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.

This content is only available as a PDF.