FIG. 1.

Central reactions of the protocatechuate branch of the β-ketoadipate pathway in bacteria. Metabolic steps or sequences are represented by designations for the associated genes. Plant products such as quinate and phenylpropenoid compounds (ferulate, caffeate, and coumarate) are metabolized to protocatechuate (in the box with the thin black border). Enzymes encoded by the pca genes convert protocatechuate to common metabolites. The intermediate carboxymuconate (in the box with the thick black border) is very toxic, and cells lacking a functional pcaB fail to grow when protocatechuate or metabolic precursors of this compound are added to growth media. Secondary mutations preventing formation of carboxymuconate from aromatic compounds can be selected, which has facilitated genetic analysis of vanAB, pobA, and pcaHG. Exposure of cells to high concentrations of protocatechuate also impedes growth, and characterization of strains resisting this toxicity led to discovery of functions associated with the transporters VanK and PcaK in Acinetobacter. The β-ketoadipate transporter encoded by pcaT is found in fluorescent Pseudomonas species and has no known counterpart in Acinetobacter.