Relationships among Enzymes of the β-Ketoadipate Pathway

The enzymes of the β-ketoadipate pathway are governed by different induction mechanisms in Acinetobacter and Pseudomonas; muconolactone isomerase is one of the enzymes of the catabolic pathway. In order to determine whether the different inductive controls reflect the independent evolutionary acquisition of the pathway in the two bacterial genera, the properties of muconolactone isomerase purified from a representative of each genus were compared. Procedures previously used to crystallize the enzyme from extracts of Pseudomonas putidia were modified to permit crystallization of the enzyme from extracts of Acinetobacter calcoaceticus. The isofunctional enzymes were found to be serologically remote, and some differences in their amino acid compositions were observed. Nevertheless, the muconolactone isomerases possessed similar molecular weights (100,000 for the Acinectobacter enzyme and 94,000 for the Pseudomonas enzyme) and similar subunit sizes (12,000 for the Acinetobacter subunit and 11,500 for the Pseudomonas subunit). At least 12 (and perhaps 13) of the first 15 residues in the NH2-terminal amino acid sequences of the proteins are identical. On the basis of this evidence we conclude that the muconolactone isomerases from the two genera share a common ancestral gene. The apparently close evolutionary homology of the muconolactone isomerases and the wide evolutionary separation of Acinetobacter and Pseudomonas suggest that the structural gene for the catabolic enzyme may have been exchanged well after the divergence of other genes that dictated the evolutionary separation of the two genera. Conservation of the distinctive inductive controls that are characteristic of each genus may have resulted from selective pressures imposed by the nutritional opportunities available to each bacterial group in its natural environment.