The yeast assimilatory sulfite reductase is a complex enzyme that is responsible for conversion of sulfite into sulfide. To obtain information on the nature of this enzyme, we In Saccharomyces cerevisiae, sulfate taken up from the surrounding environment is converted into sulfite through three enzymatic steps. Sulfite is further reduced to sulfide, which in turn combines with O-acetyl homoserine to form homocysteine and eventually methionine. A central step in this assimilation pathway is conversion of sulfite into sulfide, a six-electron transfer reaction catalyzed by the yeast assimilatory sulfite reductase (YSiR). YSiR was found to be NADPH dependent (35) and to contain the prosthetic groups flavin adenine dinucleotide (FAD), flavin mononucleotide (FMN), an iron-sulfur cluster and a nonflavin chromophore (61) later identified as siroheme (46), a uroporphyrinogen III derivative also found in bacterial sulfite reductases (33,34), and nitrite reductase (21). Some methionine auxotrophs of S. cerevisiae were found to possess a defective sulfite reductase devoid of FAD or of FAD and FMN (62). The mutants devoid only of FAD were later assigned to the metlO complementation group and shown to be affected solely in the sulfite reduction step (30,31). The results of Kobayashi and Yoshimoto (25) suggested that YSiR has an ct212 structure and that the molecular mass of