Zero-valent sulfur is the product of assimilatory sulfite reductase and a substrate of cysteine synthase

Sulfate reduction is an important link in the global sulfur cycle. Assimilatory sulfate reduction consists of sulfate reduction to hydrogen sulfide (H₂S) and then H₂S assimilation for cysteine synthesis. This concept was founded in the 1950s and has not been challenged until today. In this study, we re-examined four representative assimilatory sulfite reductases (aSiRs) and found that these enzymes directly reduced sulfite to zero-valent sulfur (S⁰). A small portion of produced S⁰ was reduced to H₂S via aSiR-independent chemical reactions. Further, we re-examined six representative cysteine synthases and found that these enzymes used S⁰ derivative compounds (thiosulfate and glutathione persulfide) as substrates to synthesize cysteine. Thus, S⁰ is the key intermediate of assimilatory sulfate reduction process, rather than H₂S.