THE BETA-KETOADIPATE PATHWAY AND THE BIOLOGY OF SELF-IDENTITY

The beta-ketoadipate pathway is a chromosomally encoded convergent pat hway for aromatic compound degradation that is widely distributed in s oil bacteria and fungi. One branch converts protocatechuate, derived f rom phenolic compounds including p-cresol, 4-hydroxybenzoate and numer ous lignin monomers, to beta-ketoadipate. The other branch converts ca techol, generated from various aromatic hydrocarbons, amino aromatics, and lignin monomers, also to beta-ketoadipate. Two additional steps a ccomplish the conversion of beta-ketoadipate to tricarboxylic acid cyc le intermediates. Enzyme studies and amino acid sequence data indicate that the pathway is highly conserved in diverse bacteria, including P seudomonas putida, Acinetobacter calcoaceticus, Agrobacterium tumefaci ens, Rhodococcus erythropolis, and many others. The catechol branch of the beta-ketoadipate pathway appears to be the evolutionary precursor for portions of the plasmid-borne ortho-pathways for chlorocatechol d egradation. However, accumulating evidence points to an independent an d convergent evolutionary origin for the eukaryotic beta-ketoadipate p athway. In the face of enzyme conservation, the beta-ketoadipate pathw ay exhibits many permutations in different bacterial groups with respe ct to enzyme distribution (isozymes, points of branch convergence), re gulation (inducing metabolites, regulatory proteins), and gene organiz ation. Diversity is also evident in the behavioral responses of differ ent bacteria to beta-ketoadipate pathway-associated aromatic compounds . The presence and versatility of transport systems encoded by beta-ke toadipate pathway regulons is just beginning to be explored in various microbial groups. It appears that in the course of evolution, natural selection has caused the beta-ketoadipate pathway to assume a charact eristic set of features or identity in different bacteria. Presumably such identities have been shaped to optimally serve the diverse lifest yles of bacteria.