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Summary

Although Escherichia coli and Salmonella enterica inhabit similar niches and employ similar genetic regulatory programmes, we find that they differ significantly in their DNA supercoiling responses to environmental and antibiotic challenges. Whereas E. coli demonstrates large dynamic transitions in supercoiling in response to growth phase, osmotic pressure and novobiocin treatment, supercoiling levels are much less variable in S. enterica. The FIS protein is a global regulator of supercoiling in E. coli, but it was found to have less influence over supercoiling control in S. enterica. These inter-species differences fine-tune gene promoters to endogenous supercoiling and FIS levels. Transferring a Salmonella virulence gene promoter (PssrA) into a new enteric host (E. coli) caused aberrant expression in response to stimulatory signals. Reciprocal horizontal transfer of topA promoters, which control expression of topoisomerase I, between E. coli and S. enterica revealed how these orthologous promoters have evolved to respond differentially to FIS and supercoiling levels in their cognate species. This also identified a previously unrecognized osmoregulation of topA expression that is independent of FIS and supercoiling in both E. coli and S. enterica. These findings suggest that E. coli and S. enterica may be unexpectedly divergent in their global regulation of cellular physiology.