Multiple cis‐acting sites positively regulate Escherichia coli nrd expression

Regulation of nrd expression in Escherichia coli by cis‐acting elements was found to be more complex than previously reported. At least five upstream sites appear to positively regulate nrd expression including a Fis binding site, a DnaA binding site, an AT‐rich region, an inverted repeat and a 10 bp site between the AT‐rich region and the inverted repeat. Double mutants defective in these sites indicate that all sites tested act independently when regulating nrd expression. As the decrease in nrd expression in exponentially growing cultures paralleled the decrease observed in DNA synthesis‐inhibited cultures for all single and double mutants, we concluded that nrd is regulated by the same mechanism in these physiological states. As mutants unable to induce nrd expression during inhibition of DNA synthesis also fail to exhibit cell cycle‐regulated nrd expression, we conclude that cell cycle nrd regulation is controlled by these same sites. Site‐directed mutagenesis was used to show that the absence of an increase in nrd expression during DNA inhibition previously observed for deletion of the AT‐rich region results from deletion of both the Fis binding site and the AT‐rich region.