Bacterial resistance to antibiotics has focused primarily on the role of acquired genetic elements contained in transposons and plasmids. In contrast to this view, many bacterial species are equipped with intrinsic mechanisms to survive exposure to a wide variety of antimicrobial compounds. This form of resistance is mediated through regulated expression of efflux pump systems, reducing enzymes, and enzymes in cellular metabolism. Modulating this response in Escherichia coli are three homologous, transcription factors: MarA, SoxS, and Rob. Together, these transcription factors serve as master regulators of the extensive mar/sox/rob regulon that has been directly implicated in multidrug resistance found in clinical and laboratory isolates. In this work, we examine the degree of genetic cross-talk between these regulatory systems and the cooperative role of these three transcriptional regulators in activating downstream targets.
The overarching goal of this work is to provide an integrated model for the mar/sox/rob regulatory network. First, the role of MarA, SoxS, and Rob in cross-regulating and auto-regulating expression from the marRAB, soxRS, and rob loci is explored. Previous evidence has suggested the potential for a fully interconnected transcriptional regulatory network between marRAB, soxRS, and rob. Using a genetic approach, the transcription-level interaction between the marRAB, soxRS, and rob systems was dissected and a more complete model is proposed. As a corollary, evidence is presented to support a model where MarA serves a conditional auto-repressor of its own expression. Similarly, genetic and biochemical evidence is presented showing the global nutritional regulator, cyclic AMP receptor protein (CRP) interacts directly with the marRAB promoter region. Second, the role of MarA and Rob in coordinately regulating the reduction of OmpF expression during drug exposure is examined. The canonical model for this event argues that up-regulated expression of MicF (a small RNA regulator of OmpF translation), mediated by MarA, SoxS, and Rob, is the causal agent of OmpF reduction. Evidence is here provided that MarA and Rob function as independent pathways for micF promoter activation. Likewise, data is presented to suggest the possibility of a MicF-independent pathway for OmpF reduction that is regulated by MarA. Additionally, the reduction in OmpF expression in tolC mutants is found to be the result of Rob-dependent activation of MicF. Finally, genetic and biochemical data is presented that demonstrates the role of aromatic metabolites in activating the mar/sox/rob regulon through direct interaction with the repressor protein, MarR. Collectively, these results provide continuing steps towards an integrated view of the mar/sox/rob regulon and cellular physiology
