Aerobic metabolism of phenylacetic acids in Azoarcus evansii

Abstract.

The aerobic metabolism of phenylacetic acid (PA) and 4-hydroxyphenylacetic acid (4-OHPA) was investigated in the β-proteobacterium Azoarcus evansii. Evidence for the existence of two independent catabolic pathways for PA and 4-OHPA is presented. 4-OHPA metabolism involves the formation of 2,5-dihydroxyphenylacetate (homogentisate) and maleylacetoacetate catalyzed by specifically induced 4-OHPA 1-monooxygenase and homogentisate 1,2-dioxygenase. The metabolism of PA starts by its activation to phenylacetyl-CoA (PA-CoA) via an aerobically induced phenylacetate-coenzyme A ligase. Phenylalanine (Phe) aerobic metabolism in this bacterium proceeds also via PA and PA-CoA. Whole cells of A. evansii transformed [1-¹⁴C]PA to ¹⁴C-phenylacetyl-CoA and subsequently to a number of unknown labeled products, which were also observed in PA-degrading bacteria from different phylogenetic groups, i.e. Escherichia coli, Rhodopseudomonas palustris and Bacillus stearothermophilus. A chromosomal region from A. evansii of 11.5 kb containing a cluster of 11 phenylacetic acid catabolic (paa) genes (paaYZGHIKABCDE) was sequenced and characterized. The derived gene products were similar to the characterized putative gene products involved in PA catabolism in E. coli and Pseudomonas putida and to other putative PA catabolic gene products of diverse bacteria. RT-PCR analysis of the paa genes of A. evansii growing aerobically with PA showed a probable organization of the paa genes in three operons. The similarity of the PA metabolic products pattern and of gene sequences suggests a common aerobic bacterial PA pathway.