Compact form of SV40 viral minichromosome is resistant to nuclease: possible implications for chromatin structure

Abstract

We report two new findings bearing on the “supranucleosomal” level of the structure of the Simian Virus 40 mini-chromosome. I) Isolated SV40 minichromosone which contains all five histones including HI /I exists in solutionunder approximately physiological ionic conditions as a compact roughly spherical particle∼300 Å in diameter which is capable of fitting within the virus capsid. In spite of such a compact conformation of the minichromosome individual nucleosomes can be readily visualized within the particle. Compact state of SV40 minichromosome depends on both the presence of histone HI and maintenance of approximately physiologicalionic strength of solution (μ≈0.15). Removal of HI results in a conversion of the compact minichromosome into an extended (circular beaded) structure. 2) The compact form of the SV40 minichromosome in contrast to its circular beaded form is virtually completely resistant to staphylococcal nuclease, strongly suggesting that in particular nucleasesensitive, parts of the internucleosomal DNA regions are not exposed on the outside of the compact SV40 minichromosome. On the other hand, DNase I which is known to attack both inter- and intranucleosomal DNA in the chromatin /2, 3/ readily digests the compact form of the SV40 minichromosome.Possible models of the compact minichromosome and implications for higher order structures of the cellular chromatin are discussed.