and cell wall construction, respectively.