Evidence that enzymes of a novel aerobic 2‐amino‐benzoate metabolism in denitrifying Pseudomonas are coded on a small plasmid

Abstract

A new pathway for aerobic metabolism of 2‐aminobenzoate which proceeds via anthranoyl‐CoA has recently been revealed in a Pseudomonas strain KB740. This bacterial strain was found to contain a small 8.1‐kbp plasmid pKB740 which appears to harbour the genes encoding for two key enzymes catalyzing the initial reactions of the pathway, 2‐aminobenzoate coenzyme A ligase and 2‐aminobenzoyl‐coenzyme A monooxygenase/reductase. The evidence is as follows: The plasmid content of the culture varied by a factor of ten depending on the growth substrates; it was highest when cells were grown aerobically on 2‐aminobenzoate. The plasmid pKB740 could be introduced into Escherichia coli strain JM83 by transformation. Wild‐type E. coli and E. coli JM83 are unable to metabolize 2‐aminobenzoate whereas the transformed E. coli JM83 cells could grow with this aromatic compound as sole organic substrate and oxidize it completely to CO2. The plasmid recovered from E. coli had the same restriction map as the original plasmid, but was dimerized. The two key enzyme activities were demonstrated in the transformed E. coli in sufficiently high amounts to explain growth. They appear to be regulated on the transcription level by induction; they were formed only during aerobic growth in the presence of 2‐aminobenzoate, as in the parent Pseudomonas. The N‐terminal amino acid sequence of 2‐aminobenzoyl‐CoA monooxygenase/reductase was similar to the consensus sequence of the FAD binding site of different flavoenzymes. The data also prove that the enzyme with two flavin functions is an α2 homodimer. Southern blotting of digested chromosomal and plasmid DNA and hybridization against a labelled 15‐base oligonucleotide derived from the N‐terminal amino acid sequence of 2‐aminobenzoyl‐CoA monooxygenase/ reductase revealed that the gene for this enzyme is coded on the plasmid rather than on the chromosome. The gene was localized on a 3.2‐kbp restriction fragment. The formation of 2‐aminobenzoyl‐CoA monooxygenase/reductase protein in transformed E. coli was demonstrated by Western blotting of proteins of cell extracts separated by SDS/PAGE. The enzyme protein band, which was stained by a procedure based on antibodies against 2‐aminobenzoyl‐CoA monooxygenase/reductase, was demonstrated in transformed E. coli. Copyright © 1990, Wiley Blackwell. All rights reserved