Two genetically-distinct and differentially-regulated aconitases (AcnA and AcnB) in Escherichia coli

An acnA mutant of Escherichia coli was constructed by replacing the chromosomal acnA gene by an internally deleted derivative containing a kanR cassette. Southern and Western blotting confirmed that the acnA gene had been replaced by the disrupted gene and that the aconitase A protein was no longer expressed. However, the mutant failed to exhibit the anticipated glutamate auxotrophy and it retained a residual aconitase activity. This activity was due to an analogous unstable enzyme(s) designated aconitase B. Studies on the regulation of aconitase A synthesis using an acnA-lacZ translational fusion showed that the acnA gene resembles other citric acid cycle genes in being subject to CRP-mediated catabolite repression and ArcA-mediated anaerobic repression. In addition to being activated by the SoxRS oxidative stress regulatory system, the acnA gene appeared to be activated by the ferric uptake regulator (Fur). It was concluded that the acnA gene belongs to at least four global regulatory networks, crp, arcA, fur and soxRS. In contrast, the aconitase B activity decreased after exposure to oxidative stress and was less affected by anaerobiosis. Comparable studies with the fumarase genes (fumA, B and C) indicated that fumA (encoding the unstable aerobic iron-sulphur-containing fumarase) is activated by the ferric uptake regulator (Fur) and fumC (encoding the stable fumarase) is activated by the SoxRS oxidative stress regulatory system.