Fig. 6.

Proposed mechanism of transcriptional regulation of the srpABC operon in P. putida S12. (A) In the absence of toluene, SrpR (antirepressor) binding to SrpS (repressor) reduces the ability of SrpS to bind the operator site, permitting low-level expression from the srpS and srpA promoters. (B) In the presence of toluene, toluene binds SrpS, limiting its ability to bind the operator site. This derepression (in addition to the derepression caused by SrpR) strongly increases the transcription level from the srpS and srpA promoters. The opposing black arrows represent the promoters of srpS and srpA, with the thickness of the arrows indicating the level of transcription. The black triangle represents toluene.