PROJECT SUMMARY Bacterial survival requires a nearly constant and specific global DNA supercoiling status. This is achieved by the opposing functions of the DNA-relaxing topoisomerase I and DNA-compacting DNA gyrase. We propose the first unbiased high-throughput exploration of genes governing the abundance and activity of these enzymes, which hold great untapped promise for therapeutic manipulation. How pathogens change DNA supercoiling during infection conceals vulnerabilities critical to overcoming the toxic effects of current DNA gyrase inhibitors and emergence of bacteria resistant to these agents. We will examine mutants of the pathogen Salmonella enterica serovar Typhimurium for altered DNA supercoiling in infection-relevant conditions by deploying Fluorescence Evaluation of DNA Supercoiling – or FEDS – a new high-throughput method developed in our laboratory that reports the global DNA supercoiling status of living bacteria. To expose the genetic basis for altered DNA supercoiling, we will identify the mutated genes and explore genetic interactions among the mutations to reveal novel instances of synthetic lethality, thereby uncovering both novel genetic pathways and novel ways to manipulate DNA supercoiling. We will solve the bases for the DNA supercoiling behavior of the identified mutants by determining the amounts of DNA gyrase and topoisomerase I as well as the abundance of adenosine triphosphate, adenosine diphosphate, and specific polyamines – all critical regulators of DNA gyrase activity. The proposed research program pioneers the physiological understanding of an essential cellular property exploitable in a wide range of bacterial species to identify novel targets for antibacterial agents.