Measurement of Atom Resolvability in CryoEM Maps with Q-scores

Abstract CryoEM density maps are now at the point where resolvability of individual atoms can be achieved. However, resolvability is not necessarily uniform throughout the map. We introduce a quantitative parameter to characterize the resolvability of individual atoms in cryoEM maps, the map Q-score. Q-scores can be calculated for atoms in proteins, nucleic acids, water, ligands, and other solvent atoms, using models fitted to or derived from cryoEM maps. Q-scores can also be averaged to represent larger features such as entire residues and nucleotides. In several examples, Q-scores are shown to correlate well to resolvability. Q-scores averaged over entire models are also shown to correlate closely to the estimated resolution of cryoEM maps for both protein and RNA. Assuming the models they are calculated from are well-fitted to the map, Q-scores can thus be used as another measure to indicate the quality or resolvability in entire maps and also of features as small as individual atoms.