Catalyzes the reversible isomerization of citrate to isocitrate via cis-aconitate. The apo form of AcnA functions as a RNA-binding regulatory protein which plays a role as a maintenance or survival enzyme during nutritional or oxidative stress. During oxidative stress inactive AcnA apo-enzyme without iron sulfur clusters binds the acnA mRNA 3' UTRs (untranslated regions), stabilizes acnA mRNA and increases AcnA synthesis, thus mediating a post-transcriptional positive autoregulatory switch. AcnA also enhances the stability of the sodA transcript. citrate = D-threo-isocitrate Binds 1 [4Fe-4S] cluster per subunit. Optimum pH is 7.4. It retains a high specific activity over a broad pH range. Carbohydrate metabolism; tricarboxylic acid cycle; isocitrate from oxaloacetate: step 2/2. Monomer. Induced upon entry into stationary phase by iron, oxidative and salt stress under aerobic conditions. AcnA is subject to CRP-mediated catabolite repression and ArcA-mediated anaerobic repression. AcnA is negatively regulated by ryhB RNA. Cells lacking this gene are more sensitive to peroxide stress. The acnAB double mutant does not grow on unsupplemented glucose minimal medium and does not respond under aerobic conditions to glutamate. The acnAB double mutant retains a low but significant aconitase activity. The AcnA activity over a broad pH range might be a useful adaptation for specifically expression in the stationary phase, where the intracellular pH may vary over a wider range. AcnA is resistant to oxidation in vivo. Belongs to the aconitase/IPM isomerase family.