A NOVEL FUNCTION OF THE DNA-REPAIR GENE RHP6 IN MATING-TYPE SILENCINGBY CHROMATIN REMODELING IN FISSION YEAST

Recent studies have indicated that the DNA replication machinery is co upled to silencing of mating-type loci in the budding yeast Saccharomy ces cerevisiae, and a similar silencing mechanism may operate in the d istantly related yeast Schizosaccharomyces pombe. Regarding gene regul ation, an important function of DNA replication may be in coupling of faithful chromatin assembly to reestablishment of the parental states of gene expression in daughter cells. We have been interested in isola ting mutants that are defective in this hypothesized coupling. An S. p ombe mutant fortuitously isolated from a screen for temperature-sensit ive growth and silencing phenotype exhibited a novel defect in silenci ng that was dependent on the switching competence of the mating-type l oci, a property that differentiates this mutant from other silencing m utants of S. pombe as well as of S. cerevisiae. This unique mutant phe notype defined a locus which we named sng1 (for silencing not governed ). Chromatin analysis revealed a switching-dependent unfolding of the donor loci mat2P and mat3M in the sng1(-) mutant, as indicated by incr eased accessibility to the in vivo-expressed Escherichia coli dam meth ylase. Unexpectedly, cloning and sequencing identified the gene as the previously isolated DNA repair gene rhp6. RAD6, an rhp6 homolog in S. cerevisiae, is required for postreplication DNA repair and ubiquitina tion of histones H2A and H2B. This study implicates the Rad6/rhp6 prot ein in gene regulation and, more importantly, suggests that a transien t window of opportunity exists to ensure the remodeling of chromatin s tructure during chromosome replication and recombination. We propose t hat the effects of the sng1(-)/rhp6(-) mutation on silencing are indir ect consequences of changes in chromatin structure.