Nα-acetylated Sir3 stabilizes the conformation of a nucleosome-binding loop in the BAH domain

Acetylation of the Sir3 N terminus is important for transcriptional silencing in budding yeast and is thought to promote binding of the Sir3 BAH domain to the nucleosome. Structural and biochemical analyses now demonstrate that the acetylated Sir3 N terminus does not interact with the nucleosome directly but instead stabilizes a nucleosome-binding loop in the BAH domain. In Saccharomyces cerevisiae , acetylation of the Sir3 N terminus is important for transcriptional silencing. This covalent modification promotes the binding of the Sir3 BAH domain to the nucleosome, but a mechanistic understanding of this phenomenon is lacking. By X-ray crystallography, we show here that the acetylated N terminus of Sir3 does not interact with the nucleosome directly. Instead, it stabilizes a nucleosome-binding loop in the BAH domain.