Superhelical stress and nucleosome-mediated repression of 5S RNA gene transcription in vitro.

Nucleosomes were assembled on a plasmid carrying a Xenopus somatic 5S RNA gene prepared at different superhelix densities. The gene was preferentially assembled into a positioned nucleosome which was stable to superhelical stress. No evidence for a conformational change in the nucleosome was found, even under extreme negative superhelical stress. Transcription in an extract from Xenopus oocyte nuclei was repressed to a degree which depended on the number of nucleosomes assembled. Topoisomerase activity in the extract was effectively inhibited by camptothecin, which had no effect on transcription. Transcription of reconstitutes remained repressed relative to naked plasmids, and was independent of superhelix density. Transcripts from reconstitutes were derived solely from nucleosome-free genes. Thus, a histone octamer positioned on the gene was sufficient to block its transcription. Tryptic removal of the core histone tail domains had no effect on transcription at any superhelix density. Transcription of reconstitutes containing H3/H4 tetramers was also repressed, but not eliminated (unlike reconstitutes containing octamers), and repression was independent of superhelix density. We suggest that removal of histones H2A/H2B from the nucleosome facilitates activation of transcription in the extract. We conclude that superhelical stress alone does not activate transcription of a 5S RNA gene assembled into a nucleosome in vitro.