Repetitions in the NH2-terminal amino acid sequence of beta-ketoadipate enol-lactone hydrolase from Pseudomonas putida.

Muconolactone delta-isomerase (EC 5.3.3.4) and beta-ketoadipate enol-lactone hydrolase (EC 3.1.1.24) mediate consecutive reactions in the beta-ketoadipate pathway of bacteria. An earlier investigation (Yeh, W.K., Davis, G., Fletcher, P., and Ornston, L.N. (1978) J. Biol. Chem. 253, 4920-4923) revealed that the respective NH2-terminal amino acid sequences of Pseudomonas putida muconolactone isomerase and Acinetobacter calcoaceticus beta-ketoadipate enol-lactone hydrolase II are evolutionarily homologous. In this report, we describe the purification of Pseudomonas beta-ketoadipate enol-lactone hydrolase and present evidence indicating that the protein is a trimer composed of identical 11,000-dalton subunits. The NH2-terminal amino acid sequences of Pseudomonas muconolactone isomerase and Pseudomonas enol-lactone hydrolase have diverged widely from each other, yet the two sequences contain different fragments of an ancestral sequence which is represented in Acinetobacter enol-lactone hydrolase II. The widely divergent Pseudomonas muconolactone isomerase and Pseudomonas enol-lactone hydrolase sequences each contain unique sets of repeated peptides. In principle, the repetitive sequences might have been introduced by elongation mutations which occurred early in the evolution of the proteins. However, the divergence of Pseudomonas muconolactone isomerase and Pseudomonas enol-lactone hydrolase is so extreme that the observed sequence repetitions cannot have been conserved from ancestral duplication mutations. Rather, the data favor the interpretation that copies of DNA were substituted into structural genes for the enzymes as they diverged.