ENHANCEMENT OF THE METHOD OF MOLECULAR REPLACEMENT BY INCORPORATION OF KNOWN STRUCTURAL INFORMATION

Crystals of macromolecules often have two or more molecules per asymme tric unit, or contain domains of a macromolecule or a macromolecular c omplex that are structurally independent. In such cases the convention al molecular-replacement method attempts to determine the position of each structural unit independently. Typically, some parts of the struc ture can be determined more easily or more reliably than other parts. Methods are proposed whereby information from a part of a crystal stru cture that has been determined can be used to help determine the struc ture of the remainder. Two different strategies are discussed, 'subtra ction' and 'addition'. With 'subtraction' strategy the Patterson funct ion of the known part of the structure is subtracted from the 'observe d' Patterson. This approach is found to be most effective in the conte xt of the rotation function in that it eliminates peaks that are irrel evant to the desired solution. With 'addition' strategy the structure factors of the known component are added to those of the search model. This procedure is most effective in the context of the translation fu nction because it brings the structure factors calculated from the sea rch model closer to those observed. Methods of applying the fast Fouri er transform to facilitate these calculations are described. A number of examples are provided including structures of mutants of T4 lysozym e that might not have been solved without recourse to the proposed met hods. A method of including information from a heavy-atom derivative i n a translation function is also developed and shown to be superior in some situations to the conventional translation function.