Current understanding of sulfur assimilation metabolism to biosynthesize L-cysteine and recent progress of its fermentative overproduction in microorganisms.

To all organisms, sulfur is an essential and important element. The assimilation of inorganic sulfur molecules such as sulfate and thiosulfate into organic sulfur compounds such as L-cysteine and L-methionine (essential amino acid for human) is largely contributed by microorganisms. Of these, special attention is given to thiosulfate (S2O32-) assimilation, because thiosulfate relative to often utilized sulfate (SO42-) as a sulfur source is proposed to be more advantageous in microbial growth and biotechnological applications like L-cysteine fermentative overproduction toward industrial manufacturing. In Escherichia coli as well as other many bacteria, the thiosulfate assimilation pathway is known to depend on O-acetyl-L-serine sulfhydrylase B. Recently, another yet-unidentified CysM-independent thiosulfate pathway was found in E. coli. This pathway is expected to consist of the initial part of the thiosulfate to sulfite (SO32-) conversion, and the latter part might be shared with the final part of the known sulfate assimilation pathway [sulfite → sulfide (S2-) → L-cysteine]. The catalysis of thiosulfate to sulfite is at least partly mediated by thiosulfate sulfurtransferase (GlpE). In this mini-review, we introduce updated comprehensive information about sulfur assimilation in microorganisms, including this topic. Also, we introduce recent advances of the application study about L-cysteine overproduction, including the GlpE overexpression.