Calf and Pea Histone IV

Abstract

Thirteen tryptic peptides, which accounted for all of the 102 amino acid residues of calf thymus histone IV, were obtained from the maleylated protein. In addition, eight chymotryptic peptides were isolated from the unmaleylated protein. Six of the chymotryptic peptides were smaller fragments of the other two chymotryptic peptides (37 and 65 residues, respectively). All of the residues of the protein were present in the two large chymotryptic peptides as well as in the six smaller chymotryptic peptides. The sequence studies performed on these peptides, together with the sequence of the COOH-terminal cyanogen bromide peptide previously reported, established the complete amino acid sequence of calf thymus histone IV. Two unusual amino acid residues, e-N-methyllysine (Lys(Me)) and e-N-acetyllysine (Lys(Ac)), were identified and positioned in a cluster of 5 basic residues (Lys(Ac)-Arg-His-Arg-Lys(Me)) starting at residue 16 in the protein. The acetylated lysyl residue was apparently present in only half of the calf thymus histone IV fraction. Other clusters of 2 to 3 basic residues were found throughout the protein with the longest consecutive sequence, lacking a basic residue, being 11 residues (residues 80 to 90). The distribution of residues of various types in histone IV is uneven and suggests a high degree of specificity and a unique conformation. The NH2-terminal portion of the molecule is exceedingly rich in positive charges and the COOH-terminal part contains most of the aromatic, other hydrophobic, threonine residues and negative charges. This distribution suggests that the NH2-terminal part of the histone may involve the major binding sites to DNA whereas the COOH-terminal region appears to be capable of possessing a specific protein conformation.