Locating proper non-crystallographic symmetry in low-resolution electron-density maps with the program GETAX

Non-crystallographic symmetry averaging for improving and extending an initial set of phases can be crucial at an early stage of a protein structure analysis. A method is described which detects the position of a proper rotation axis in a surprisingly poor electron-density map and is fast enough to run through a large number of axis orientations. It uses a simple multimer mask to define the searching unit, which is then shifted through the whole unit cell looking for the position with the highest correlation coefficient between the interrelated parts. Appropriate weighting and averaging enhances the signal-to-noise ratio. Examples of the application of this algorithm are given. The use of the local rotation axis for phasing is commented on. A search of the Protein Data Bank showed that 27% of the unique crystal forms contain proper local n-fold axes, which could have been located with the presented method.