The function of the histone H3 N terminal tail in the formation of heterochromatin in the budding yeast, Saccharomyces cerevisiae

In budding yeast, telomeres and the mating type (HM) loci are found in a heterochromatin-like silent structure initiated by Rap1 and extended by the interaction of Sir (Silencing Information Regulator) proteins with histones. Binding data demonstrate that both the H3 and H4 N terminal domains required for silencing in vivo interact directly with Sir3 and Sir4 in vitro. The role of H4 lysine 16 deacetylation is well established in Sir3 protein recruitment, however that of the H3 N terminal tail has remained unclear. In order to characterize the role of H3 in silent chromatin formation and compare it to H4 we have generated comprehensive high resolution genome-wide binding maps of heterochromatin proteins. We find that H4 lysine 16 deacetylation is required for the recruitment and spreading of heterochromatin proteins at all telomeres and HM loci. In contrast the H3 N terminus is required for neither recruitment nor spreading of Sir proteins. Instead, deletion of the H3 tail leads to increased accessibility within heterochromatin of an ectopic bacterial dam methylase and the decreased mobility of an HML heterochromatic fragment in sucrose gradients. These findings indicate a less compact chromatin structure. We propose that Sir proteins recruited by the H4 tail are then condensed by the interaction of the H3 tail with Sir3 and Sir4 to form compact silent heterochromatin.