Abstract
Two sensor proteins, NarX and NarQ, mediate nitrate regulation of anaerobic respiratory gene expression. Either of these sensors is sufficient to signal the presence of nitrate to the response regulator protein, NarL, a transcriptional activator and repressor. Two observations suggested the existence of a second response regulator that is also involved in nitrate regulation. First, narL null mutants retain residual nitrate 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 strains (M.H. Choe and W.S. Reznikoff, J. Bacteriol. 173:6139-6146, 1991). We found that this nitrate induction requires either the NarX or the NarQ protein, consistent with the existence of a second response regulator. We designate this second regulator NarP. We isolated insertion mutants that are defective in aeg-46.5 operon expression. These insertions are in the narP gene, which encodes a response regulator that is 44% identical to the NarL protein. Null alleles of narP abolished aeg-46.5 induction and also eliminated the residual NarL-independent nitrate induction of fdnG operon expression. Both the NarX and NarQ proteins communicate with both the NarP and NarL proteins. We found that the primary signal for NarP-dependent aeg-46.5 operon induction is nitrite rather than nitrate. By contrast, nitrite is a relatively weak signal for NarL-dependent induction. In narX null strains, nitrate was an efficient signal for NarL-dependent induction, and this induction required the NarQ protein. We conclude that, in wild-type strains, the NarQ protein communicates the presence of nitrite to both the NarP and NarL proteins and that the NarX protein inhibits this communication with the NarL protein.
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