The arrangement of proteins on the deoxyribonucleic acid in chromatin

Abstract

The arrangement of the protein component on the DNA of the chromatin complex was studied by comparing the rate of release of oligonucleotides and of protein after addition of deoxyribonuclease I and deoxyribonuclease II to rat thymus chromatin. Also the action of deoxyribonuclease I on normal chromatin and on chromatin depleted of non-histone protein was compared, to elucidate the role of the latter protein in chromatin structure. As a preliminary to the above, the rate of action of deoxyribonuclease I on DNA and on chromatin at the same DNA concentration, and the dependence of the action of this enzyme on the Mg2+ concentration, were studied. It was found that: (1) little if any DNA in chromatin is present in extensive, truly ‘free’ zones, i.e. completely uncovered by protein; (2) at relatively low concentrations of added Mg2+, deoxyribonuclease I degrades chromatin more rapidly than DNA; (3) the non-histone protein is not attached directly to the DNA in chromatin.