An application of the molecular replacement technique in direct space to a known protein structure

The molecular replacement technique in real space (successive cycles of electron density calculations, averaging among non-crystallographically equivalent subunits, and structure factor calculations based upon the improved averaged density) was applied to glyceraldehyde-3-phosphate dehydrogenase. It was shown possible (i) to extend the phases from a known 6.0 Å single isomorphous replacement set to at least 4.9 Å resolution; (ii) to determine a reasonable structure at low resolution given the molecular envelope and non-crystallographic symmetry; and (iii) to use the resulting low-resolution phases in the determination of heavy-atom sites. The application of such procedures to the determination of virus structures and heavy-atom derivative sites is discussed.