Chemical Studies of Histone Acetylation

Histone fractions, f1, f2a1, f2a2, f2b, and f3, were prepared from calf thymus nuclei which had been incubated in the presence of sodium acetate-1-14C. Each of the histone fractions was digested with trypsin and with Pronase, and the resulting peptides and amino acids were separated by ion exchange and exclusion chromatography. Only the arginine-rich fractions, f2a1 and f3, were found to be appreciably labeled with 14C-acetate. All of their radioactivity could be recovered as a single chromatographic peak which was identified as e-N-acetyllysine. The comparatively lysine-rich fractions, f1 and f2b, were unlabeled and did not contain e-N-acetyllysine. The moderately lysine-rich f2a2 fraction contained small amounts of radioactive acetate and of acetyllysine, a result which may indicate either very little acetylation of this fraction or contamination of the f2a2 by small amounts of the more acetylated histones.