Anaerobic activation of <em>arcA</em> transcription in <em>Escherichia coli</em>: roles of Fnr and ArcA.

The ArcA and Fnr regulators of Escherichia coli, both of which are activated in anaerobic conditions, negatively regulate the sodA gene (coding for manganese superoxide dismutase), but Fnr has no effect on anaerobic sodA expression in a ΔarcAΔAfnr back-ground (Compan and Touati, 1993). We show here that the sdh gene (coding for succinate dehydrogenase) is also negatively regulated by Fnr, but again Fnr exerts no control in a ΔarcA background. One interpretation of these results is that Fnr activates arcA transcription. Using arcA-lac transcriptional and translational fusions, we show that arcA expression increases (about fourfold) in anaerobiosis and that both Fnr and ArcA are required for full expression, in a Δfnr background, there is no auto-activation, suggesting that ArcA enhances activation by Fnr. Transcript and sequence analyses reveal that the arcA upstream regulatory region lies within a 530 bp non-coding DNA fragment, which contains five putative promoter sequences and a putative Fnr-binding site, identification of the transcription start sites indicates that transcription occurs in aerobiosis from three constitutive upstream promoters (P_e, P_d, P_c). In anaerobiosis an additional completely Fnr-dependent transcript starting at P_a, is present; expression from P_a is reduced in the absence of ArcA, and Fnr activation at P_a blocks the weak anaerobic-dependent expression from P_b- Fnr activation of arcA transcription may play an important role in the coordination of expression of genes associated with aerobic and anaerobic metabolism during environmental changes. [ABSTRACT FROM AUTHOR]