The subunit structure of sea urchin sperm chromatin: A kinetic approach

Abstract

Many reports have appeared showing that digestion of chromatin DNA by endonucleases results in formation of fragments, apparently multiples of a unit length Hewish and Burgoyne [l] initially reported the formation of these fragments in experiments of autodigsstion of rat liver nuclei. The evidence for an oligomeric structure of the histones [2] led to the proposal of a repeating unit for the chromatin structure consisting of eight histones and 200 base pairs of DNA [3]. DNA fragments of about 200 base pairs in length have been obtained by adding micrococcal nuclease to rat liver nuclei [4], to mouse liver nuclei [5] and to developing trout testis nuclei [6] . In experiments with chromatin from duck reticulocyte nuclei [7] , with chromatin from pea seedlings [8] and with chromatin from yeast [9], the monomer DNA has been found to be about 130 base pairs in length. Recently it has been shown that digestion of nuclei or of native chromatin from rat liver gives similar results [lo] At variance with these results are those on trout sperm chromatin that show no fragmentation pattern upon digestion [6] . We wish to compare here the DNA fragmentation pattern, obtained by digestion of the chromatin of nuclei of sea urchin sperms with different DNases, and the time course of the digestions. The electrophoretic size distributions of the DNA fragments observed at different digestion times with micrococcal nuclease suggest that sperm chromatin is homogeneously structured as a highly regular series of histone oligomers along the DNA molecule. The DNA length, protected by each oligomer, depends on the percent DNA digested and ranges from about 160 base pairs, when about 3% DNA is