Histone III

Abstract

Abstract The complete amino acid sequence of calf thymus histone III has been established by studies on the tryptic peptides from the maleylated protein and on chymotryptic and cyanogen bromide peptides from the S-carboxymethylated protein. Calf thymus histone III has 135 residues in a single polypeptide chain with NH2-terminal and COOH-terminal alanine and a molecular weight of 15,324. The NH2-terminal region (residues 1 to 53) of the protein is strongly basic, having a net positive charge of +18 and containing lysine residues 14 and 23 which are e-N-acetylated in a fraction of the molecules, as well as lysine residues 9 and 27 which are partially e-N-methylated. All three e-N-methyl derivatives, e-N-monomethyllysine, e-N-dimethyllysine, and e-N-trimethyllysine, are present at each site. This is the first time that the trimethyl derivative has been identified at specific sites in histones. The COOH-terminal region (residues 54–135) is only slightly basic (net charge of +4), has a 29-residue acidic sequence that lacks a basic residue and contains most of the hydrophobic residues of the protein. Histone III is the only histone in calf thymus which contains cysteine, both residues of which are in the nonbasic region at positions 96 and 110. There are some indications of sequence similarities between histones III and IV.