Involved in both the tricarboxylic acid (TCA) and methylcitric acid cycles (PubMed:28956599). Catalyzes the reversible isomerization of citrate to isocitrate via cis-aconitate (PubMed:3110133, PubMed:23354745). Also catalyzes the rehydration of 2-methyl-cis-aconitate to produce 2-methylisocitrate (PubMed:28956599). The apo form of AcnA functions as a RNA-binding regulatory protein which plays a role in the regulation of citrate concentration and in the sporulation. To prevent the accumulation of excessive levels of citrate, it binds near the 5' end of the citZ mRNA, decreasing its stability and thereby limiting the concentration of citrate synthase in the cell. Aconitase also binds to the gerE transcript late in sporulation and stabilizes it for translation, thereby increasing the rate and level of GerE protein accumulation (PubMed:10468622, PubMed:16923907, PubMed:23354745, PubMed:9393699). citrate = D-threo-isocitrate 3-hydroxybutane-1,2,3-tricarboxylate = 2-methyl-cis-aconitate + H2O Binds 1 [4Fe-4S] cluster per subunit. Carbohydrate metabolism; tricarboxylic acid cycle; isocitrate from oxaloacetate: step 2/2. Monomer. By citrate via CcpC. When citrate is absent, CcpC binds to the sites near the citB promoter and blocks expression. When citrate is present, it causes a change in the interaction of CcpC with its binding sites, resulting in derepression of citB. When citrate is very abundant, CcpC activates citB expression, presumably reflecting a change in the interaction of CcpC with RNA polymerase. Also induced by decoyinine and nutrient depletion. Mutagenesis of different positions (F662L, R741F, Q745E, I810T and V853A) on the C-terminal region of citB gene product shows that even B.subtilis mutant has high aconitase catalytic activity, it is defective in sporulation. The defect is at a late stage of sporulation, specifically affecting expression of sigma-K-dependent genes, many of which are important for spore coat assembly and require transcriptional activation by GerE. It strongly suggests that aconitase RNA binding activity may stabilize gerE mRNA in order to allow efficient GerE synthesis and proper timing of spore coat assembly. Cells lacking this gene show a dramatic increase in the concentration of citrate. This accumulation of citrate prevents sporulation due to chelation by citrate of divalent cations required for proper functioning of the Spo0A-initiated phosphorelay. Whether aconitase is active as an enzyme or an RNA-binding protein is determined by the status of an iron-sulfur (4Fe-4S) cluster that is essential for the catalytic activity of all aconitases. Citrate is both cotransported with iron and a chelator of divalent cations, including iron. If the intracellular citrate level becomes excessive, iron will be sequestered away from iron-containing proteins, including aconitase. Since excess citrate greatly stimulates aconitase synthesis via the positive regulatory effect of CcpC, the cell will gain the ability to metabolize citrate at a higher rate. If so much iron has been sequestered that aconitase loses enzymatic activity, the cell will acquire a high concentration of enzymatically inactive but RNA-binding-competent aconitase molecules. These aconitase proteins can bind to the citZ mRNA and reduce the rate of citrate accumulation by restricting the synthesis of citrate synthase protein. Belongs to the aconitase/IPM isomerase family.