DUAL RESPONSE REGULATORS (NARL AND NARP) INTERACT WITH DUAL SENSORS (NARX AND NARQ) TO CONTROL NITRATE-REGULATED AND NITRITE-REGULATED GENE-EXPRESSION IN ESCHERICHIA-COLI K-12

Two sensor proteins, NarX and NarQ, mediate nitrate regulation of anae robic respiratory gene expression. Either of these sensors is sufficie nt to signal the presence of nitrate to the response regulator protein , NarL, a transcriptional activator and repressor. Two observations su ggested the existence of a second response regulator that is also invo lved in nitrate regulation. First, narL null mutants retain residual n itrate induction of fdnG operon expression; this residual induction is absent in narX narQ double-null strains. Second, nitrate induction of aeg-46.5 operon expression is substantially enhanced in narL null str ains (M. H. Choe and W. S. Reznikoff, J. Bacteriol. 173:6139-6146, 199 1). We found that this nitrate induction requires either the NarX or t he NarQ protein, consistent with the existence of a second response re gulator. We designate this second regulator NarP. We isolated insertio n mutants that are defective in aeg-46.5 operon expression. These inse rtions are in the narP gene, which encodes a response regulator that i s 44% identical to the NarL protein. Null alleles of narP abolished ae g-46.5 induction and also eliminated the residual NarL-independent nit rate induction of fdnG operon expression. Both the NarX and NarQ prote ins communicate with both the NarP and NarL proteins. We found that th e primary signal for NarP-dependent aeg-46.5 operon induction is nitri te rather than nitrate. By contrast, nitrite is a relatively weak sign al for NarL-dependent induction. In narX null strains, nitrite was an efficient signal for NarL-dependent induction, and this induction requ ired the NarQ protein. We conclude that, in wild-type strains, the Nar Q protein communicates the presence of nitrite to both the NarP and Na rL proteins and that the NarX protein inhibits this communication with the NarL protein.