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Journal of Bacteriology, May 2007, p. 3502-3514, Vol. 189, No. 9
0021-9193/07/$08.00+0     doi:10.1128/JB.01098-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Characterization of a Pseudomonad 2-Nitrobenzoate Nitroreductase and Its Catabolic Pathway-Associated 2-Hydroxylaminobenzoate Mutase and a Chemoreceptor Involved in 2-Nitrobenzoate Chemotaxis ^{, ,}

Department of Biotechnology, Faculty of Engineering and High Technology Research Center, Kansai University, Suita, Osaka 564-8680, Japan,¹ Biotechnology Research Institute, National Research Council Canada, Montreal, Quebec H4P 2R2, Canada²

Received 24 July 2006/ Accepted 25 January 2007

Pseudomonas fluorescens strain KU-7 is a prototype microorganism that metabolizes 2-nitrobenzoate (2-NBA) via the formation of 3-hydroxyanthranilate (3-HAA), a known antioxidant and reductant. The initial two steps leading to the sequential formation of 2-hydroxy/aminobenzoate and 3-HAA are catalyzed by a NADPH-dependent 2-NBA nitroreductase (NbaA) and 2-hydroxylaminobenzoate mutase (NbaB), respectively. The 216-amino-acid protein NbaA is 78% identical to a plasmid-encoded hypothetical conserved protein of Polaromonas strain JS666; structurally, it belongs to the homodimeric NADH:flavin mononucleotide (FMN) oxidoreductase-like fold family. Structural modeling of complexes with the flavin, coenzyme, and substrate suggested specific residues contributing to the NbaA catalytic activity, assuming a ping-pong reaction mechanism. Mutational analysis supports the roles of Asn40, Asp76, and Glu113, which are predicted to form the binding site for a divalent metal ion implicated in FMN binding, and a role in NADPH binding for the 10-residue insertion in the ß5-2 loop. The 181-amino-acid sequence of NbaB is 35% identical to the 4-hydroxylaminobenzoate lyases (PnbBs) of various 4-nitrobenzoate-assimilating bacteria, e.g., Pseudomonas putida strain TW3. Coexpression of nbaB with nbaA in Escherichia coli produced a small amount of 3-HAA from 2-NBA, supporting the functionality of the nbaB gene. We also showed by gene knockout and chemotaxis assays that nbaY, a chemoreceptor NahY homolog located downstream of the nbaA gene, is responsible for strain KU-7 being attracted to 2-NBA. NbaY is the first chemoreceptor in nitroaromatic metabolism to be identified, and this study completes the gene elucidation of 2-NBA metabolism that is localized within a 24-kb chromosomal locus of strain KU-7.

Published ahead of print on 2 February 2007.

This publication is issued as NRCC number 49029.

Supplemental material for this article may be found at http://jb.asm.org/.