The effects of DNA supercoiling on the expression of operons of the ilv regulon of Escherichia coli suggest a physiological rationale for divergentlytranscribed operons

Transcriptional activities of closely spaced divergent promoters are affect ed by the accumulation of local negative superhelicity in the region betwee n transcribing RNA polymerase molecules (transcriptional coupling). The eff ect of this transcription-induced DNA supercoiling on these promoters depen ds on their intrinsic properties. As the global superhelical density of the chromosome is controlled by the energy charge of the cell, which is affect ed by environmental stresses and transitions from one growth state to anoth er, the transcriptional coupling that occurs between divergently transcribe d promoters is likely to serve a physiological purpose. Here, we suggest th at transcriptional coupling between the divergent promoters of the ilvYC op eron of Escherichia coli serves to co-ordinate the expression of this opero n with other operons of the ilv regulon during metabolic adjustments associ ated with growth state transitions. As DNA supercoiling-dependent transcrip tional coupling between the promoters of other divergently transcribed oper ons is investigated, additional global gene regulatory mechanisms and physi ological roles are sure to emerge.