[Various novel properties of transcriptionally active mini-chromosomes of the SV40 virus].

The treatment of isolated SV40 mini-chromosomes by DNA-topoisomerase I leads to relaxation of DNA within a small fraction (2-5%) of mini-chromosomes strongly enriched in endogenous RNA-polymerase. The DNA supercoiling in the bulk of mini-chromosomes remained unchanged. The relaxable fraction proved to be specifically hypersensitive to DNAase I, but lost hypersensitivity after prior topoisomerase treatment. The DNA relaxation induced either by topoisomerase or DNAase I nicking and breaking led to almost a complete loss of proteins from this fraction while the DNA-protein interactions in the bulk of mini-chromosomes remained unchanged. Endogenous RNA-polymerase remained specifically enriched in these uncoated mini-chromosomes. It is concluded that (1) there is an elastic torsional strain in DNA within transcriptionally active mini-chromosomes, (2) DNA-protein interactions are altered within transcriptionally active mini-chromosomes, (3) there is evidence to indicate that local DNA conformational transitions in transcriptionally active chromatin are caused by DNA torsional strain.