DNA supercoiling-dependent transcriptional coupling between the divergently transcribed promoters of the ilvYC operon of Escherichia coli is proportional to promoter strengths and transcript lengths

The twin-domain model of Liu and Wang suggested that high levels of DNA sup ercoiling generated in the region between closely spaced divergently transc ribed promoters could serve to couple the activities of these promoters tra nscriptionally. In this report, we use topoisomer sets of defined superheli cal densities as DNA templates in a purified in vitro transcription system to demonstrate transcriptional coupling between the divergently transcribed ilvY and ilvC promoters of the ilvYC operon of Escherichia coli. Current e vidence for this type of DNA supercoiling-dependent transcriptional couplin g, based largely on the in vivo activities of promoters contained in engine ered DNA constructs, suggests that the transcription complex must be physic ally hindered to generate DNA supercoils and to prevent their diffusion thr oughout the DNA duplex. However, the in vitro results presented here demons trate that (i) transcriptional coupling is observed between the divergent p romoters of the ilvYC operon in the absence of transcript anchoring; (ii) t he magnitude of the negative DNA supercoiling generated in the divergent pr omoter region is proportional to the sum of the global and transcription-in duced superhelicity; and (iii) the magnitude of this transcription-induced superhelicity is proportional to promoter strengths and transcript lengths.