T7 RNA polymerase cannot transcribe through a highly knotted DNA template.

The ability of T7 RNA polymerase to transcribe a plasmid DNA in vitro in its linear, supercoiled, relaxed and knotted forms was analysed. Similar levels of transcription were found on each template with the exception of plasmids showing varying degrees of knotting (obtained using stoichiometric amounts of yeast topoisomerase II). A purified fraction of knotted DNA with a high number of nodes (crosses) was found to be refractory to transcription. The unknotting of the knotted plasmids, using catalytic amounts of topoisomerase II, restored their capacity as templates for transcription to levels similar to those obtained for the other topological forms. These results demonstrate that highly knotted DNA is the only topological form of DNA that is not a template for transcription. We suggest that the regulation of transcription, which depends on the topological state of the template, might be related to the presence of knotted DNA with different number of nodes.