Nitrate repression of the Escherichia coli pfl operon is mediated by the dual sensors NarQ and NarX and the dual regulators NarL and NarP.

The pfl operon is expressed at high levels anaerobically. Growth of Escherichia coli in the presence of nitrate or nitrite led to a 45% decrease in expression when cells were cultivated in rich medium. Nitrate repression, however, was significantly enhanced (sevenfold) when the cells were cultured in minimal medium. Regulation of pfl expression by nitrate was dependent on the NarL, NarP, NarQ, and NarX proteins but independent of FNR, ArcA, and integration host factor, which are additional regulators of pfl expression. Strains unable to synthesize any one of the NarL, NarP, NarQ, or NarX proteins, but retaining the capacity to synthesize the remaining three, exhibited essentially normal nitrate regulation. In contrast, narL narP and narX narQ double null mutants were devoid of nitrate regulation when cultured in rich medium but they retained some nitrate repression (1.3-fold) when grown in minimal medium. By using lacZ fusions, it was possible to localize the DNA sequences required to mediate nitrate repression to the pfl promoter-regulatory region. DNase I footprinting studies identified five potential binding sites for the wild-type NarL protein in the pfl promoter-regulatory region. Specific footprints were obtained only when NarL was phosphorylated with acetyl phosphate before the binding reaction was performed. Each of the protected regions contained at least one heptamer sequence which has been deduced from mutagenesis studies to be essential for NarL binding (K. Tyson, A. Bell, J. Cole, and S. Busby, Mol. Microbiol. 7:151-157, 1993).