Spherically averaged phased translation function and its application to the search for molecules and fragments in electron-density maps

The molecular-replacement method has been extended to locate molecules and their fragments in an electron-density map. The approach is based on a new spherically averaged phased translation function. The position of the centr e of mass of a search model is found prior to determination of its orientat ion. The orientation is subsequently found by a phased rotation function. T he technique also allows superposition of distantly related macromolecules. The method has been implemented in a computer program MOLREP and successfu lly tested using experimental data sets.