OVERCOMING NONISOMORPHISM BY PHASE PERMUTATION AND LIKELIHOOD SCORING - SOLUTION OF THE TRPRS CRYSTAL-STRUCTURE

Entropy maximization to maximum likelihood, constrained jointly by the best available experimental phases and by a sufficiently good envelope, can bring about substantial model-independent map improvement, even at medium (3.1 angstrom) resolution [Xiang, Carter, Bricogne & Gilmore (1993). Acta Cryst. D49, 193-212]. In the crystal structure determination of the Bacillus stearothermophilus tryptophanyl-tRNA synthetase (TrpRS), however, the following had to be dealt with simultaneously: (1) a serious lack of isomorphism in the heavy-atom derivatives, resulting in large starting-phase errors; and (2) an initially poorly known molecular envelope. Because the constraints - both phases and envelope - were insufficiently well determined at the outset, maximum-entropy solvent flattening as previously applied was unsuccessful. Rather than improving the maps, it led to a deterioration of their quality, accompanied by a dramatic decrease of die log-likelihood gain as phases were extended from about 5 angstrom resolution to the 2.9 angstrom limit of the diffraction data. This deadlock was broken by the identification of strong reflections, which were initially unphased and which were inaccessible by maximum-entropy extrapolation from the phased ones, and by permutation of the phases of these reflections so as to sample the space of possible electron-density and envelope modifications they represented. Permutation was carried out by successive full and incomplete factorial designs [Carter & Carter (1979). J. Biol. Chem. 254, 12219-12223] for 28 strong reflections selected in decreasing order of their 'renormalized' structure-factor amplitudes. The permuted reflections included one reflection for which the probability distribution from multiple isomorphous replacement with anomalous scattering (MIRAS) indicated an incorrect phase with a high figure of merit and which consequently had a large renormalized structure factor. A similar permutation was carried out for six different binary choices related to the calculation and description of the molecular envelope. Permutation experiments were scored using the log-likelihood gain and contrasts for each main effect were analyzed by multiple-regression least squares. Student t tests provided significant and reliable indications for a large majority of the permuted reflections and for all six hypotheses related to the molecular envelope. The resulting phase improvement made it possible to assign positions (hitherto unobtainable) for nine of the ten selenium atoms in an isomorphous difference Fourier map for selenomethionine-substituted TrpRS crystals and hence to solve the structure. Phase-permutation methods continued to be useful in producing improved maps from all the available isomorphous-replacement phase information and therefore played a critical role in solving the structure. This process rescued phases for the tetragonal TrpRS structure (now solved) from an otherwise crippling lack of isomorphism. It represents the first application of a fully fledged Bayesian phase-determination process [Bricogne (1988). Acta Cryst. A44, 517-5451 to the solution of an unknown structure and demonstrates the feasibility of using these methods with low-to-medium-resolution data.