A sulfurtransferase is required in the transfer of cysteine sulfur in the in vitro synthesis of molybdopterin from precursor Z in Escherichia coli

It has been shown that conversion of precursor Z to molybdopterin (MPT) by Escherichia coli MPT synthase entails the transfer of the sulfur atom of the C-terminal thiocarboxylate from the small subunit of the synthase to generate the dithiolene group of MPT and that the moeB mutant E. coli contains inactive MPT synthase devoid of the thiocarboxylate. The data presented here demonstrate that L-cysteine can serve as the source of the sulfur for the biosynthesis of MPT in vitro but only in the presence of a persulfide containing sulfurtransferase such as IscS, cysteine sulfinate desulfinase (CSD), or CsdB. A fully defined in vitro system has been developed in which an inactive form of MPT synthase can be activated by incubation with MoeB, Mg ATP, L-cysteine, and one of the NifS-like sulfurtransferases, and the addition of precursor Z to the in vitro system gives rise to MPT formation. The use of radiolabeled L-[S-35]cysteine has demonstrated that both sulfurs of the dithiolene group of MPT originate from L-cysteine. It was found that MPT can be produced from precursor Z in an E. coli iscS mutant strain, indicating that IscS is not required for the in vivo sulfuration of RIFT synthase. A comparison of the ability of the three sulfurtransferases to provide the sulfur for MPT formation showed the highest activity for CSD in the in vitro system.