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 multiwavelength anomalous diffraction (MAD) data. The first component of the software is a program for finding heavy-atom sites when presented with SIR (i.e. native and derivative) structure-factor amplitudes or MAD-FA structure-factor amplitudes (i.e. the derived structure factors corresponding to the anomalously scattering atoms). The principle on which this program is based is a reciprocal-space maximization of a Patterson correlation coefficient between the SIR or MAD-FA data and the calculated intensities from search atoms placed at trial positions. The program has been successfully tested on two sets of SIR data for the cytochrome c' protein and on MAD-FA data from the DnaK protein. The second component of the software is a three-dimensional viewer which may be used to assess difference Patterson maps and which provides peak-picking and atom-editing facilities for interpreting cross-difference Fourier maps.