The Rtt109-Vps75 histone acetyltransferase complex acetylates non-nucleosomal histone H3

Acetylation of lysine 56 of histone H3 (H3-Lys-56) occurs in S phase and disappears during G₂/M phase of the cell cycle. However, it is not clear how this modification is regulated during the progression of the cell cycle. We and others have shown that the histone acetyltransferase (HAT) Rtt109 is the primary HAT responsible for acetylating H3-Lys-56 in budding yeast. Here we show that Rtt109 forms a complex with Vps75 and that both recombinant Rtt109-Vps75 complexes and native complexes purified from yeast cells acetylate H3 present in H3/H4/H2A/H2B core histones but not other histones. In addition, both recombinant and native Rtt109-Vps75 HAT complexes exhibited no detectable activity toward nucleosomal H3, suggesting that H3-Lys-56 acetylation is at least in part regulated by the inability of Rtt109-Vps75 complexes to acetylate nucleosomal H3 during G₂/M phase of the cell cycle. Further, Rtt109 bound mutant H3/H4 tetramers composed of histones lacking their N-terminal tail domains less efficiently than wild-type H3/H4 tetramers, and Rtt109-Vps75 complexes displayed reduced HAT activity toward these mutant H3/H4 tetramers. Thus, the N termini of H3/H4 tetramers are required for efficient acetylation of H3 by the Rtt109-Vps75 complex. Taken together, these studies provide insights into how H3-Lys-56 acetylation is regulated during the cell cycle.