Transcriptional coupling between the divergent promoters of a prototypic LysR-type regulatory system, the ilvYC operon of Escherichia coli

The twin-domain model [Liu, L. F. & Wang, J. C. (1987) Proc. Natl. Acad. Sr i. USA 84, 7024-7027] suggests that closely spaced, divergent, superhelical ly sensitive promoters can affect the transcriptional activity of one anoth er by transcriptionally induced negative DNA supercoiling generated in the divergent promoter region, This gene arrangement is observed for many LysR- type-regulated operons in bacteria. We have examined the effects of diverge nt transcription in the prototypic LysR-type system, the ilvYC operon of Es cherichia coil. Double-reporter constructs with the lacZ gene under transcr iptional control of the ilvC promoter and the galK gene under central of th e divergent ilvY promoter were used to demonstrate that a down-promoter mut ation in the ilvY promoter severely decreases in vivo transcription from th e ilvC promoter. However, a down-promoter mutation in the ilvC promoter onl y slightly affects transcription from the ilvY promoter. In vitro transcrip tion assays with DNA topoisomers skewed that transcription from the ilvC pr omoter increases over the entire range of physiological superhelical densit ies, whereas transcription initiation from the ilvY promoter exhibits a bro ad optimum at a midphysiological superhelical density. Evidence that this p romoter coupling is DNA supercoiling-dependent is provided by the observati on that a novobiocin-induced decrease in global negative superhelicity resu lts in an increase in ilvY promoter activity and a decrease in ilvC promote r activity predicted by the in vitro data. We suggest that this transcripti onal coupling is important for coordinating basal level expression of the i lvYC operon with the nutritional and environmental conditions of cell growt h.