Fig. 4. Biosynthesis pathway of s4U (position 8). In T. thermophilus (black arrow), the synthesis involves desulfurase IscS or SufS initiates the persulfide transfer to TtuB which is acyl-adenylated by TtuC. TtuA is the final modifying enzyme. In E coli (green arrow), IscS transfers perfulfide sulfur to ThiI in the Rhd domain as the sulfur acceptor site. In B. subtilis (red arrow), NifZ acts as desulfurase enzyme and ThiI can directly receive persulfide sulfur from NifZ despite lacking Rhd domain.
