Degradation of Aromatic Compounds by Purple Nonsulfur Bacteria

Abstract

Aromatic compounds are among the more difficult groups of naturally occurring organic compounds to degrade because of the high resonance stability of benzene rings. Some purple nonsulfur bacteria have a well-developed ability to degrade green plant-derived aromatic compounds including a variety of lignin monomers as well as some man-made compounds, including chlorobenzoates and toluene. Peripheral pathways modify these compounds to form a small number of common intermediates that enter pathways leading to ring cleavage. Depending on the compound and the species involved, degradation can occur aerobically under chemoheterotrophic conditions or anaerobically under photoheterotrophic conditions. Two different biochemical strategies, one involving oxygenases and the other involving aromatic ring reduction, take place depending on the availability of oxygen. The best-studied aromatic compound-degrading species, Rhodopseudomonas (Rps.) palustris, has served as a model organism to elucidate a central reductive pathway of benzoate degradation that is used to process most compounds anaerobically. The main features of this pathway appear to apply to other metabolic groups of bacteria that degrade aromatic compounds under anaerobic conditions. These include a novel enzyme, benzoyl-CoA reductase, that relieves the resonance stability of the aromatic ring, and a sequence of β-oxidation-like reactions leading to ring cleavage by a new type of ring cleavage enzyme. The genes for anaerobic benzoate and 4-hydroxybenzoate degradation have been located on the sequenced genome of Rps. palustris strain CGA009. A similar gene cluster is present in three other recently sequenced strains of Rhodopseudomonas. The genome sequence of one of the strains, BisB5, revealed that this strain can degrade an expanded set of aromatic compounds converging on and including phenylacetate under photoheterotrophic conditions.