Bacillus subtilis (Bs) citrate synthase CS-I_like. CS catalyzes the condensation of acetyl coenzyme A (AcCoA) and oxalacetate (OAA) to form citrate and coenzyme A (CoA), the first step in the citric acid cycle (TCA or Krebs cycle). 2MCS catalyzes the condensation of propionyl-coenzyme A (PrCoA) and OAA to form 2-methylcitrate and coenzyme A (CoA) during propionate metabolism. The overall CS reaction is thought to proceed through three partial reactions and involves both closed and open conformational forms of the enzyme: a) the carbanion or equivalent is generated from AcCoA by base abstraction of a proton, b) the nucleophilic attack of this carbanion on OAA to generate citryl-CoA, and c) the hydrolysis of citryl-CoA to produce citrate and CoA. This group contains proteins similar to BsCS-I, one of two CS isozymes in the gram-positive B. subtilis. The majority of CS activity in B. subtilis is provided by the other isozyme, BsCS-II (not included in this group). BsCS-I has a lower catalytic activity than BsCS-II, and has a Glu in place of a key catalytic Asp residue. This change is conserved in other members of this group. For E. coli CS (not included in this group), site directed mutagenesis of the key Asp residue to a Glu converts the enzyme into citryl-CoA lyase which cleaves citryl-CoA to AcCoA and OAA. A null mutation in the gene encoding BsCS-I (citA) had little effect on B. subtilis CS activity or on sporulation. However, disruption of the citA gene in a strain null for the gene encoding BsCS-II resulted in a sporulation deficiency, a characteristic of strains defective in the Krebs cycle. This group contains proteins which functions exclusively as either a CS or a 2MCS, as well as those with relaxed specificity which have dual functions as both a CS and a 2MCS. Many of the gram-negative species represented in this group have a second CS isozyme which is in another group.