Acetylation of the Yeast Histone H4 N Terminus Regulates Its Binding to Heterochromatin Protein SIR3

Heterochromatin at yeast telomeres and silent mating ( HM) loci represses adjacent genes and is formed by the binding and spreading of s ilencing i nformation r egulators (SIR proteins) along histones. This involves the interaction between the C terminus of SIR3 and the N terminus of histone H4. Since H4 is hypoacetylated in heterochromatin we wished to determine whether acetylation is involved in regulating the contacts between SIR3 and H4. Binding of H4 peptide (residues 1–34) acetylated at lysines Lys-5, Lys-8, Lys-12, and Lys-16 to an immobilized SIR3 protein fragment (residues 510–970) was investigated using surface plasmon resonance. We find that acetylation of H4 lysines reduces binding ( K a ) of H4 to SIR3 in a cumulative manner so that the fully acetylated peptide binding is decreased ∼50-fold relative to unacetylated peptide. Thus, by affecting SIR3-H4 binding, acetylation may regulate the formation of heterochromatin. These data help explain the hypoacetylated state of histone H4 in heterochromatin of eukaryotes.