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

Figure 9

General scheme for FeS cluster biosynthesis in E. coli. Shown is the isc operon containing the genes iscR-iscSUA-hscAB-fdx-iscX. IscR is the regulator for expression of the operon by sensing the FeS status of the cell. FeS clusters assemble on the scaffold protein IscU, which receives the sulfur from the L-cysteine desulfurase IscS. The iron donor in this reaction is not identified so far. CyaY is believed to stabilize a conformation of the IscS-IscU complex. The role of IscX is unclear. Ferredoxin is proposed to participate in the reductive coupling of two [2Fe2S] clusters to form a single [4Fe4S] cluster on IscU. Release of the clusters is catalyzed by the chaperones HscAB. The carrier protein IscA delivers the formed FeS clusters to the final target proteins.