A newly isolated denitrifying bacterium, Thauera sp. strain DNT-1, grew on toluene as the sole carbon and energy source under both aerobic and anaerobic conditions. When this strain was cultivated under oxygenlimiting conditions with nitrate, first toluene was degraded as oxygen was consumed, while later toluene was degraded as nitrate was reduced. Biochemical observations indicated that initial degradation of toluene occurred through a dioxygenase-mediated pathway and the benzylsuccinate pathway under aerobic and denitrifying conditions, respectively. Homologous genes for toluene dioxygenase (tod) and benzylsuccinate synthase (bss), which are the key enzymes in aerobic and anaerobic toluene degradation, respectively, were cloned from genomic DNA of strain DNT-1. The results of Northern blot analyses and real-time quantitative reverse transcriptase PCR suggested that transcription of both sets of genes was induced by toluene. In addition, the tod genes were induced under aerobic conditions, whereas the bss genes were induced under both aerobic and anaerobic conditions. On the basis of these results, it is concluded that strain DNT-1 modulates the expression of two different initial pathways of toluene degradation according to the availability of oxygen in the environment.Benzene, toluene, ethylbenzene, and xylenes (BTEX) are one of the most common groups of groundwater contaminants. Of these contaminants, toluene is degraded by many strains of aerobic bacteria, and five different aerobic toluene degradation pathways have been identified. Burkholderia cepacia G4, Ralstonia pickettii PKO1, and Pseudomonas mendocina KR1 first oxidize toluene using specific monooxygenases to form o-, m-, and p-cresol, respectively (34, 35, 52). The cresols formed by strains G4 and PKO1 undergo a second monooxygenation to form 3-methylcatechol, which is then degraded by a meta ring fission pathway (35,45). In strain KR1, the methyl group of p-cresol is oxidized, and the resulting 4-hydroxybenzoate is degraded by an ortho cleavage pathway (51). On the other hand, Pseudomonas putida mt-2 oxidizes the methyl group of toluene to form benzoic acid, which is further metabolized through a meta cleavage pathway via catechol (4). P. putida F1 carries the chromosomally encoded tod pathway. Toluene dioxygenase (TodC1C2BA) oxidizes toluene to cis-toluene dihydrodiol. Then, toluene dihydrodiol dehydrogenase (TodD) transforms the dihydrodiol to 3-methylcatechol, which is cleaved by the meta fission enzyme 3-methylcatechol 2,3-
