SOLVE -- The Next Generation of Automated Heavy Atom Location and Phasing for MIR and MAD. Thomas C. Terwilliger* and Joel Berendzen# *Structural Biology (Mail Stop M888) and #Biophysics (Mail Stop D454) Groups, Los Alamos National Laboratory, Los Alamos, NM 87545

We have developed a fully automated procedure for determination of heavy atom sites and calculation of native phases in a MAD experiment or in all derivatives in an MIR experiment. The input required consists of the space group and cell constants, resolution limits, the type of heavy atoms present, and the scaled data. The output consists of ranked sets of refined heavy atom parameters and native electron density maps.

The most important new feature of the procedure is the scoring of potential solutions. Four criteria are used in scoring. The first corresponds to peak heights at expected positions in the Patterson function. The second is peak heights at heavy atom positions in "omit" heavy atom difference Fouriers. The third is the figure of merit of the phasing obtained. The fourth is the non-randomness of the native Fourier obtained. Each of these criteria are evaluated based on the Z-score obtained from a comparison of the solution being tested and the mean over many solutions.

The procedure uses a Patterson search procedure (a modified version of HASSP) to obtain plausible starting solutions. The solutions are scored and ranked. One at a time, solutions are considered as test starting solutions for generation of new solutions by addition or deletion of individual sites. After calculation of native phases from a test solution, difference Fourier maps (and cross-difference Fourier maps for the MIR case) are used to identify possible additional sites. New solutions are scored and included in the ranked list of solutions. The procedure is iterated until no additions or deletions to existing highly-ranked solutions yield an improvement in score.

The procedure has been applied to MAD data on gene 5 protein (2 sites) and to MIR data on granulocyte-macrophage colony stimulating factor (4 derivatives, 2 to 4 sites per derivative). The top-ranked solutions in both cases contain the major heavy atom sites and no incorrect sites.