GENES-CODING FOR THE BENZOYL-COA PATHWAY OF ANAEROBIC AROMATIC METABOLISM IN THE BACTERIUM THAUERA-AROMATICA

Many aromatic compounds are anaerobically oxidized to CO, via benzoyl- CoA as the common aromatic intermediate. In Thauera aromatica, the cen tral benzoyl-CoA pathway comprises the ATP-driven two-electron reducti on of the benzene ring, this reaction uses a ferredoxin as electron do nor and is catalyzed by benzoyl-CoA reductase. The first intermediate, cyclohex-1,5-diene-1-carboxyl-CoA, is subsequently hydrated by dienoy l-CoA hydratase to 6-hydroxycyclohex-1-ene-1-carboxyl-CoA. Formation o f the main product produced by cell extracts, 3-hydroxypimelyl-CoA, re quires at least two further steps; the oxidation of a hydroxyl group a nd the hydrolytic carbon ring cleavage of a CoA-activated beta-oxoacid . In addition, enoyl-CoA hydratase may come into play. A cluster of ei ght adjacent genes, which are transcribed in the same direction and ma y form an operon, was found in this bacterium. The cluster codes for p roven and postulated enzymes of the benzoyl-CoA pathway. The genes for the enzymes code for ferredoxin, four subunits of benzoyl-CoA reducta se, dienoyl-CoA hydratase, 6-hydroxycyclohex-1-ene-1-carboxyl-CoA dt:h ydrogenase (NAD(+)), and the ring hydrolyzing enzyme. The deduced amin o acid sequences of these proteins were 35-86% similar to the correspo nding sequences found in Rhodopseudomonns palustris. Benzoyl-CoA reduc tase subunits exhibit distinct similarities with 2-hydroxyglutaryl-CoA dehydratase and its ATP-hydrolysing activase protein of Acidaminococc us fermentans as well as with open reading frames of unknown function in other bacteria. Conversion of benzoyl-CoA to 3-hydroxypimelyl-CoA c an be explained by a minimal model of the benzoyl-CoA pathway assuming the four enzymes whose genes were characterized and an additional eno yl-CoA hydratase. In R. palustris the dienoyl-CoA hydratase gene is la cking suggesting the operation of a modified benzoyl-CoA pathway with cyclohex-1-ene-1-carboxyl-CoA as intermediate.