Transcriptional regulation in response to oxygen and nitrate of the operons encoding the [NiFe] hydrogenases 1 and 2 of Escherichia coli

Synthesis of the [NiFe] hydrogenases 1 and 2 of Escherichia coli is induced in response to anaerobiosis and is repressed when nitrate is present in th e growth medium. The hydrogenase 1 and hydrogenase 2 enzymes are encoded by the polycistronic hyaABCDEF and hybOABCDEFG operons, respectively. Primer extension analysis was used to determine the initiation site of transcripti on of both operons. This permitted the construction of single-copy lacZ ope ron fusions, which were used to examine the transcriptional regulation of t he two operons. Expression of both was induced by anaerobiosis and represse d by nitrate, which is in complete accord with earlier biochemical studies. Anaerobic induction of the hyb operon was only partially dependent on the FNR protein and, surprisingly, was enhanced by an arcA mutation. This latte r result indicated that ArcA suppresses anaerobic hyb expression and that a further factor, which remains to be identified, is involved in controlling anaerobic induction of operon expression. Nitrate repression of hyb expres sion was mediated by the NarL/NarX and NarP/NarQ two-component regulatory s ystems. Remarkably, a narP mutant lacked anaerobic induction of hyb express ion, even in the absence of added nitrate. Anaerobic induction of hya expre ssion was dependent on the ArcA and AppY regulators, which confirms earlier observations by other authors. Nitrate repression of the hya operon was me diated by both NarL and NarP. Taken together, these data indicate that alth ough the hya and hyb operons share common regulators, there are important d ifferences in the control of expression of the individual operons.