Fig. 5. Regulation of gene expression by redox-sensitive Arc and Fnr pathways. Under aerobic conditions, ArcB and ArcA are sequentially activated by phosphorylation. ArcA negatively regulates the transcription of the cyoABCDE operon, which encodes cytochrome bo oxidase (high V_max, low oxygen affinity) and positively regulates cydAB encoding cytochrome bd oxidase (low V_max, high oxygen affinity). Fnr is inactive under aerobic conditions, but at low oxygen it undergoes a conformational change, probably mediated by reduction of ferric to ferrous iron at an iron-sulphur center. Conformationally activated Fnr binds DNA at sites with the inverted repeat sequences TTGAT —— ATCAA. Fnr binding represses cyoABCDE and cydAB operons and induces transcription from the operons dmsABC (dimethyl sulfoxide/trimethylamine-N-oxide reductase), frdABCD (fumarate reductase), and narGHJI (nitrate reductase). Cross-talk between the two pathways at cyoABCDE and cydAB is indicated by the broken arrow. Under microaerophilic conditions as oxygen becomes limiting, cydAB is optimally active and ArcA may successfully compete Fnr to activate the regulator under these conditions (Bunn and Poyton, 1996).