Combining SAD/SIR iteration and MR iteration in partial-model extension of proteins

There are two kinds of dual-space partial-model extensions which involve the direct-method program OASIS. The first kind, named SAD/SIR iteration, uses SAD/SIR information, while the second kind, named molecular replacement (MR) iteration, does not use that information. In general, the SAD/SIR iteration is more powerful since more experimental information is used. However, in most cases when protein structures are solved with the molecular replacement method, SAD/SIR information is not available. Thus the MR iteration is particularly useful for the completion of models from molecular replacement. The SAD/SIR iteration will be automatically used in OASIS for data sets containing SAD/SIR signals, while the MR iteration will be dedicated to data sets without SAD/SIR signals. The present paper shows that for data containing SAD/SIR signals, a combination of SAD/SIR iteration and MR iteration could lead to significantly better results than that obtained from the SAD/SIR iteration alone.