Genes coding for the benzoyl-CoA pathway of anaerobic aromatic metabolism in the bacterium Thauera aromatica.

Many aromatic compounds are anaerobically oxidized to CO2 via benzoyl-CoA as the common aromatic intermediate. In Thauera aromatica, the central benzoyl-CoA pathway comprises the ATP-driven two-electron reduction of the benzene ring; this reaction uses a ferredoxin as electron donor and is catalyzed by benzoyl-CoA reductase. The first intermediate, cyclohex-1,5-diene-1-carboxyl-CoA, is subsequently hydrated by dienoyl-CoA hydratase to 6-hydroxycyclohex-1-ene-1-carboxyl-CoA. Formation of the main product produced by cell extracts, 3-hydroxypimelyl-CoA, requires at least two further steps; the oxidation of a hydroxyl group and the hydrolytic carbon ring cleavage of a CoA-activated beta-oxoacid. In addition, enoyl-CoA hydratase may come into play. A cluster of eight adjacent genes, which are transcribed in the same direction and may form an operon, was found in this bacterium. The cluster codes for proven and postulated enzymes of the benzoyl-CoA pathway. The genes for the enzymes code for ferredoxin, four subunits of benzoyl-CoA reductase, dienoyl-CoA hydratase, 6-hydroxycyclohex-1-ene-1-carboxyl-CoA dehydrogenase (NAD+), and the ring hydrolyzing enzyme. The deduced amino acid sequences of these proteins were 35-86% similar to the corresponding sequences found in Rhodopseudomonas palustris. Benzoyl-CoA reductase subunits exhibit distinct similarities with 2-hydroxyglutaryl-CoA dehydratase and its ATP-hydrolysing activase protein of Acidaminococcus fermentans as well as with open reading frames of unknown function in other bacteria. Conversion of benzoyl-CoA to 3-hydroxypimelyl-CoA can be explained by a minimal model of the benzoyl-CoA pathway assuming the four enzymes whose genes were characterized and an additional enoyl-CoA hydratase. In R. palustris the dienoyl-CoA hydratase gene is lacking suggesting the operation of a modified benzoyl-CoA pathway with cyclohex-1-ene-1-carboxyl-CoA as intermediate.