A Structural Comparison of Different Lysine-rich Histones of Calf Thymus

Abstract

A structural study was made of the four lysine-rich histones isolated previously. Two of these histones (Fraction I and Fraction II) were studied individually, but the other two were studied as a mixture (Fraction III). Amino acid analysis established only a few conclusive differences among these fractions. Fraction I had 4 moles of arginine per mole of histone (mol wt 21,000), Fraction II had 3, and Fraction III had 3.5. Fractions I and II had 10 moles of valine per mole of histone, while Fraction III had 8. Peptide fractionation by ion exchange chromatography and paper electrophoresis resolved 50 to 55 components in the tryptic digest of each histone fraction. There was a coincidence of 45 of these components when the three fractions were compared, suggesting that the four histones in these fractions had very closely related primary structures. All these fractions appeared to have the same amino acid sequence around the tyrosine residue and around the phenylalanine residue found in each fraction. All seemed to have identical sequences around 3 arginine residues, but Fraction II and one of the histones in Fraction III apparently lacked a fourth arginine-containing sequence. Although much of the primary structure of these histones is common to all of them, a number of differences were established. There was one peptide unique to Fraction I, four unique to Fraction II, and seven unique to Fraction III. One peptide found in Fractions II and III was missing from Fraction I; two peptides found in Fractions I and III were missing from Fraction II; two peptides found in Fractions I and II were missing from Fraction III. It was concluded that these four lysine-rich histones have closely related primary structures, but exist endogenously as distinct molecular species.