NITRATE REGULATION OF ANAEROBIC RESPIRATORY GENE-EXPRESSION IN ESCHERICHIA-COLI

Synthesis of most anaerobic respiratory pathways is subject to dual re gulation by anaerobiosis and nitrate. Anaerobic induction is mediated by the FNR protein. Dual interacting two-component regulatory systems mediate nitrate induction and repression. The response regulator prote in NARL binds DNA to control nitrate induction and repression of genes encoding nitrate respiration enzymes and alternate anaerobic respirat ory enzymes, respectively. The homologous protein NARP controls nitrit e induction of at least two operons. Nitrate and nitrite signalling ar e both mediated by the homologous sensor proteins NARX and NARQ. Recen t mutational analyses have defined a heptamer sequence necessary for s pecific DNA binding by the NARL protein. These heptamers are located a t different positions in the control regions of different operons. The NARL protein-binding sites in the narG (nitrate reductase) and narK ( nitrate-nitrite antiporter) operon control regions are located approxi mately 200bp upstream of the transcription initiation site. The integr ation host factor (IHF) greatly stimulates nitrate induction of these operons, indicating that a specific DNA loop brings NARL protein, boun d at the upstream region, into the proximity of the promoter for trans cription activation. Other NARL protein-dependent operons do not requi re IHF for nitrate induction, and the arrangement of NARL heptamer seq uences in these control regions is quite different. This complexity of signal transduction pathways, coupled with the diversity of control r egion architecture, combine to provide many interesting areas for futu re investigation. An additional challenge is to determine how or if th e FNR and NARL proteins interact to mediate dual positive control of t ranscription initiation.