We show that transcription modulation of a plasmid sequence in exponentially growing Escherichia coli cells leads to a rapid change in the linking number of plasmid DNA. Activation of transcription is accompanied by an increase in the plasmid's level of negative supercoiling. The added superhelical turns, whose number is proportional to the strength of the promoter and to the length of the transcript, are promptly removed when transcription is turned off. The transcription-induced increase of template supercoiling can still be detected in the presence of an inhibitor of ATP-dependent DNA gyrase [DNA topoisomerase (ATP-hydrolyzing), EC 5.99.1.3]. Altogether, our results indicate that, in addition to being under a general control, DNA superhelicity can be modulated locally in response to the topological perturbations associated with DNA tracking processes. We discuss a model in which supercoiling changes are produced by differential swiveling activities on the opposite sides of a transcriptional flow during transcriptional modulation.
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