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Molecular and Cellular Biology, September 1998, p. 5511-5522, Vol. 18, No. 9
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

A Novel Function of the DNA Repair Gene rhp6 in Mating-Type Silencing by Chromatin Remodeling in Fission Yeast

Jagmohan Singh,1,* Vintoo Goel,1 and Amar J. S. Klar2

Institute of Microbial Technology, Sector 39 A, Chandigarh 160 036, Punjab, India,1 and Developmental Genetics Section, Gene Regulation and Chromosome Biology Laboratory, ABL-Basic Research Program, NCI-Frederick Cancer Research and Development Center, Frederick, Maryland 21702-12012

Received 11 February 1998/Returned for modification 31 March 1998/Accepted 15 June 1998

Recent studies have indicated that the DNA replication machinery is coupled to silencing of mating-type loci in the budding yeast Saccharomyces cerevisiae, and a similar silencing mechanism may operate in the distantly related yeast Schizosaccharomyces pombe. Regarding gene regulation, 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 isolating mutants that are defective in this hypothesized coupling. An S. pombe mutant fortuitously isolated from a screen for temperature-sensitive growth and silencing phenotype exhibited a novel defect in silencing that was dependent on the switching competence of the mating-type loci, a property that differentiates this mutant from other silencing mutants of S. pombe as well as of S. cerevisiae. This unique mutant phenotype 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 increased accessibility to the in vivo-expressed Escherichia coli dam methylase. 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 ubiquitination of histones H2A and H2B. This study implicates the Rad6/rhp6 protein in gene regulation and, more importantly, suggests that a transient window of opportunity exists to ensure the remodeling of chromatin structure during chromosome replication and recombination. We propose that the effects of the sng1-/rhp6- mutation on silencing are indirect consequences of changes in chromatin structure.

* Corresponding author. Mailing address for Jagmohan Singh: Institute of Microbial Technology, Sector 39 A, Chandigarh 160 036, Punjab, India. Phone: 91-172-690908, ext. 443. Fax: 91-172-690585 or 91-172-690632. E-mail: Jag{at}koel.imtech.ernet.in. Mailing address for Amar J. S. Klar: NCI-Frederick Cancer Research and Development Center, ABL-Basic Research Program, Gene Regulation and Chromosome Biology Laboratory, Developmental Genetics Section, P.O. Box B, Frederick, MD 21702-1201. Phone: (301) 846-5149. Fax: (301) 846-6911. E-mail: klar{at}ncifcrf.gov.

Molecular and Cellular Biology, September 1998, p. 5511-5522, Vol. 18, No. 9
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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