Topoisomerase I mutants: the gene on pBR322 that encodes resistance to tetracycline affects plasmid DNA supercoiling.

Plasmid pBR322 DNA isolated from topoisomerase I mutants of Escherichia coli and Salmonella typhimurium exhibits a distinctive supercoiling distribution characterized by an extremely heterogeneous distribution of linking numbers that contains highly negatively supercoiled topoisomers. Analysis of the supercoiling distributions of deletion and insertion derivatives of pBR322 shows that the presence of the gene on pBR322 encoding resistance to tetracycline is responsible for the unusual supercoiling distribution. Both an intact promoter and a portion of the remainder of the gene, but not the gene product, are required. However, no particular section of the gene outside the promoter appears to be necessary; only the size of the section remaining appears to be important. These observations suggest that transcription of this gene may be responsible for its effect on DNA supercoiling.