Cell Cycle-Dependent Regulation of Saccharomyces cerevisiae Donor Preference during Mating-Type Switching by SBF (Swi4/Swi6) and Fkh1

Abstract

Saccharomyces mating-type switching occurs through a double-strand break-initiated gene conversion event at MAT, using one of two donors located distantly on the same chromosome, HMLα and HMRa. MATa cells preferentially choose HMLα, a decision that depends on the recombination enhancer (RE) that controls recombination along the left arm of chromosome III. We previously showed that an fhk1Δ mutation reduces HMLα usage in MATa cells, but not to the level seen when RE is deleted. We now report that donor preference also depends on binding of the Swi4/Swi6 (SBF) transcription factors to an evolutionarily conserved SCB site within RE. As at other SCB-containing promoters, SBF binds to RE in the G₁ phase. Surprisingly, Fkh1 binds to RE only in G₂, which contrasts with its cell cycle-independent binding to its other target promoters. SBF and Fkh1 define two independent RE activation pathways, as deletion of both Fkh1 and SCB results in nearly complete loss of HML usage in MATa cells. These transcription factors create an epigenetic modification of RE in a fashion that apparently does not involve transcription. In addition, the putative helicase Chl1, previously involved in donor preference, functions in the SBF pathway.

We are grateful to Brenda Andrews, Kim Nasmyth, Guy-Franck Richard, and Bik Tye for strains, plasmids, and antibodies. We also thank Xuan Wang for technical advice and Miyuki Yamaguchi for technical help.

E.C. was supported in part by grants from l'Association pour la Recherche sur le Cancer and the Philippe Foundation. Research was supported by NIH grant GM20056.