Ferredoxin reductase BphA4 was well known as a component of biphenyl dioxygenase. However, there was little information about whether it could utilize nonphysiological oxidants as electron acceptors. In the present study, we reported the novel nitroreductase activity of BphA4(LA-4). The homology model of ferredoxin reductase BphA4 from Dyella ginsengisoli LA-4 was constructed. According to the alignment of three-dimensional structures, it was supposed that BphA4(LA-4) could function as nitroreductase. Recombinant His-tagged BphA4(LA-4) was purified with a molecular mass of 49.6 +/- 1 kDa. Biochemical characterization of purified BphA4(LA-4) possessed the nitroreductase activity with the optimal temperature 50A degrees C and pH 8.0. The substrate spectrum and kinetics indicated BphA4(LA-4) could reduce several nitroaromatics with different apparent K (m) values: m-dinitrobenzene (560 mu M), o-dinitrobenzene (1,060 mu M), o-nitroaniline (1,570 mu M), m-nitrobenzoic acid (1,300 mu M) and m-nitrophenol (67 mu M). The nitroreductase activity was further explained by docking studies, which was indicated that Arg 288 should play an important role in binding nitroaromatics. Moreover, there existed a good linear correlation between lnK (m) and calculated binding energy.