1. Department of Molecular Genetics and Microbiology, University of New Mexico School of Medicine, 915 Camino de Salud Albuquerque, New Mexico 87131, USA

Correspondence to: Mary Ann Osley¹ Correspondence and requests for materials should be addressed to M.A.O. (Email: mosley@salud.unm.edu).

Abstract

The repair of DNA double-strand breaks (DSBs) is crucial for maintaining genome stability. Eukaryotic cells repair DSBs by both non-homologous end joining and homologous recombination. How chromatin structure is altered in response to DSBs and how such alterations influence DSB repair processes are important issues. In vertebrates, phosphorylation of the histone variant H2A.X occurs rapidly after DSB formation¹, spreads over megabase chromatin domains, and is required for stable accumulation of repair proteins at damage foci². In Saccharomyces cerevisiae, phosphorylation of the two principal H2A species is also signalled by DSB formation, which spreads 40 kb in either direction from the DSB³. Here we show that near a DSB phosphorylation of H2A is followed by loss of histones H2B and H3 and increased sensitivity of chromatin to digestion by micrococcal nuclease; however, phosphorylation of H2A and nucleosome loss occur independently. The DNA damage sensor MRX⁴ is required for histone loss, which also depends on INO80, a nucleosome remodelling complex⁵. The repair protein Rad51 (ref. 6) shows delayed recruitment to DSBs in the absence of histone loss, suggesting that MRX-dependent nucleosome remodelling regulates the accessibility of factors directly involved in DNA repair by homologous recombination. Thus, MRX may regulate two pathways of chromatin changes: nucleosome displacement for efficient recruitment of homologous recombination proteins; and phosphorylation of H2A, which modulates checkpoint responses to DNA damage².

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