Were male X chromosomes to function at the same level of transcriptional activation as females, males would have only half the level of X chromosome coded gene products as females. Histone H4 plays a role in dosage compensation. The specifically acetylated isoform of histone H4, H4Ac16, is detected predominantly on the male X chromosome. No H4HAc16 is found on X chromosomes in mutants of MSL genes. Control of gene accessability to transcription factors is not the only role of acetylation of H4 in the biology of the cell.
Acetylation is involved in the process of histone assembly into nucleosomes.
Hat B has been characterized from yeast, and it appears in the cytoplasm as a dimer consisting of two subunits, Hat1p and Hat2p. Hat1p is the histone transacetylase, while Hat2p is a member of an evolutionarily conserved family of p48 proteins. Members of the p48 family are histone escorts, accompanying newly synthesized histones from cytoplasm to nucleus. A second histone transacetylase activity is found in the nucleus of yeast. GCN5p, a yeast protein involved in transcriptional activation, is homologous to tetrahymena HAT A, a nuclear histone acetyltransferase.