Effects of nitrate respiration on expression of the Arc-controlled operons encoding succinate dehydrogenase and flavin-linked L-lactate dehydrogenase.

Expression of sdhCDAB (encoding succinate dehydrogenase) and lctD (encoding the flavin-linked L-lactate dehydrogenase) is elevated aerobically and repressed anaerobically in Escherichia coli. The repression is initiated by autophosphorylation of the sensor protein ArcB, followed by phosphoryl group transfer to the regulator ArcA. ArcA-P, a global transcriptional regulator, then prevents sdh and lct expression. The stimulus for ArcB is not O2 deficiency per se. In vitro experiments showed that ArcB phosphorylation is enhanced by pyruvate, D-lactate, acetate, and NADH, the concentrations of which are likely to increase with the lack of an effective exogenous electron sink. In addition to their aerobic function, the two primary dehydrogenases also have roles in anaerobic nitrate respiration. Results presented here indicate that the increase of sdh and lct expression by nitrate depended on its chemical reduction, which in turn diminished the ArcA-P pool. Unexpectedly, a mutation in the fnr gene (encoding a global regulator involved in anaerobic metabolism) also alleviated the anaerobic repressions. Mutations in arcB or arcA were epistatic over that of fnr. Moreover, since this relief was counteracted by pyruvate in the growth medium, Fnr appears to affect formation of stimuli for ArcB. It is possible that Fnr also indirectly affects some of the other members of the arcA modulon, e.g., cyoABCDE (encoding the cytochrome o complex), cydAB (encoding the cytochrome d complex), and sodA (encoding the manganese-dependent superoxide dismutase).

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