Electron density recovered for the SFX LCP structure of RCvir. Diffraction data were recorded and processed to 2.25 Å resolution. Electron density recovered for the mobile ubiquinone (QB) was of similar quality to that recovered for other cofactors. 2Fobs − Fcalc electron-density maps (blue) are contoured at 1σ, where σ represents the root-mean-square electron density of the map. The A-branch leading to QA is on the right, whereas the M-branch leading to QB is on the left.