Comamonas strain CNB-1 was isolated from a biological reactor treating wastewater from a p-chloronitrobenzene production factory. Strain CNB-1 used p-chloronitrobenzene as sole source of carbon, nitrogen, and energy. A 2-aminophenol 1,6-dioxygenase was purified from cells of strain CNB-1. The purified 2-aminophenol 1,6-dioxygenase had a native molecular mass of 130 kDa and was composed of alpha- and beta-subunits of 33 and 38 kDa, respectively. This enzyme is different from currently known 2-aminophenol 1,6-dioxygenases in that it: (a) has a higher affinity for 2-amino-5-chlorophenol (K(m)=0.77 microM) than for 2-aminophenol (K(m)=0.89 microM) and (b) utilized protocatechuate as a substrate. These results suggested that 2-amino-5-chlorophenol, an intermediate during p-chloronitrobenzene degradation, is the natural substrate for this enzyme. N-terminal amino acids of the alpha- and beta-subunits were determined to be T-V-V-S-A-F-L-V and M-Q-G-E-I-I-A-E, respectively. A cosmid library was constructed from the total DNA of strain CNB-1 and three clones (BG-1, BG-2, and CG-13) with 2-aminophenol 1,6-dioxygenase activities were obtained. DNA sequencing of clone BG-2 revealed a 15-kb fragment that contained two ORFs, ORF9 and ORF10, with N-terminal amino acid sequences identical to those of the beta- and alpha-subunits, respectively, from the purified 2-aminophenol 1,6-dioxygenase. The enzyme was actively synthesized when the genes coding for the ORF9 and ORF10 were cloned into Escherichia coli.