Nucleosomes inhibit both transcriptional initiation and elongation by RNA polymerase III in vitro.

To examine the effect of nucleosomes on in vitro transcription, purified chicken erythrocyte core histones and plasmid DNA bearing the Xenopus 5S RNA gene were assembled into nucleosomes and used as templates for transcription in a Xenopus oocyte nuclear extract. Plasmids having a nucleosome incorporating a specific region of the gene were selected by treating the reconstituted molecules with restriction endonucleases. In this way, it was shown that a nucleosome on or close to the internal control region of the 5S RNA gene inhibits transcription. Furthermore, experiments with 5S maxigenes showed that RNA polymerase III, in contrast to SP6 RNA polymerase, will not transcribe through a nucleosome in vitro.