DNAaseI-hypersensitive minichromosomes of SV40 possess an elastic torsional strain in DNA

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Abstract
Previously, we have shown that DNA in a small fraction (2–5%) of SV40 minichromosomes was torsionally strained and could be relaxed by treating minichromosomes with topoisomerase I. This fraction was enriched with endogoneous RNA polymerase II (Luchnik et al 1982, EMBO J., 1, 1353). Here we show that one and the same fraction of SV4O 1 minichromosomes is hypersensitive to DNAase I and is relaxable by topoisomerase I. Moreover, this fraction completely loses its hypersensitivity to 1 DNAase I upon relaxation. The possibility that this fraction of minichromosomes can be represented by naked DNA is ruled out by the results of studying the kinetics of minichromosome digestion by DNAase I in comparison to digestion of pure SV40 DNA and by measuring the buoyant density of SV40 chromatin in equilibrium CaCl gradient. Our data obtained with SV4O minichromosomes may be relevant to the mechanism responsible for DNAase I hypersensitivity in the loops or domains of cellular chromatin.