MOLECULAR REPLACEMENT: A NEW DENSITY MODIFICATION STRATEGY USING A SUBTRACTIVE ALGORITHM. Philip D. Martin, Jacqueline Vitali, Brian F. P. Edwards, Wayne State University, Detroit, MI 48201

When more than one molecule lies in the crystallographic asymmetric unit, more often than not only one of the unknown molecules is readily found by molecular replacement. Repeating the search using Fo-Fc coefficients is frequently unsuccessful because of the noise, due to inaccurate positions and B values of the known molecule, at this level of analysis. However, if the known molecule is refined crystallographically it can be used to phase a 2Fo-Fc map which may be 'correct enough' to roughly visualize the unknown portions of the asymmetric unit plus the image of the known molecule used for phasing. If all the electron density within 2 Å of the known molecule is then zeroed from the map, and the map inverted, a new, modified, set of structure factors that does not contain noise from the known molecule is produced. If another molecular replacement search is then initiated with the modified structure factors, the search can reveal the next molecule, and the process can be repeated until the structure is solved. We will show how the method was used to find: (1) a second thrombin molecule in a mutant peptide/thrombin complex with a dimer in the asymmetric unit; (2) the kringle 2 portion of a des-F1-meizothrombin structure with a dimer in the asymmetric unit; and (3) a second molecule of a thrombin dimer with an exocite peptide bound ( Vitali, J., Martin, P. D., Malkowski, M. G., et al., Acta Crystallogr . (1996). In Press). This work was supported in part by NIH grant GM33192. J. V. was supported by NIH training grant T32 HL07602.