The mar regulon: multiple resistance to antibiotics and other toxic ...

R EVIEWScounterpart in Salmonella typhimurium 21 ,marC homologs are also found inMethanococcus jannaschii, Aquifex aeolicus,Pyrococcus horikoshii, Archaeoglobusfulgidus and Treponema pallidum, and allhave unknown functions. The only knowngene homologous to E. coli marB is thatfrom S. typhimurium 21 .(a)Pmar I Pmar II marOmarC marR marA marBMarR and MarA: novel transcriptionregulatorsMarR homologs are widespread amongmany different bacterial genera and someare believed to interact with differentphenolic compounds 22 . For example, the invitro activity of CinR, a MarR homologfrom Butyrivibrio fibrisolvens E14 thatnegatively regulates the expression of anenzyme involved in releasing cinnamicacids (salicylate precursors in plants) duringplant cell wall catabolism, is affected bycertain products of this metabolic pathway23 . Moreover, recent experiments havedemonstrated that many of the chemicalsthat induce mar expression in vivo antagonizethe repressor activity of MarR invitro 16 . Thus, it appears that both MarRand CinR have DNA-binding and inducerrecognitionproperties.Data pertaining to the functional regionsof MarR have begun to emerge. Pointmutations at particular residues result in a‘super-repressor’ protein that has increasedDNA-binding affinity 24 . Whether these mutationsaffect specific or non-specific DNA–protein interactions is currently unknown.Until recently, there was no biophysicalcharacterization of the DNA-binding domain(s)of any of the XylS/AraC familymembers, as these proteins are severely unstablein solution 25 . However, Rhee andcoworkers recently solved the 3-D structureof a MarA–DNA co-crystal, providing the firstdirect proof for the existence of two helix-turn-helix(HTH) DNA-binding motifs 26 . Both HTH motifs inthis structure appear to participate in DNA bindingand are organized in such a manner that the helicesresponsible for DNA recognition extend from thesame side of the protein. As MarA is assumed tofunction as a monomer 18 , this assembly allows theactivator to bind to one face of its target DNA 26 .(b)(c)FisFis-binding site−35 −10Site ImarORob SoxSMarboxmarR marA marBSite IImarR marA marBmarOPmar IItrends in MicrobiologyFig. 1. Genetic organization of the Escherichia coli mar locus. (a) Expression of marCand marRAB is controlled by independent promoters Pmar I and Pmar II , respectively.MarR (144 residues) and MarA (127 residues) encode the Mar repressor and activator.MarB (72 residues) has an unknown function, as does MarC, a 221 amino acid,putative inner-membrane protein with multiple transmembrane-spanning helices.(b) MarR negatively regulates marRAB expression by binding to two sites in the operatormarO. (c) Following inactivation of MarR, marRAB expression is constitutive, butits level of expression depends on the interaction of MarA, Rob and SoxS (wheninduced) with a sequence (the marbox) proximal to the 35/10 hexamers. Fis playsan accessory role in this positive control 17 .agents is conferred by increased expression of cytoplasmicenzymes that counteract the damaging effectsinflicted upon the cell 30 .Inactivation of the AcrAB homolog in Haemophilusinfluenzae confers hypersusceptibility to manystructurally diverse chemicals 31 . Whether the functionof this multidrug efflux system is positivelyregulated by Ya52, the sole MarA homolog in H. influenzae32 , remains to be determined.Mechanisms of mar-mediated resistanceAntibiotic resistance arises in E. coli mar mutantsboth by decreasing influx and increasing efflux oftoxic chemicals from the cell. The former is accomplishedpartially by downregulating the synthesis ofOmpF (Ref. 27) and, possibly, by altering the expressionof other membrane proteins 4 . Increased efflux inboth E. coli and S. typhimurium is achieved primarilyby increased synthesis of the AcrAB–TolC multidrugefflux system 4,28,29 . Resistance to oxidative stressAn array of resistance phenotypesThe mar locus was originally identified by its abilityto confer multiple antibiotic resistance 2 . mar mutantsshow enhanced saturable active efflux of tetracyclineand chloramphenicol 2,33 . These findings were extendedwith the discovery that expression of the AcrAB effluxsystem 28 , in addition to putative drug-specificefflux pumps 10 , was increased in mar mutants andthat its removal led to increased susceptibility totetracycline, norfloxacin, chloramphenicol, ampicillin,TRENDS IN MICROBIOLOGY 411VOL. 7 NO. 10 OCTOBER 1999