The Biosynthesis of the Molybdenum Cofactor in Escherichia coli and Its Connection to FeS Cluster Assembly and the Thiolation of tRNA

Figure 11

Model for the effect of elevated concentrations of TusA or IscU on Moco biosynthesis. IscS interacts with IscU, TusA, and ThiI for sulfur transfer to FeS clusters, tRNA, Moco biosynthesis, and thiamine/tRNA, respectively. FeS clusters are inserted into target proteins. Among these proteins is MoaA, involved in Moco biosynthesis (conversion of 5′GTP to cPMP). TusA is involved in the sulfur transfer for the conversion of cPMP to MPT. Increased levels of IscU reduce the level of active molybdoenzymes in E. coli. When IscU is present in high amounts, it forms a complex with IscS, making it unavailable for the interaction with TusA, thus, resulting in a lack of sulfur transfer for the conversion of cPMP to MPT. When the amount of TusA is increased, the levels of FeS clusters are decreased which further result in an inactive MoaA protein and thus in a decreased activity of molybdoenzymes. Detailed descriptions are given in the text.