Fig. 1. Proposed mechanism of the transcriptional regulation of the mtrCDE multidrug efflux pump genes by MtrR.

A MtrR bound to the mtrCDE operator site blocks recognition of the −35 and −10 elements by RNA polymerase thereby precluding transcription of the mtrC, mtrD, and mtrE genes, which encode the periplasmic accessory, efflux, and outer membrane channel proteins, respectively, of the MtrCDE efflux system. The dark blue arrows depict the coding regions of each gene. The transcription start site is represented by a bent arrow. Transcriptional repression is shown as a red X, cytotoxic molecules are represented as red spheres, and MtrR is represented as a blue surface. B The entirely helical structure of MtrR is shown as cylinders and the biochemical and biological functions of each helix is described. C Structure of the homodimeric MtrR bound to the mtrCDE operator site with the helices colored as in B (PDB: 7JU3)²⁵.