DNA supercolling and aerobic regulation of transcription from theKlebsiella pneumoniae nifLApromoter

Abstract
Expression from the K. pneumoniae nifLA promoter is oxygen sensitive and is also inhibited by the DNA gyrase inhibitor coumermycin Al under anaerobic growth conditions. The activity of this promoter was found to be highly sensitive to changes in DNA topology in vitro. Transcription was completely dependent on negative supercoiling at physiological salt concentrations although transcription from linear or fully relaxed closed circular templates was detectable at KCl concentrations lower than 50 mM. These observations suggest that aerobic regulation of nif transcription may be mediated through the level of DNA supercoiling.