SAD phasing by OASIS-2004: case studies of dual-space fragment extension.

The principle of dual-space phasing is used in dealing with protein SAD data. Four programs are involved in iterative dual-space fragment extension to improve automatic model building. OASIS-2004 is used to break the phase ambiguity intrinsic in the SAD experiment. In the initial cycle, discrimination of SAD phase doublets is performed by the direct method incorporating the known anomalous-scattering substructure. In subsequent cycles, discrimination is performed by the direct method incorporating both the known anomalous-scattering substructure and the partial protein structure obtained from model building in the preceding cycle. DM is used to improve direct-method phases via density modification. RESOLVE is used for initial model building and ARP/wARP is used to complete the structure. Case studies with three sets of difficult SAD data showed that the procedure is beneficial to high-throughput protein-structure determination and all of the four programs involved make their unique contribution to the process.