The fluorene catabolic linear plasmid in Terrabacter sp strain DBF63 carries the β-ketoadipate pathway genes, pcaRHGBDCFIJ, also found in proteobacteria

Terrabacter sp. strain DBF63 is capable of degrading fluorene (FN) to tricarboxylic acid cycle intermediates via phthalate and protocatechuate. Genes were identified for the protocatechuate branch of the beta-ketoadipate pathway (pcaR, pcaHGBDCFIJ) by sequence analysis of a 70 kb DNA region of the FN-catabolic linear plasmid pDBF1. RT-PCR analysis of RNA from DBF63 cells grown with FN, dibenzofuran, and protocatechuate indicated that the pcaHGBDCFIJ operon was expressed during both FN and protocatechuate degradation in strain DBF63. The gene encoding beta-ketoadipate enol-lactone hydrolase (pcaD) was not fused to the next gene, which encodes gamma-carboxymuconolactone decarboxylase (pcaC), in strain DBF63, even though the presence of the pcaL gene (the fusion of pcaD and pcaC) within a pca gene cluster has been thought to be a Gram-positive trait. Quantitative RT-PCR analysis revealed that pcaD mRNA levels increased sharply in response to protocatechuate, and a biotransformation experiment with cis,cis-muconate using Escherichia coli carrying both catBC and pcaD indicated that PcaD exhibited beta-ketoadipate enol-lactone hydrolase activity. The location of the pca gene cluster on the linear plasmid, and the insertion sequences around the pca gene cluster suggest that the ecologically important beta-ketoadipate pathway genes, usually located chromosomally, may be spread widely among bacterial species via horizontal transfer or transposition events.