Figure 6.

NMR signal perturbation of [U-¹⁵N]-IscX upon adding 3-fold Fe³⁺ as ferric ammonium citrate (blue) or upon adding 3-fold Fe²⁺ as ferrous ammonium sulfate (purple). Residues from IscX, whose signals are broadened beyond detection by the addition of Fe²⁺ (L3, K4, L39, and E66) are marked with red triangles; upon the addition of Fe³⁺, the signals from these residues were neither broadened nor shifted.