The ISC machinery assembles [2Fe-2S] clusters by formation and fusion of [1Fe-1S] precursors

Iron-sulfur clusters are essential metallocofactors synthesized by multiprotein machineries via an unclear multistep process. Here we report a step-by-step dissection of the [2Fe-2S] cluster assembly process by the Escherichia coli iron-sulfur cluster (ISC) assembly machinery using an in vitro reconstituted system and a combination of biochemical and spectroscopic techniques. We show that this process is initiated by iron binding to the scaffold protein IscU, which triggers persulfide insertion by the cysteine desulfurase IscS upon the formation of a complex with IscU. Then, the persulfide is cleaved into sulfide by the ferredoxin Fdx, leading to a [1Fe-1S] precursor. IscU dissociates from IscS, dimerizes and generates a bridging [2Fe-2S] cluster by fusion of two [1Fe-1S] precursors. The IscU dimer ultimately dissociates into a monomer, ready to transfer its [2Fe-2S] cluster to acceptors. These data provide a comprehensive description of the [2Fe-2S] cluster assembly process by the ISC assembly machinery, highlighting the formation of key intermediates through a tightly concerted process.