Mobility of histones on the chromosome of simian virus 40.

Linear simian virus 40 (SV40) chromosomes were prepared by Eco R1 nuclease cleavage of the circular SV40 chromosomes released from virions with dithiothreitol at pH 9,8. Chromatin-DNA hybrids were constructed with segments of 3H-labeled, naked SV40 DNA covalently joined via the Eco R1-generated cohesive ends to segments of linear SV40 chromosome. Upon incubation of chromatin-DNA hybrids at 37 degrees C and moderate ionic strength, histones migrated onto the labeled DNA while retaining the nucleosome structure. This was shown first, by the pattern of micrococcal nuclease digestion of labeled DNA; second by nitrocellulose filter binding of labeled DNA after redigestion of the chromatin-DNA hybrids with Eco R1; and third, by examination of chromatin-DNA hybrids in the electron microscope. Migration was slow, being apparent after several hours. Parallel experiments in which naked DNA and chromosomes were mixed without joining showed no transfer of nucleosomal histones between DNA molecules. The kinetics of Eco R1 cleavage of the DNA in virion-derived SV40 chromosomes are also consistent with the notion that nucleosomal histones, in the absence of other proteins, can move on DNA.