Use of matrix direct methods for low-resolution phase extension for tRNA

The interpretation of low-resolution electron-density maps for macromolecules frequently proves to be difficult and, usually, no progress can be made with phase extension by direct methods. An iterative procedure for map modification is described, which, together with consideration of the molecular model in terms of group scatterers rather than as single atoms, enables matrix methods of phase extension to be successfully applied. The results of extending phase information from 5 to 3 Å for yeast tRNA^Phe are shown and discussed.