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Sulfite reductase (SiR) catalyzes the reduction of sulfite to sulfide in chloroplasts and root plastids using ferredoxin (Fd) as an electron donor. Using purified maize (Zea mays L.) SiR and isoproteins of Fd and Fd-NADP+reductase (FNR), we reconstituted illuminated thylakoid membrane- and NADPH-dependent sulfite reduction systems. Fd I and L-FNR were distributed in leaves and Fd III and R-FNR in roots. The stromal concentrations of SiR and Fd I were estimated at 1.2 and 37 μm, respectively. The molar ratio of Fd III to SiR in root plastids was approximately 3:1. Photoreduced Fd I and Fd III showed a comparable ability to donate electrons to SiR. In contrast, when being reduced with NADPH via FNRs, Fd III showed a several-fold higher activity than Fd I. Fd III and R-FNR showed the highest rate of sulfite 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 high NADPH/NADP+ ratio are crucial for non-photosynthetic sulfite reduction. In accordance with this view, a cysteine-auxotrophicEscherichia coli mutant defective for NADPH-dependent SiR was rescued by co-expression of maize SiR with Fd III but not with Fd I.

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