CHROMATIN REMODELING: REGULATION OF CHROMATIN HIGHER‐ORDER FOLDING AND DNA REPAIR

Dynamic changes in chromosome structure are catalyzed by ATP‐dependent chromatin remodeling enzymes and by histone modifying enzymes, such as histone acetyltransferases. We will present two stories that illustrate distinct roles for both types of chromatin‐based enzymes. First, we will discuss our recent work on Ino80 and Swr1 which are ATP‐dependent chromatin remodeling enzymes that have been implicated in DNA repair. We have found that Ino80 is required for maintanence of high levels of histone H2AX phosphorylation and for cell cycle checkpoint adaptation in response to a persistent DNA double strand break (DSB). Our data indicate that Ino80 and Swr1 function antagonistically at chromatin surrounding a DSB, and that they regulate the incorporation of different histone H2A variants that can either promote or block cell cycle checkpoint adaptation. The second story will focus on histone H4 lysine 16 acetylation which is a prevalent and reversible post‐translational modification in eukaryotic organisms. We have used a native chemical ligation strategy to generate histone H4 homogeneously acetylated at K16. When this histone is incorporated into nucleosomal arrays, we find that acetylation of this single lysine residue inhibits the formation of compact 30 nm‐like fibers and impedes the ability of chromatin to form cross‐fiber interactions. This is the first example of a single histone modification that is sufficient to modulate higher‐order chromatin structure.