The cdc25 phosphatase is essential for the G2/M phase transition in the basidiomycete yeast Ustilago maydis.

Cdc25-related phosphatases reverse the inhibitory phosphorylation of mitotic Cyclin-dependent kinases mediated by Wee1-related kinases, thereby promoting entry into mitosis. In the fission yeast, Schizosaccharomyces pombe, Cdc25 is required for entry into mitosis, while in the budding yeast Saccharomyces cerevisiae, Mih1 (the homologue of Cdc25) is not required for entry into mitosis or for viability. As these differences were linked to the different cell division and growth mechanism of these species, we sought to analyse the roles of Cdc25 in Ustilago maydis, which as S. cerevisiae divides by budding, but relies in a polar growth. This basidiomycete yeast is perfectly suited to analyse the relationships between cell cycle and morphogenesis. We show that U. maydis contains a single Cdc25-related protein, which is essential for growth. Loss of Cdc25 function results in a specific G2 arrest that correlated with high level of Tyr15 phosphorylation of Cdk1. Moreover, we show genetic interactions of cdc25 with wee1 and clb2 that support the notion that in U. maydis Cdc25 counteracts the Wee1-mediated inhibitory phosphorylation of Cdk1-Clb2 complex. Our results supports a model in which inhibitory phosphorylation of Cdk1 is a primary mechanism operating at G2/M transition in this fungus.