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The role of DNA gyrase,an ATP-dependent type Ⅱ topoisomerase,in altering DNA topology in bacteria through the introduction of negative supercoils is well established,as is the impact of the changes in DNA supercoiling on gene transcription.It is becoming clear that different bacterial species exhibit different DNA supercoiling set points under the same sets of growth conditions and that this is a contributing factor in determining the distinct nature of bacterial species.This lecture will review our recent work on the pH stress response in two closely related Gram-negative bacterial species: the commensal organism Escherichia coli K-12 and the facultative intracellular pathogen Salmonella enterica serovar Typhimurium.Both bacteria possess identical copies of the global regulatory protein OmpR and both use it to alter transcription of key genes during adaptation to acid stress.However,OmpR plays a much more restricted role in E.coli compared to S.Typhimurium where it regulates scores of genes,including many of the horizontally-acquired A+T-rich genes that are involved in the establishment of infection by Salmonella.We have discovered that the binding affinity of OmpR to its DNA targets is modulated by changes in DNA conformation.Specifically,DNA relaxation,especially of A+T-rich DNA,creates an ideal substrate for OmpR binding.When S.Typhimurium experiences acid stress(pH 4.5)it relaxes its DNA globally and this facilitates the binding of OmpR to its target genes;E.coli does not relax its genomic DNA following exposure to pH 4.5.A molecular mechanism linking DNA relaxation of A+T-rich DNA to efficient binding by proteins such as OmpR that rely on a winged helix-turn-helix motif will be presented.We have discovered a role for the Salmonella-specific cytoplasmic protein MgtC in mediating acid-stress-dependent DNA relaxation.MgtC is a known inhibitor of ATP synthase and therefore controls the energy source of DNA gyrase.Elimination of the mgtC gene from Salmonella removes the ability to relax DNA under acid stress condition,making Salmonella E.coli-like;introducing the Salmonella mgtC gene into E.coli confers on that species the characteristic DNA relaxation behaviour that is exhibited by Salmonella at pH 4.5.The implications of these findings for the production ofdesignermicrobial species will be discussed.