Reconstitution of mononucleosomes: characterization of distinct particles that differ in the position of the histone core.

Reconstitution of mononucleosomes from DNA and core histones was carried out to study the positioning of histone octamers on the DNA. Using random DNA molecules in the 200 to 250 bp size range we found that the reconstitution products consisted of a mixture of three different types of particles that could be separated by low ionic strength gel electrophoresis. In one particle, DNA was complexed with histones along its entire length indicating the binding of more than one histone octamer. The second particle contained only one histone core that was always associated, however, with the terminal 145 bp of the DNA regardless of its sequence which can be ascribed to a DNA end effect. Only the third particle consisted of histone octamers bound at internal positions of the DNA and is therefore the only particle suitable for investigating the influence of the DNA sequence on the positioning of the histone cores. A defined 154 bp pBR 322 restriction fragment that contains three BspRI restriction sites was also reconstituted with core histones. The accessibility of these sites to BspRI was measured in order to delineate the utility of restriction nucleases as probes for the structure of chromatin. Two sites located close to the center of the DNA were less susceptible by at least a factor of 1000 as compared to free DNA while the susceptibility of the third site in the terminal section of the DNA decreased about 50 fold after reconstitution.