Differential interaction of maize root ferredoxin : NADP(+) oxidoreductasewith photosynthetic and non-photosynthetic ferredoxin isoproteins

In higher plants ferredoxin (Fd):NADP(+) oxidoreductase (FNR) and Fd are ea ch distributed in photosynthetic and nonphotosynthetic organs as distinct i soproteins. We have cloned cDNAs for leaf FNR (L-FNR I and L-FNR II) and ro ot FNR (R-FNR) from maize (Zen mays L.), and produced recombinant L-FNR I a nd R-FNR to study their enzymatic functions through kinetic and Fd-binding analyses. The K-m value obtained by assay for a diaphorase activity indicat ed that R-FNR had a 10-fold higher affinity for NADPH than L-FNR I. When we assayed for NADPH-cytochrome c reductase activity using maize photosynthet ic Fd (Fd I) and non-photosynthetic Fd (Fd III), the R-FNR showed a marked difference in affinity between these two Fd isoproteins; the F-m for Fd III was 3.0 mu M and that for Fd I was 29 mu M. Consistent with this, the diss ociation constant for the R-FNR:Fd III complex was 10-fold smaller than tha t of the R-FNR:Fd I complex. This differential binding capacity was confirm ed by an affinity chromatography of R-FNR on Fd-sepharose with stronger bin ding to Fd III. L-FNR I showed no such differential interaction with Fd I a nd Fd III. These data demonstrated that R-FNR has the ability to discrimina te between these two types of Fds. We propose that the stronger interaction of R-FNR with Fd III is crucial for an efficient electron flux of NADPH-FN R-FIL cascade, thus supporting Fd-dependent metabolism in non-photosyntheti c organs.