In order to elucidate the importance of a ferredoxin (Fd) Arg-Glu pair involved in dynamic exchange from intra-to intermolecular salt bridges upon complex formation with ferredoxin-NADP + oxidoreductase (FNR), Equisetum arvense FdI and FdII were investigated as normal and the pair-lacking Fd, respectively. The FdI mutant lacking this pair was unstable and rapidly lost the [2Fe^2S] cluster. The catalytic constant (k cat ) of the electron transfer for FdI is 5.5 times that for FdII and the introduction of this pair into FdII resulted in the increase of k cat to a level comparable to that for FdI, demonstrating directly that the Arg-Glu pair is important for e⁄cient electron transfer between Fd and FNR.