ARP/wARP and molecular replacement

The aim of ARP/wARP is improved automation of model building and refinement in macromolecular crystallography. Once a molecular-replacement solution h as been obtained, it is often tedious to refine and rebuild the initial (se arch) model. ARP/wARP offers three options to automate that task to varying extents: (i) autobuilding of a completely new model based on phases calcul ated from the molecular-replacement solution, (ii) updating of the initial model by atom addition and deletion to obtain an improved map and (iii) doc king of a structure onto a new (or mutated) sequence, followed by rebuildin g and refining the side chains in real space. A few examples are presented where ARP/wARP made a considerable difference in the speed of structure sol ution and/or made possible refinement of otherwise difficult or uninterpret able maps. The resolution range allowing complete autobuilding of protein s tructures is currently 2.0 Angstrom, but for map improvement considerable a dvances over more conventional refinement techniques are evident even at 3. 2 Angstrom spacing.