Ab initio phasing using diffraction data from different crystal forms The crystallographic phase problem is routinely solved when imaging small molecules but remains a key component and difficult step in protein crystallography. Current phasing approaches are either experimentally based or rely on the knowledge of an homologous structure. With the recent advances in phasing algorithms called iterative projections algorithms (IPAs), ab initio phasing methods, where only minimal extra information is needed are gaining interest in protein crystallography, as they potentially offer a dramatic reduction in effort over experimental methods and are not affected by model bias. Two versions of a new ab initio phase retrieval method are presented in this paper. These rely on the measurement of the diffraction from two or more crystal forms of the same molecule. The required IPA constraint projections are derived and 2D simulations using the difference map algorithm to illustrate the convergence properties of the methods in presence of object overlap. A 3D simulation illustrates potential use and effectiveness of the algorithm.

Matching of EM Map Segments to StructurallyRelevant Biomolecular Regions Electron microscopy is a technique used to determine the structure of biomolecular machines via threedimensional images (called maps). The stateoftheart is able to determine structures at resolutions that allow us to identify up to secondary structural features, in some cases, but it is not widespread. Furthermore, because molecular interactions often require atomiclevel details to be understood, it is still necessary to complement current maps with techniques that provide finergrain structural details. We applied segmentation techniques to maps in the Electron Microscopy Data Bank (EMDB), the standard community repository for these data. We assessed the potential of these algorithms to match functionally relevant regions in their atomicresolution image counterparts by comparing against three protein systems, each with multiple atomicdetailed domains. We found that at least 80% of amino acid residues in 7 out of 12 domains were assigned to single segments, suggesting there is potential to match the lower resolution segmented regions to the atomic counterparts. We also qualitatively analyzed the potential on other EMDB structures, as well as generating the raw segmentation information for the complete EMDB, for interested researchers to use. Results can be accessed online and the library developed is provided as part of an opensource project.