Analysis of Reductant Supply Systems for Ferredoxin-Dependent Sulfite Reductase in Photosynthetic and Nonphotosynthetic Organs of Maize1

摘要

Sulfite reductase (SiR) catalyzes thereduction of sulfite to sulfide in chloroplasts and root plastids usingferredoxin (Fd) as an electron donor. Using purified maize (Zeamays L.) SiR and isoproteins of Fd and Fd-NADP+reductase (FNR), we reconstituted illuminated thylakoid membrane- andNADPH-dependent sulfite reduction systems. Fd I and L-FNR weredistributed in leaves and Fd III and R-FNR in roots. The stromalconcentrations of SiR and Fd I were estimated at 1.2 and 37μm, respectively. The molar ratio of Fd III to SiR inroot plastids was approximately 3:1. Photoreduced Fd I and Fd IIIshowed a comparable ability to donate electrons to SiR. In contrast,when being reduced with NADPH via FNRs, Fd III showed a several-foldhigher activity than Fd I. Fd III and R-FNR showed the highest rate ofsulfite reduction among all combinations tested. NADP+decreased the rate of sulfite reduction in a dose-dependent manner.These results demonstrate that the participation of Fd III and highNADPH/NADP+ ratio are crucial for non-photosyntheticsulfite reduction. In accordance with this view, a cysteine-auxotrophicEscherichia coli mutant defective for NADPH-dependentSiR was rescued by co-expression of maize SiR with Fd III but not withFd I.