Regulation of the Escherichia coli DNA topoisomerase I gene by DNA supercoiling.

The transcriptional control region of THE E. coli DNA topoisomerase I (topA) gene has been fused to the galactokinase (galK) gene coding region in a recombinant plasmid. In vivo synthesis of the galactokinase produced from such a plasmid has been measured and found to be reduced when mutations in the genes coding for DNA gyrase subunits are introduced into the cell or when gyrase inhibitors are present. In vitro transcription-translation of the galactokinase gene product confirms that a supercoiled DNA template is required for efficient transcription from the topA gene promoter. These results indicate that the amount of DNA topoisomerase I activity in E. coli is regulated by the extent of DNA supercoiling and can contribute to the overall modulation of DNA superhelicity and the expression of other genes.