| Acetylation of histone within the nucleoprotein complex known as chromatin is one method to regulate access to the eukaryotic genome by factors involved in DNA metabolism. Acetylation is the function of proteins known as Histone AcetyItransferases (HATs). These proteins occur in large multi-subunit complexes, however the mechanism of acetyl-transfer nucleosome recognition, and the effects of the subunits on acetyl-transfer are unknown.;Kinetic analysis of one HAT complex, piccolo NuA4, found no evidence for an acetyl-cysteine in the acetyl-transfer as was previously proposed. Additional analysis would show that like the GNAT family of HATs, piccolo NuA4 uses a sequential, direct attack mechanism of acetyl-transfer. Much of the literature on nucleosome recognition by HAT complexes is based on studies using short histone tail peptides. Whether these peptide studies accurately represent recognition of chromatin is unclear. Nucleosome recognition by Esa1 was enhanced by interactions with the Yng2 and Epl1 subunits of the piccolo NuA4 complex. In addition, Esa1 was found to contact the histone-fold domain of histone H4, enhancing the efficiency of acetylation over three orders of magnitude. The interaction of Esa1 with Epl1, Yng2, and the histone fold domain acts synergistically to increase the efficiency of nucleosome acetylation nearly six orders of magnitude (106) to near diffusion limited rates of acetyl-transfer.;Acetylation of histones is not limited to the unstructured tail regions. Acetylation of H3-K56, which occurs on nascent histone during replication, transcription and DNA damage repair, has been found in many eukaryotic organisms from yeast to humans. In S. cerevisiae, the Rtt109·•histone chaperone complexes are responsible for H3-K56 acetylation. Through a mechanism of reciprocal stimulation, Rtt109 and the associated chaperone stimulate activity of the other subunit, coupling histone acetylation to histone deposition in a single complex.;Thus, the non-catalytic subunits of HAT complexes serve to enhance catalysis and thereby the rate of acetylation by HATs allowing for more efficient acetylation of complex substrates such as the nucleosome. |
