<p><b>Upper panel</b>: Intermediates of cysteine biosynthesis in mammals and bacteria. The red arrows indicate the biosynthetic pathway in mammals and the yellow arrows the biosynthetic pathway in bacteria. <b>Lower panel</b>: Sulfur assimilation in bacteria. Sulfate and thiosulfate are the most abundant forms of extracellular sulfur, the latter being predominant under less oxidizing conditions. Inorganic sulfur enters the cells through specific transporters. In contrast to OASS-A, OASS-B can directly use thiosulfate for cysteine biosynthesis. The product S-sulfo-L-cysteine is reduced by glutaredoxins to cysteine and sulfide that enters in the last step of the sulfate reduction pathway <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077558#pone.0077558-Sekowska1" target="_blank">[120]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077558#pone.0077558-Hatzios2" target="_blank">[121]</a>.</p
