A CLUSTER OF BACTERIAL GENES FOR ANAEROBIC BENZENE-RING BIODEGRADATION

A reductive benzoate pathway is the central conduit for the anaerobic biodegradation of aromatic pollutants and lignin monomers. Benzene rin g reduction requires a and this metabolic capability has, so far, been reported only in bacteria, To determine the molecular basis for this environmentally important process, we cloned and analyzed genes requir ed for the anaerobic degradation of benzoate and related compounds fro m the phototrophic bacterium, Rhodopseudomonas palustris. A cluster of 24 genes was identified that includes twelve genes likely to be invol ved in anaerobic benzoate degradation and additional genes that conver t the related compounds 4-hydroxybenzoate and cyclohexanecarboxylate t o benzoyl-CoA. Genes encoding benzoyl-CoA reductase, a novel enzyme ab le to overcome the resonance stability of the aromatic ring, were iden tified by directed mutagenesis. The gene encoding the ring-cleavage en zyme, 2-ketocyclohexanecarboxyl-CoA hydrolase, was identified by assay ing the enzymatic activity of the protein expressed in Escherichia col i. Physiological data and DNA sequence analyses indicate that the benz oate pathway consists of unusual enzymes for ring reduction and cleava ge interposed among enzymes homologous to those catalyzing fatty acid degradation. The cloned genes should be useful as probes to identify b enzoate degradation genes from other metabolically distinct groups of anaerobic bacteria, such as denitrifying bacteria and sulfate-reducing bacteria.