UV-visible absorption spectra and iron/sulfur contents of wild-type and mutant QhpD complexes with sQhpC. UV-visible absorption spectra of the as-purified (gray) and reconstituted (black) complexes of sQhpC·QhpD (A), sQhpC·QhpDC116S (B), sQhpC·QhpDG161A (C), sQhpC·QhpDC353S (D), and sQhpC·QhpDR373A (E) are shown after normalization to a protein concentration of 1 mg/ml. F, numbers of iron and sulfur atoms per each sQhpC·QhpD complex are represented by black and gray bars, respectively. Dashed line indicates the number of iron and sulfur atoms expected for the content in three [4Fe-4S] clusters.