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.
Cao et al. (Mon,) studied this question.