In vitro evidence that transcription-induced stress causes nucleosome dissolution and regeneration

During transcription, positive and negative superhelical stresses are generated on a DNA template which could potentially affect nucleosomal structure. When transcription was performed on a closed circular plasmid containing nucleosomes, using T7 RNA polymerase and topoisomerase I, nucleosomal structure was lost from the DNA. Nucleosome content was assayed by analyzing both the topological state of the DNA and the nuclease-resistant fragments produced by micrococcal nuclease and DNase I treatment. This nucleosome dissolution required positive superhelical stress as evidenced by the requirement that the extended RNA transcript remain associated with the polymerase during the transcription process. Rates of transcription were found to be independent of whether the nucleosomes dissolved. When transcription was performed in the absence of topoisomerase I, nucleosome reformation occurred very rapidly. This observation suggests that negative superhelical stress, induced in the wake of polymerase action, facilitates nucleosome reformation.