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The coenzyme A (CoA)-activated C₅-dicarboxylic acids mesaconyl-CoA and β-methylmalyl-CoA play roles in two as yet not completely resolved central carbon metabolic pathways in bacteria. First, these compounds are intermediates in the 3-hydroxypropionate cycle for autotrophic CO₂ fixation in Chloroflexus aurantiacus, a phototrophic green nonsulfur bacterium. Second, mesaconyl-CoA and β-methylmalyl-CoA are intermediates in the ethylmalonyl-CoA pathway for acetate assimilation in various bacteria, e.g., in Rhodobacter sphaeroides, Methylobacterium extorquens, and Streptomyces species. In both cases, mesaconyl-CoA hydratase was postulated to catalyze the interconversion of mesaconyl-CoA and β-methylmalyl-CoA. The putative genes coding for this enzyme in C. aurantiacus and R. sphaeroides were cloned and heterologously expressed in Escherichia coli, and the proteins were purified and …