Plant sulfite reductase contains the siroheme and the [4Fe-4S] cluster as catalytically active redox centers and catalyzes the six-electron reductions of sulfite and nitrite using electrons donated from ferredoxin. A heterologous expression of a cDNA for maize sulfite reductase in E. coli has enabled us to produce the wild-type and mutant enzymes. Putative substrate-binding basic residues, located at the siroheme distal side, have been substituted for other residues with neutral or negatively charged side chains. Kinetic studies of the resulting mutant enzymes have demonstrated that substrate specificity for the two anions is remarkably changed by amino acid substitutions at a single site. We have also produced two classes of ferredoxin mutants with less ability to donate electrons to sulfite reductase: one with a defect in the recognition of the partner enzyme and the other with an unfavorable redox property. This article summarizes our knowledge about the structure function relationships of plant sulfite reductase.