Decisions, Decisions: Donor Preference during Budding Yeast Mating-Type Switching

Abstract

Mating-type (MAT) switching in the budding yeast Saccharomyces cerevisiae has been particularly well studied, both in terms of the homologous recombination mechanism that leads to the switch itself and the remarkable donor preference system that ensures efficient exchange of mating types. For example, mutations that prevent acetylation of four lysines in histone H4 lead to the complete unsilencing of HMLa but have much less effect on the silencing of HMRa. MAT switching occurs by a mechanism of DSB repair known as synthesis-dependent strand annealing (SDSA). There are a number of distinct, slow steps in MAT switching that have been identified by a combination of Southern blot and PCR analyses, as well as by chromatin immunoprecipitation techniques, that monitor the recruitment of recombination proteins to the sequences undergoing recombination/repair. Analysis of various temperature-sensitive mutations in essential DNA replication genes has shown that the short patch of new DNA synthesis in MAT switching does not require the Mcm helicase proteins or Pol-primase or the Cdc7 kinase. The binding of both Fkh1 and Swi4/6 to recombination enhancer (RE) has been evaluated by chromatin immunoprecipitation at different times in the cell cycle. RE represents one of the most fascinating and best-defined cis-acting loci that control aspects of chromosome positioning, folding, or tethering. The availability of new cytological resources as well as a wide variety of molecular and genetic tools opens the way to a detailed understanding of the ways chromosomes are arranged and constrained in the nucleus.