Determinants of Sir2-Mediated, Silent Chromatin Cohesion

Abstract

Cohesin associates with distinct sites on chromosomes to mediate sister chromatid cohesion. Single cohesin complexes are thought to bind by encircling both sister chromatids in a topological embrace. Transcriptionally repressed chromosomal domains in the yeast Saccharomyces cerevisiae represent specialized sites of cohesion where cohesin binds silent chromatin in a Sir2-dependent fashion. In this study, we investigated the molecular basis for Sir2-mediated cohesion. We identified a cluster of charged surface residues of Sir2, collectively termed the EKDK motif, that are required for cohesin function. In addition, we demonstrated that Esc8, a Sir2-interacting factor, is also required for silent chromatin cohesion. Esc8 was previously shown to associate with Isw1, the enzymatic core of ISW1 chromatin remodelers, to form a variant of the ISW1a chromatin remodeling complex. When ESC8 was deleted or the EKDK motif was mutated, cohesin binding at silenced chromatin domains persisted but cohesion of the domains was abolished. The data are not consistent with cohesin embracing both sister chromatids within silent chromatin domains. Transcriptional silencing remains largely intact in strains lacking ESC8 or bearing EKDK mutations, indicating that silencing and cohesion are separable functions of Sir2 and silent chromatin.

Supplemental material for this article may be found at http://dx.doi.org/10.1128/MCB.00057-16.

ACKNOWLEDGMENTS

We thank Rolf Sternglanz, Jasper Rine, and David Shore for sharing yeast strains and plasmids, Mike Hampsey and Steven Zheng for sharing supplies and equipment, and Tom Ellenberger for sharing unpublished data. We thank John Everett for help with the figures.

M.R.G. is member of The Rutgers Cancer Institute of New Jersey, New Brunswick, NJ.

Additional information

Funding

This work was funded by HHS | NIH | National Institute of General Medical Sciences (NIGMS) (51402).