The mar Regulon

This chapter reviews the current understanding of the mar regulon, and focusing on some of the more interesting features of regulation. It highlights the considerable homology and cross-regulation that exists between MarA and the related transcriptional regulators SoxS and Rob. The extensive overlap observed in the regulons and phenotypes associated with these proteins is discussed in this chapter. The mar regulon consists of a large group of chromosomal genes directly or indirectly regulated by MarA. This regulon is also frequently referred to as the mar/sox/rob regulon because the two Escherichia coli MarA homologues, SoxS and Rob, recognize the same regulatory DNA element in the promoter of regulated genes. More recently, with the advent of genome-wide transcriptome analysis, two independent macroarray studies have provided new insights into the multitude of genes that constitute the transcriptional network of the regulon. Importantly, although multiple marboxes can be found in the promoter regions of mar regulon genes, in vivo studies have demonstrated that it is the one closest to the promoter signatures that plays the major role in transcriptional control. While understanding the molecular cross-talk that underlies regulon expression and how this is elicited by natural stresses remains an important goal for the future, the prevailing long-term objective is to understand the physiological relevance of the changes within the cell and to more accurately map the development of the phenotype. This will offer new possibilities for identifying targets for novel antimicrobial therapies to better deal with the growing problem of antimicrobial resistance.