Regulation of the Hypha-Inducing Transcription Factor Ume6 by the CDK1 Cyclins Cln3 and Hgc1.

The ability to switch between proliferation as yeast cells and development into hyphae is a hallmark of . The switch to hyphal morphogenesis depends on external inducing conditions, but its efficiency is augmented in stationary-phase cells. Ume6, a transcription factor that is itself transcriptionally induced under hypha-promoting conditions, is both necessary and sufficient for hyphal morphogenesis. We found that Ume6 is regulated posttranslationally by the cell cycle kinase Cdc28/Cdk1, which reduces Ume6 activity via different mechanisms using different cyclins. Together with the cyclin Hgc1, Cdk1 promotes degradation of Ume6 via the SCF ubiquitin ligase. Since is a key transcriptional target of Ume6, this results in a negative-feedback loop between Hgc1 and Ume6. In addition, we found that Cln3, a G cyclin that is essential for cell cycle progression and yeast proliferation, suppresses hyphal morphogenesis and that Cln3 suppresses Ume6 activity both in the heterologous system and in itself. This activity of Cln3 may provide the basis for the antagonistic relationship between yeast proliferation and hyphal development in . The yeast to hypha (mold) morphogenetic switch of plays a role in its virulence and constitutes a diagnostic trait for this organism, the most prevalent systemic fungal pathogen in industrialized countries. It has long been known that hyphae are most efficiently induced from stationary cultures. Here, a molecular basis for this observation is provided. The G cyclin Cln3, an essential promoter of yeast proliferation, was found to suppress hyphal induction. Suppression of hyphal induction is achieved by inhibition of the activity of the central activator of hyphal morphogenesis, the transcription factor Ume6. Thus, levels of Cln3 control the switch between proliferation of as individual yeast cells and development into extended hyphae, a switch that may preface the proliferation/differentiation switch in multicellular organisms.