Biosynthesis and Enzymatic Modifications of Histones

Abstract

In higher organisms DNA is closely associated with a class of basic proteins, the histones. During the last few years it has become evident that there are only five major histones found in all nucleated cells regardless of their origin. The primary sequences of four histones, F2al, F2b, F3, and F2a2, have been determined [1–4], and the sequence of one of the subfractions of histone fraction F1 has been worked out partially [5]. A comparison of the sequence of histone F2a1 derived from two species, pea and calf, revealed that only two conservative changes in amino acid composition occurred since the divergence of the two species 1.5 billion years ago [6]. From the nearly identical peptide maps of histone F3 derived from pea and calf, a high degree of sequence conservation has also been inferred for this histone. In contrast, the rates of mutation acceptance of cytochrome C is 50-times faster, that of pancreatic ribonuclease is 500-times faster [7]. This is only to illustrate the exceptional evolutionary stability of the histones suggesting that the entire polypeptide chains are required for the function of these proteins in the chromosomes of higher organisms.