Ferredoxin-NADP + Reductase from Pseudomonas putida Functions as a Ferric Reductase

Pseudomonas putida harbors two ferredoxin-NADP+ reductases (Fprs) on its chromosome, and their functions remain largely unknown. Ferric reductase is structurally contained within the Fpr superfamily. Interestingly, ferric reductase is not annotated on the chromosome ofP. putida . In an effort to elucidate the function of the Fpr as a ferric reductase, we used a variety of biochemical and physiological methods using the wild-type and mutant strains. In both the ferric reductase and flavin reductase assays, FprA and FprB preferentially used NADPH and NADH as electron donors, respectively. Two Fprs prefer a native ferric chelator to a synthetic ferric chelator and utilize free flavin mononucleotide (FMN) as an electron carrier. FprB has a higherk cat /Km value for reducing the ferric complex with free FMN. The growth rate of thefprB mutant was reduced more profoundly than that of thefprA mutant, the growth rate of which is also lower than the wild type in ferric iron-containing minimal media. Flavin reductase activity was diminished completely when the cell extracts of thefprB mutant plus NADH were utilized, but not thefprA mutant with NADPH. This indicates that other NADPH-dependent flavin reductases may exist. Interestingly, the structure of the NAD(P) region of FprB, but not of FprA, resembled the ferric reductase (Fre) ofEscherichia coli in the homology modeling. This study demonstrates, for the first time, the functions of Fprs inP. putida as flavin and ferric reductases. Furthermore, our results indicated that FprB may perform a crucial role as a NADH-dependent ferric/flavin reductase under iron stress conditions.