AUTOMATED AND GRAPHICAL METHODS FOR LOCATING HEAVY-ATOM SITES FOR ISOMORPHOUS REPLACEMENT AND MULTIWAVELENGTH ANOMALOUS DIFFRACTION PHASE DETERMINATION

We have developed new software for locating heavy-atom sites for phase determination from single isomorphous replacement (SIR) and multiwave length anomalous diffraction (MAD) data. The first component of the so ftware is a program for finding heavy-atom sites when presented with S IR (ie. native and derivative) structure-factor amplitudes or MAD-FA s tructure-factor amplitudes (i.e. the derived structure factors corresp onding to the anomalously scattering atoms). The principle on which th is program is based is a reciprocal-space maximization of a Patterson correlation coefficient between the SLR or MAD-FA data and the calcula ted intensities from search atoms placed at trial positions. The progr am has been successfully tested on two sets of SIR data for the cytoch rome c' protein and on MADFA data from the DnaK protein. The second co mponent of the software is a three-dimensional viewer which may be use d to assess difference Patterson maps and which provides peak-picking and atom-editing facilities for interpreting cross-difference Fourier maps.