The napF and narG nitrate reductase operons in Escherichia coli are differentially expressed in response to submicromolar concentrations of nitrate but not nitrite

Escherichia coli synthesizes two biochemically distinct nitrate reductase e nzymes, a membrane-bound enzyme encoded by the narGHJI operon and a peripla smic cytochrome c-linked nitrate reductase encoded by the napFDAGHBC operon , To address why the cell makes these two enzymes, continuous cell culture techniques were used to examine napF and narG gene expression in response t o different concentrations of nitrate and/or nitrite. Expression of the nap F-lacZ and narG-lacZ reporter fusions in strains grown at different steady- state levels of nitrate revealed that the two nitrate reductase operons are differentially expressed in a complementary pattern. The napF operon appar ently encodes a "low-substrate-induced" reductase that is maximally express ed only at low levels of nitrate, Expression is suppressed under high-nitra te conditions. In contrast, the narGHJI operon is only weakly expressed at low nitrate levels but is maximally expressed when nitrate is elevated. The narGHJI operon is therefore a "high-substrate-induced" operon that somehow provides a second and distinct role in nitrate metabolism by the cell. Int erestingly, nitrite, the end product of each enzyme, had only a minor effec t on the expression of either operon, Finally, nitrate, but not nitrite, wa s essential for repression of napF gene expression. These studies reveal th at nitrate rather than nitrite is the primary signal that controls the expr ession of these two nitrate reductase operons in a differential and complem entary fashion. In light of these findings, prior models for the roles of n itrate and nitrite in control of narG and napF expression must be reconside red.