Red IscS also accumulates in wild-type E. coli cells with depletion of intracellular iron. Recombinant IscS was expressed in wild-type E. coli cells grown in LB medium supplemented with 0 (spectrum 1), 0.4 mm 2,2′-dipyridyl (spectrum 2), or 0.4 mm 2,2′-dipyridyl and 0.4 mm Fe(NH4)2(SO4)2 (spectrum 3). Purified IscS proteins (20 μm) were subjected to UV-visible absorption measurements. Inset, a photograph of IscS proteins purified from wild-type E. coli cells grown in LB medium supplemented with 0 (1) or 0.4 mm 2,2′-dipyridyl (2).