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The protein components of the 2-nitrotoluene (2NT) and nitrobenzene dioxygenase enzyme systems from Acidovorax sp. strain JS42 and Comamonas sp. strain JS765, respectively, were purified and characterized. These enzymes catalyze the initial step in the degradation of 2-nitrotoluene and nitrobenzene. The identical shared reductase and ferredoxin components were monomers of 35 and 11.5 kDa, respectively. The reductase component contained 1. Nitroaromatic compounds are relatively rare in nature but are widely used in the chemical industry for the production of dyes, resins, pesticides, herbicides, explosives, and other useful materials such as polyurethane foams (27). Because these compounds have been introduced into the environment quite recently, bacteria have had little time to adapt to the presence of these new chemicals and evolve new pathways for their degradation. Numerous aerobic bacteria that are capable of utilizing specific nitroaromatic compounds as sole carbon, nitrogen, and energy sources have been isolated from contaminated soils and groundwaters (27). Acidovorax (formerly Pseudomonas [25]) sp. strain JS42 was isolated from a nitrobenzene-contaminated industrial facility in Mississippi by selection for growth on 2-nitrotoluene (2NT) (20). Comamonas sp. strain JS765, which was isolated from an industrial waste treatment plant in New Jersey, utilizes nitrobenzene as a sole carbon and nitrogen source (28). In each strain, only a single new enzymatic reaction is required to convert the nitroarene compound to an easily degraded, naturally occurring product. Nitrobenzene and 2NT degradation by Comamonas sp. strain JS765 and Acidovorax sp. strain JS42 is initiated by closely related multicomponent dioxygenase enzyme systems. The nitrobenzene dioxygenase system (NBDO) and 2-nitrotoluene dioxygenase system (2NTDO) add both atoms of molecular oxygen to the aromatic ring (1, 24). The proposed unstable nitrohydrodiol products rearrange, forming catechol or 3-methylcatechol with simultaneous release of nitrite (Fig. 1).NBDO and 2NTDO are members of the Rieske nonheme iron dioxygenase family, which includes the well-studied naphthalene dioxygenase enzyme system (NDO) (42). Enzymatic attack at the nitro-substituted carbon by NBDO and 2NTDO is critical for the elimination of nitrite, and this type of reaction has not been demonstrated for any of the related aromatic hydrocarbon dioxygenases. Each enzyme system is composed of three protein components: an iron-sulfur flavoprotein reductase, a Rieske [2Fe-2S] ferredoxin, and a Rieske [2Fe-2S] nonheme iron dioxygenase. The reductase and ferredoxin components of NBDO and 2NTDO transfer electrons from NADH to the oxygenase and are identical in sequence (24). Although the oxygenases from these enzyme systems have very similar deduced amino acid sequences, they have distinct but * Corresponding author. Section

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