Gene Cloning, Purification, and Characterization of NfsB, a Minor Oxygen-Insensitive Nitroreductase from Escherichia coli, Similar in Biochemical Properties to FRase I, the Major Flavin Reductase in Vibrio fischeri

nfsB, encoding a minor oxygen-insensitive nitroreductase, was isolated by PCR using primers corresponding to two amino acid sequences conserved among the major flavin reductase from Vibrio fischeri and classical nitroreductases from Salmonella typhimurium and Enterobacter cloacae. The gene product, NfsB, was purified to homogeneity from extracts of Escherichia coli cells overexpressing it. NfsB was found to be situated at 13 min on the E. coli map. Biochemical analysis indicated NfsB to be a polypeptide having a calculated molecular weight of 23,904, capable of forming a homodimer and associated tightly with FMN as a prosthetic group. Although it exhibited a lower affinity to the NfsB apoenzyme than FMN, FAD could serve as an effective substitute for FMN. It was also shown that NfsB has a broad electron acceptor specificity and is associated with a low level of the NAD(P)H-flavin oxidoreductase. The NfsB catalysis obeys the ping pong Bi-Bi mechanism. The Km value for NADH varied depending on the second substrate used.