A scaffold on which IscS assembles Fe-S clusters. Subsequently gives the nascent cluster to other proteins. It is likely that Fe-S cluster coordination is flexible as the role of this complex is to build and then hand off Fe-S clusters (By similarity). Binds 1 Zn(2+) ion; this is probably a substitute for Fe-S centers. Forms a heterotetramer with IscS; each subunit of the IscS dimer contacts an IscU monomer (By similarity). Upon overexpression can be isolated a monomer, dimers and other oligomers. Belongs to the NifU family.