Inter-sigmulon communication through topological promoter coupling.

Divergent transcription from within bacterial intergenic regions frequently involves promoters dependent on alternative σ-factors. This is the case for the non-overlapping σ- and σ-dependent promoters that control production of the substrate-responsive regulator and enzymes for (methyl)phenol catabolism. Here, using an array of in vivo and in vitro assays, we identify transcription-driven supercoiling arising from the σ-promoter as the mechanism underlying inter-promoter communication that results in stimulation of the activity of the σ-promoter. The non-overlapping 'back-to-back' configuration of a powerful σ-promoter and weak σ-promoter within this system offers a previously unknown means of inter-sigmulon communication that renders the σ-promoter subservient to signals that elicit σ-dependent transcription without it possessing a cognate binding site for the σ-RNA polymerase holoenzyme. This mode of control has the potential to be a prevalent, but hitherto unappreciated, mechanism by which bacteria adjust promoter activity to gain appropriate transcriptional control.