Subunit Structure of Simian‐Virus‐40 Minichromosome

Abstract

Electron microscopic evidence indicates that Simian virus 40 (SV40) minichromosomes extracted from infected 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 about 1 5 x of the viral genome, irrespective of the ionic strength. Micrococcal nuclease digestion studies show that there is about 200 base pairs of DNA in the biochemical unit of SV40 chromatin. Therefore, the visible internucleosomal DNA of the SV40 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 digestion suggest that an SV40 nucleosome consists of a 'core' containing a DNA segment'of about 135 base pairs associated to a DNA fragment more susceptible to nuclease attack. The addition of histone H1 results in a striking condensation of the SV40 minichromosome, which supports the assumption that histone HI is involved in the folding of chromatin fibers.

Biochemical evidence for a DNA repeat length in the chromatin of Dictyostelium discoideum

“…155-160 base pairs and may represent the so-called nucleosome core. A core size of 160 base pairs has recently been reported for the great majority of polynucleosomes in bovine thymus and rat liver [13,16], in contrast to the previous estimation of 140 base pairs (e.g., [11 ] ). The bovine thymus cores are processed further to 140 pairs and small DNA fragments.…”

“…The larger monomer may be a precursor of the smaller monomer since longer digestion times do tend to reduce the con- Length of rat liver DNA fragments according to [ 13] (e.g., monomers 183 -+ 15).…”

“…Figure 1 shows gels of DNA fragments obtained following micrococcal nuclease digestion ofD. discoideum and Physarum [12] nuclei and rat liver chromatin [11,13]. The D. discoideum pattern differs from the other two in a number of aspects.…”

New procedure using a psoralen derivative for analysis of nucleosome associated DNA sequences in chromatin of living cells

“…Actually, quantitative analysis of the number of protected Hind III sites in minichromosomes is consistant with the prediction that randomly distributed nucleosomes are the major source of protection. Since the internucleosomal DNA of SV40 amounts to about 60/200s0.3 of the 200 base pair repeat (31), one would expect randomly distributed nucleosomes to protect about 70% of a specific Hind III site. When all non-protected Hind III sites are inactivated at conditions of saturating me3psoralen, a first order DNA fragment requiring two adjacent protected Hind III sites for its production would comprise 0.7x0.7t0.5 of the amount produced when no Hind III sites were inactivated.…”

Fine Structure of Virus Chromatin in Thin Sections of SV40-infected Cells: A Cytochemical and Autoradiographical Study

“…After staining or rotary shadow-casting, it generally consists of widely packed nucleosomes which are more or less regularly spaced (Bellard et al, 1976;Zentgraf et al, 1978). However, a few SV40 minichromosomes display a short region of their DNA which is not organized into nucleosomes (Jakobovits et al, 1980;Saragosti et al, 1980).…”