(A) Time dependent increase in the intensity of the 523 nm absorbance due to the formation of Fe[(bipy)3]2+ upon addition of NADPH (1.5 mM final concentration) to a solution containing BfrB (0.375 µM), FPR (15 µM), bipy (3 mM) and (●) no Bfd, (○) 15 µM wild type holo-Bfd, and (▲) wild type apo-Bfd. (B) Before the arrow: time dependent absorption changes in the intensity of the 523 nm caused by sequestration of the iron in the [2Fe-2S] cluster of (●) 15 µM wild type holo-Bfd and (○) 15 µM holo-C43S Bfd by bipy (3 mM); the solution contained 15 µM sodium dithionite to reduce the two iron ions in holo-Bfd. After the arrow: the resultant solution containing 15 µM apo-Bfd (●) or apo-C43S-Bfd (○) was made 0.375 µM in BfrB and 15 µM in FPR, before addition of NADPH (1.5 mM) started the rapid time dependent increase in the 523 nm absorption.