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pubmed:abstractText |
The TOL catabolic genes in Pseudomonas putida (pWW0) are clustered in the upper operon, encoding enzymes for the conversion of toluene and xylenes to benzoate and toluates, and the meta-cleavage operon, encoding enzymes for the conversion of the benzoate and toluates to tricarboxylic acid cycle intermediates. In this study, it was shown that cells growing in a chemostat under succinate growth-limiting conditions express both the upper and meta-cleavage pathways in response to o-xylene, a nonmetabolizable effector of the XylR regulatory protein. The dilution rate maintained in the succinate-limited chemostat cultures influenced the synthesis levels of TOL pathway enzymes, their steady-state levels, and their turnover rates. Cells growing in the presence of nonlimiting concentrations of succinate in continuous culture did not express 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 nonmetabolizable effector of the XylS regulatory protein, was 93% lower in cultures exposed to succinate at nonlimiting concentrations than in the succinate-limited chemostats. The mRNA level of xylS during nonlimited growth on succinate was very low compared with that in succinate-limited cultures, suggesting that suppression of expression of the meta-cleavage pathway is regulated mainly by the level of the XylS regulator.
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