Going soft and SAD with manganese.
Salgado PS., Walsh MA., Laurila MRL., Stuart DI., Grimes JM.
SAD phasing has been revisited recently, with experiments being carried out using previously unconventional sources of anomalous signal, particularly lighter atoms and softer X-rays. A case study is reported using the 75 kDa RNA-dependent RNA polymerase of the bacteriophase phi6, which binds a Mn atom and crystallizes with three molecules in the asymmetric unit. X-ray diffraction data were collected at a wavelength of 1.89 A and although the calculated anomalous signal from the three Mn atoms was only 1.2%, SHELXD and SOLVE were able to locate these atoms. SOLVE/RESOLVE used this information to obtain SAD phases and automatically build a model for the core region of the protein, which possessed the characteristic features of the right-hand polymerase motif. These results demonstrate that with modern synchrotron beamlines and software, manganese phasing is a practical tool for solving the structure of large proteins.