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

UNLABELLED: The process of the assimilatory sulfate reduction consists of sulfate reduction to hydrogen sulfide (H2S) and then H2S assimilation for cysteine synthesis. However, recent studies indicate that both H2S and zero-valent sulfur (S0) are produced during dissimilatory sulfate reduction, which leads to the question of whether S0 is also involved in assimilatory sulfate reduction. In this study, phylogenetic analysis grouped assimilatory sulfite reductases into five clusters, and we re-examined four representatives from two major clusters. These enzymes produced both S0 and H2S from sulfite. However, S0 was a direct product, while H2S was a derivative of S0 via chemical reduction. Phylogenetic analysis of cysteine synthases grouped them into six clusters, and we re-examined six representatives from two major clusters. These enzymes used both glutathione persulfide (GSSH, a common intracellular compound containing a S0 atom) and H2S as substrates to synthesize cysteine, but preferred to use GSSH. These findings indicate that S0, rather than H2S, is the key intermediate during the process of inorganic sulfur conversion to organic sulfur.

IMPORTANCE: 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.