Transposon Tn5 genomic mutants of plant-growth-promoting Pseudomonas putida strain WCS358 have been isolated which no longer utilize ferulic and coumaric acids as sole sources of carbon and energy. Genetic studies confirmed previous biochemical data showing that ferulic acid is degraded via vanillic acid, and coumaric acid via hydroxybenzoic acid. The genes involved in these enzymic steps were cloned and characterized. Two proteins designated Fca (26.5 kDa) and Vdh (50.3 kDa) were identified as responsible for the conversion of ferulic acid to vanillic acid; the proteins are encoded by the fca and vdh genes which are organized in an operon structure in the chromosome. The Vdh protein is 69% identical at the amino acid level to the Vdh protein recently identified in Pseudomonas sp. strain HR199 and converts vanillin to vanillic acid. Homology studies revealed that the Vdh proteins exhibited significant identity to aldehyde dehydrogenases from different organisms whereas Fca belonged to the enoyl-CoA hydratase family of proteins. Two proteins, designated VanA (39.9 kDa) and VanB (34.3 kDa), encoded by two genes, vanA and vanB, are organized in an operon in the chromosome. They were found to be responsible for the demethylation of vanillic acid to protocatechuic acid. The VanA proteins showed no homology to any other known protein, while VanB belonged to the ferredoxin family of proteins. This two-component enzyme system demethylated another phenolic monomer, veratric acid, thus indicating broad specificity. Studies of the regulation of the vanAB operon demonstrated that the genes were induced by the substrate, vanillic acid; however, the strongest induction was observed when cells were grown in the presence of the product of the reaction, protocatechuic acid.