On the ab initio solution of the phase problem for macromolecules at very low resolution: the few atoms model method

A method is proposed for the solution of the phase problem at very low resolution for macromolecules. It generates randomly a very large number of models, each consisting of a few (two to ten) pseudo-atoms. The corresponding amplitudes are used for selecting a subset of `best' models by choosing those with the highest correlation with experimental values. The phases calculated from these `best' models are analysed by a clusterization procedure leading to a few possible solutions, from which the correct one can be recognized by simple additional criteria. This method has been successfully applied to the neutron diffraction data of the AspRS-tRNA^Asp complex at 50 Å resolution and to data calculated from a model ribosome crystal at 60 Å resolution.