Figure 4.

Three-fold averaged electron density maps (45) calculated with Fo-Fc coefficients in the 20–3 Å resolution range and contoured at the 3.2 σ level (a and b) or with 2Fo-Fc coefficients and contoured at the 1.1 σ level (c). (a) Fo-Fc map calculated with the native model in which Pro-238 has been substituted by a glycine. The map confirms the presence of a mutation close to the [3Fe-4S] cluster, as there is a significant electron density peak, which is not explained by the native model. (b) Fo-Fc map calculated with the native model in which Pro-238 has been substituted by a cysteine. (c) 2Fo-Fc map calculated with the final model of the mutant including both the Cys-238 and the third [4Fe-4S] cluster.