INDUCIBILITY OF THE TOL CATABOLIC PATHWAY IN PSEUDOMONAS-PUTIDA (PWW0) GROWING ON SUCCINATE IN CONTINUOUS-CULTURE - EVIDENCE OF CARBON CATABOLITE REPRESSION CONTROL

The TOL catabolic genes in Pseudomonas putida(pWW0) are clustered in t he upper operon, encoding enzymes for the conversion of toluene and xy lenes to benzoate and toluates, and the meta-cleavage operon, encoding enzymes for the conversion of the benzoate and toluates to tricarboxy lic acid cycle intermediates. In this study, it was shown that cells g rowing in a chemostat under succinate growth-limiting conditions expre ss both the upper and meta-cleavage pathways in response to o-xylene, a nonmetabolizable effector of the XylR regulatory protein. The diluti on rate maintained in the succinate-limited chemostat cultures influen ced the synthesis levels of TOL pathway enzymes, their steady-state le vels, and their turnover rates. Cells growing in the presence of nonli miting concentrations of succinate in continuous culture did not expre ss pathway enzymes in response to the addition of o-xylene, which was due to a blockage at the transcriptional level. Expression of the meta -cleavage pathway in response to 2,3-dimethylbenzoate, a nonmetaboliza ble effector of the XylS regulatory protein, was 93% lower in cultures exposed to succinate at nonlimiting concentrations than in the succin ate-limited chemostats. The mRNA level of xylS during nonlimited growt h on succinate was very low compared with that in succinate-limited cu ltures, suggesting that suppression of expression of the meta-cleavage pathway is regulated mainly by the level of the XylS regulator.