Site-specific loss of acetylation upon phosphorylation of histone H3

Post-translational modification of histones is a central aspect of gene regulation. Emerging data indicate that modification at one site can influence modification of a second site. As one example, histone H3 phosphorylation at serine 10 (Ser¹⁰) facilitates acetylation of lysine 14 (Lys ¹⁴) by Gcn5 in vitro (1, 2). In vivo, phosphorylation of H3 precedes acetylation at certain promoters. Whether H3 phosphorylation globally affects acetylation, or whether it affects all acetylation sites in H3 equally, is not known. We have taken a genetic approach to this question by mutating Ser ¹⁰ in H3 to fix either a negative or a neutral charge at this position, followed by analysis of the acetylation states of the mutant histones using site-specific antibodies. Surprisingly, we find that conversion of Ser¹⁰ to glutamate (S10E) or aspartate (S10D) causes almost complete loss of H3 acetylation at lysine 9 (Lys⁹) in vivo. Acetylation of Lys⁹ is also significantly reduced in cells bearing mutations in the Glc7 phosphatase that increase H3 phosphorylation levels. Mutation of Ser ¹⁰ in H3 and the concomitant loss of Lys⁹ acetylation has minimal effects on expression of a Gcn5-dependent reporter gene. However, synergistic growth defects are observed upon loss of GCN5 in cells bearing H3 Ser¹⁰ mutations that are reminiscent of delays in G₂/M progression caused by combined loss of GCN5 and acetylation site mutations. Together these results demonstrate that H3 phosphorylation directly causes site-specific and opposite changes in acetylation levels of two residues within this histone, Lys⁹ and Lys¹⁴, and they highlight the importance of these histone modifications to normal cell functions.