Formation of apo and metal bound IscU complexes with IscS. Apo, Zn-bound, and a 1:1 mixture of the two forms of IscU (final concentration of 50 μM) were separately combined with IscS (50 μM) in either the presence or absence of Fe2+ (100 μM). (A) Deconvoluted zero charge MS spectra for mass ranges that correspond to IscS2 (∼97,000 Da), IscU–IscS2 (∼111,000 Da), and IscU2–IscS2 (∼125,000 Da) complexes. (B) Comparison of the different protein complex stoichiometries between IscU and IscS for the five experiments. Protein complexes with apo and metal bound forms of IscU are combined to determine the normalized intensity. The error bars are replicate errors (n = 3).