Induction of the halobenzoate catabolic pathway and cometabolism of ortho-chlorobenzoates in Pseudomonas aeruginosa 142 grown on glucose-supplemented media

The aerobic cometabolism of ortho-substituted chlorobenzoates by Pseudomonas aeruginosa strain 142 growing on glucose-supplemented medium was analyzed. The strain, which can use 2-chlorobenzoate (2-CBA) and 2,4-dichlorobenzoate (2,4-DCBA) as sole carbon and energy sources, showed high rates of 2-CBA metabolism in glucose-fed cells. In contrast, 2,4-DCBA was metabolized only after extended incubation of the full grown culture and depletion of glucose. In addition to the ortho-dehalogenation (ohb(142)) genes encoding the alpha and beta subunits of the oxygenase component of a 2-halobenzoate dioxygenase, strain 142 harbours a closely related ohbABCDFG gene cluster previously identified in P. aeruginosa JB2 (ohb(JB2)). The genes for the chlorocatechol ortho-catabolic pathway were identified and sequenced in this strain, showing a near complete identity with the clcABD operon of the pAC27 plasmid. Relative quantification of mRNA by RT-PCR shows a preferential induction of ohb(142) by 2-CBA, which is abolished in glucose-grown cultures. The alternate ohb(JB2) and clc genes were expressed preferentially in 2,4-DCBA grown cultures. Only ohb(JB2) appears to be expressed in the presence of the carbohydrate. Detection of chlorocatechol-1,2-dioxygenase activity in 2,4-DCBA plus glucose grown cultures suggests the presence of an alternate system for the ortho-cleavage of chlorobenzoates. The recruitment of elements from two halobenzoate dioxygenase systems with different induction patterns, together with a chlorocatechol degradative pathway not repressed by carbon catabolite, may allow P. aeruginosa 142 to cometabolize haloaromatics in carbohydrate grown cultures.