DNA ligase activity in chromatin and its analogs. Rejoining of DNA strands in polylysine-DNA complexes and in reconstituted chromatins.

A highly purified DNA ligase from rat liver nuclei has been tested on DNA containing single-strand breaks ("nicks"); the DNA was present in several types of complexes which were chosen to serve as models for chromatin. These model systems included complexes of polylysine or histones with DNA as well as reconstituted chromatin preparations. In all these cases, the limit of ligase sealing was measured as a function of the ratio of polypeptide or protein to DNA. With an excess of either polylysine or histones, the ligase is totally prevented from sealing nicks in the DNA. However, at ratios of histones to DNA similar to those occurring in chromatin, about half of the nicks are accessible to the ligase. In the reconstitution of chromatin, the proteins are dissociated from the DNA by exposure to high ionic strength either with or without urea. If such procedures are carried out in the presence of labeled nicked DNA, the proteins will redistribute over this ligase substrate as well. When the chromatin is reconstituted at protein/DNA ratios similar to those occurring in chromatin, once more only about half of the nicks are accessible to the ligase. Similar results were obtained with preparations reconstituted with either rat liver or duck reticulocyte chromatin. The rate of ligase action has been measured on a variety of the complexes. While the rate falls as the DNA is increasingly covered with polylysine or histones, this is largely or entirely due to the decrease in concentration of sealable sites. At saturating concentrations of these DNA complexes, the original rate on uncovered DNA is approached.