Regions of High and Low Cationic Charge in a Lysine-rich Histone

The distribution of amino acids in a homogeneous fraction of lysine-rich histone from rabbit thymus has been studied by use of chymotryptic digestion. The course of chymotryptic digestion was followed by measuring the generation of new carboxyl-terminal residues. The results allowed the choice of one set of conditions which produced a limited number of peptides in high yield. These conditions divided the molecule into four major portions. Under other conditions, more extensive digestion with chymotrypsin provided a set of peptides, 13 of which accounted for the entire molecule. These studies provided further evidence that the chromatographic fraction of rabbit thymus lysine-rich histone was molecularly homogeneous. Moreover the results indicate that most of the lysine and proline residues are closely packed in the carboxyl-terminal half of the molecule, whereas the amino acid composition of the amino-terminal half is not unlike small enzymes such as ribonuclease and lysozyme. The basic residues that are in the amino-terminal half are preferentially grouped towards the amino-terminal part of that half, leaving a continuous stretch of approximately 45 amino acids (23% of the molecule), in which the ratio of basic amino acids to acidic amino acids is close to one, and in which there are no proline residues.