Identification of five sites of acetylation in alfalfa histone H4

Radioactive acetylation in vivo of plant histone H4 of alfalfa, Arabidopsis, tobacco, and carrot revealed five distinct forms of radioactive, acetylated histone. In histone H4 of eukaryotes ranging from fungi to man, acetylation is restricted to four lysines (residues 5, 8, 12, and 16) possibly caused by a quantitative methylation of lysine-20. Chemical and proteolytic fragmentation of the amino terminally blocked alfalfa H4 protein, dynamically acetylated by radioactive acetate in vivo, allowed protein sequencing and identification of selected peptides. Peptide identification was facilitated by analyzing fully characterized calf histone H4 in parallel. Acetylation in vivo of alfalfa histone H4 was restricted to the lysines in the amino-terminal domain of the protein, residues 1-23. Lysine-20 was shown to be free of methylation, as in pea histone H4. This apparently makes lysine-20 accessible as a novel target for histone acetylation. The in vivo pattern of lysine acetylation (16 greater than 12 greater than 8 greater than or equal to 5 = 20) revealed a preference for lysines -16 and -12 without an apparent strict sequential specificity of acetylation.