The TOL catabolic genes in Pseudomonas putida(pWWO) 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 elector 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 metacleavage pathway is regulated mainly by the level of the XylS regulator.Many soil bacteria are known to be capable of mineralizing aromatic compounds. The most extensively studied organism in this respect is Pseudomonas putida(pWWO), which can utilize toluene, m-and p-xylene, pseudocumene, and mi-ethyltoluene as sole sources of carbon and energy. The genetic information for the transformation of these aromatics to central metabolites is harbored by the transmissible TOL plasmid pWWO (3). Figure 1 shows the genetic organization of the catabolic operons. Expression of the structural genes on

Bordetella holmesii is a Gram-negative bacterium first identified in 1995. It can cause pertussis-like symptoms in humans. B. holmesii contains insertion sequences IS481 and IS1001, two frequently used targets in the PCR diagnosis of Bordetella pertussis and Bordetella parapertussis infections.

Pseudomonas puida mt-2, P. cepacia G4, P. mendocina KR1, and P. putida Fl degrade toluene through different pathways. In this study, we compared the competition behaviors of these strains in chemostat culture at a low growth rate (D = 0.05 h-'), with toluene as the sole source of carbon and energy. Either toluene or oxygen was growth limiting. Under toluene-limiting conditions, P. mendocina KR1, in which initial attack is by monooxgygenation of the aromatic nucleus at the para position, outcompeted the other three strains. Under oxygen limitation, P. cepacia G4, which hydroxylates toluene in the ortho position, was the most competitive strain. P. putida mt-2, which metabolizes toluene via oxidation of the methyl group, was the least competitive strain under both growth conditions. The apparent superiority of strains carrying toluene degradation pathways that start degradation by hydroxylation of the aromatic nucleus was also found during competition experiments with pairs of strains of P. cepacia, P. fluorescence, and P. putida that were freshly isolated from contaminated soil.

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