Protein content of chromatin fractions separated by sucrose gradient centrifugation.

When sheared chromatin is centrifuged in a steep sucrose gradient, two broad peaks are resolved. DNA extracted from both fractions has approximately the same molecular weight. The basis for this fractionation seems to be differential aggregation. The slowly sedimenting material shows a lower protein/DNA ratio than the rapidly sedimenting chromatin as judged by equilbrium density centrifugation in CsCl after formaldehyde fixation or under nonionic conditions. After selective removal of histone fland further shear, most of the slowly sedimenting chromatin material appears as free DNA in steep cesium chloride gradients. The data are consistent with several recent reports concerning the subunit structure of chromatin.