Homologies in the NH2-terminal amino acid sequences of gamma-carboxymuconolactone decarboxylases and muconolactone isomerases.

gamma-Carboxymuconolactone decarobxylase (EC 4.1.1.44) and muconolactone isomerase (EC 5.3.3.4) mediate chemically analogous reactions in bacteria. The enzymes are inducible, and different metabolites trigger the respective syntheses of the decarboxylases in Acinetobacter calcoaceticus and Pseudomonas putida. The decarobxylases share similar oligomeric structures in which identical subunits of about 13,300 daltons appear to be self-associated into hexamers. Identical residues are found in 18 of the first 36 positions of the enzymes' NH2-terminal amino acid sequences. Thus, genetic rearrangements appear to have placed homologous structural genes for the decarboxylases under different transcriptional control in the two bacterial species. The NH2-terminal amino acid sequences of the decarboxylases and muconolactone isomerases are similar, suggesting that a common ancestral protein gave rise to the enzymes with different (albeit analogous) activities. In addition, the NH2-terminal amino acid sequences of the decarboxylases appear to have been conserved at a second region within the primary structure of the muconolactone isomerases. As has been observed with the two enol-lactone hydrolases (EC 3.1.3.24) of Acinetobacter, the structural genes for the decarboxylases and the isomerases appear to have diverged widely as they were co-selected within a single cell line, In part the divergence appears to have been achieved by mutations in which fragments of DNA within structural genes are replaced with fragments of DNA derived from a co-evolving sequence.