DNA supercoiling and the anaerobic and growth phase regulation of tonB gene expression

We show that several interacting environmental factors influence the topology of intracellular DNA. Negative supercoiling of DNA in vivo is increased by anaerobic growth and is also influenced by growth phase. The tonB promoter of Escherichia coli and Salmonella typhimurium was found to be highly sensitive to changes in DNA supercoiling. Expression was increased by novobiocin, an inhibitor of DNA gyrase, and was decreased by factors which increase DNA superhelicity. Expression of the plasmid-encoded tonB gene was enhanced by gamma delta insertions in cis in a distance- and orientation-independent fashion. Both the res site and the TnpR protein of gamma delta, which is known to function as a type I topoisomerase, were required for this activation. tonB expression increased during the growth cycle and was reduced by anaerobiosis. There was excellent correlation between tonB expression from a plasmid and the level of supercoiling of that plasmid under a wide range of conditions. The chromosomal tonB gene was regulated in a manner identical to that of the plasmid-encoded gene. Thus, the physiological regulation of tonB expression in response to anaerobiosis and growth phase appears to be mediated by environmentally induced changes in DNA superhelicity.