Abstract Escherichia coli NADPH-dependent assimilatory sulfite reductase is responsible for fixing sulfur for incorporation into sulfur-containing biomolecules. The oxidoreductase is composed of two subunits, an NADPH, FMN, and FAD-binding diflavin reductase and an iron siroheme and Fe 4 S 4 -containing oxidase. How they interact has been an unknown for over 50 years because the complex is highly flexible, thus has been intransigent for traditional X-ray or cryo-EM structural analysis. Using a combination of the chameleon plunging system with a fluorinated lipid we overcame the challenge of preserving the minimal dimer between the subunits for high-resolution cryo-EM analysis. Here, we report the first structure of the complex between the reductase and oxidase, revealing how they interact in a minimal interface. Further, we determined the structural elements that discriminate between the pairing of a siroheme-containing oxidase with a diflavin reductase or a ferredoxin partner to channel the six electrons that reduce sulfite to sulfide. Significance Statement Sulfur is one of the essential building blocks of life. Sulfur exists in numerous redox states but only one can be incorporated into biomass – S 2- (sulfide). In Escherichia coli , a protein enzyme called sulfite reductase reduces sulfite by six electrons to make sulfide. Typical electron transfer reactions move one or two electrons at a time, so this chemistry is unique. To do so, E. coli uses a two protein complex with unique co-enzymes. To date, how the subunits interact so the co-enzymes can transfer electrons has remained a mystery because the complex is structurally dynamic, thus difficult to analyze with traditional methods. This study shows for the first time the structure of the enzyme complex that performs this unique chemistry.

Dimerization of assimilatory NADPH-dependent sulfite reductase reveals elements for diflavin reductase binding at a minimal interface