SUBUNIT STRUCTURE OF SIMIAN-VIRUS-40 MINICHROMOSOME

Electron microscopic evidence indicates that SV-40 minichromosomes extracted from infected [African green monkey kidney CV-1] cells consist of 20 .+-. 2 nucleosomes, each containing 190-200 base pairs of DNA. About 50% of the nucleosomes are not close together, but connected by segments of DNA of irregular lengths which correspond to .apprx. 15% of the viral genome, irrespective of the ionic strength. Micrococcal nuclease digestion studies show that there is .apprx. 200 base pairs of DNA in the biochemical unit of SV-40 chromatin. Thus, the visible internucleosomal DNA of the SV-40 minichromosome does not arise from an unfolding of a fraction of the 190-200 base pairs of DNA initially wound in the nucleosome. These results support the chromatin model which proposes that the same DNA length is contained in the nucleosome and the biochemical unit. Results from extensive micrococcal nuclease [EC 3.1.4.7] digestion suggest that an SV-40 nucleosome consists of a core containing a DNA segment of .apprx. 135 base pairs associated to a DNA fragment more susceptible to nuclease attack. The addition of histone H1 [from calf thymus nuclei] results in a striking condensation of the SV-40 minichromosome, which indicates that histone H1 is involved in the folding of chromatin fibers.