To gain better understanding of the diversity and evolution of the gene regulation system in eukaryotes, the phosphate signal transduction (PHO) pathway in non-conventional yeasts has been studied in recent years. Here we characterized the PHO pathway of Hansenula polymorpha, which is genetically tractable and distantly related to Saccharomyces cerevisiae and Schizosaccharomyces pombe, in order to get more information for the diversity and evolution of the PHO pathway in yeasts. We generated several pho gene-deficient mutants based on the annotated draft genome of H. polymorpha BY4329. Except for the Hppho2-deficient mutant, these mutants exhibited the same phenotype of repressible acid phosphatase (APase) production as their S. cerevisiae counterparts. Subsequently, Hppho80 and Hppho85 mutants were isolated as suppressors of the Hppho81 mutation and Hppho4 was isolated from Hppho80 and Hppho85 mutants as the sole suppressor of the Hppho80 and Hppho85 mutations. To gain more complete delineation of the PHO pathway in H. polymorpha, we screened for UV-irradiated mutants that expressed APase constitutively. As a result, three classes of recessive constitutive mutations and one dominant constitutive mutation were isolated. Genetic analysis showed that one group of recessive constitutive mutations was allelic to HpPHO80 and that the dominant mutation occurred in the HpPHO81 gene. Epistasis analysis between Hppho81 and the other two classes of recessive constitutive mutations suggested that the corresponding new genes, named PHO51 and PHO53, function upstream of HpPHO81 in the PHO pathway. Taking these findings together, we conclude that the main components of the PHO pathway identified in S. cerevisiae are conserved in the methylotrophic yeast H. polymorpha, even though these organisms separated from each other before duplication of the whole genome. This finding is useful information for the study of evolution of the PHO regulatory system in yeasts.
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