Dual overlapping promoters control napF (periplasmic nitrate reductase) operon expression in Escherichia coli K-12.

Escherichia coli elaborates a flexible respiratory metabolism, involving differential synthesis of isoenzymes for many oxidation and reduction reactions. Periplasmic nitrate reductase, encoded by the napFDAGHBC operon, functions with concentrations of nitrate that are too low to support respiration by membrane-bound nitrate reductase. The napF operon control region exhibits unusual organization of DNA binding sites for the transcription regulators Fnr and NarP, which activate transcription in response to anaerobiosis and nitrate, respectively. Previous studies have shown that the napF operon control region directs synthesis of two transcripts whose 5' ends differ by about 3 nucleotides. We constructed mutant control regions in which either of the two promoter -10 regions is inactivated. Results indicate that the downstream promoter (P1) was responsible for Fnr- and NarP-regulated napF operon expression, whereas transcription from the upstream promoter (P2) was activated only weakly by the Fnr protein and was inhibited by phospho-NarP and -NarL proteins. The physiological function of promoter P2 is unknown. These results establish the unconventional napF operon control region architecture, in which the major promoter P1 is activated by the Fnr protein bound to a site centered at -64.5 with respect to the transcription initiation site, working in conjunction with the phospho-NarP protein bound to a site centered at -44.5.