The combination of independent phase information obtained from separate protein structure determinations of yeast hexokinase

A procedure is presented for combining in reciprocal space the information contained in protein structures which have been independently determined by multiple isomorphous replacement (MIR). Application of this method to the two crystal forms of yeast hexokinase B which have been solved at 3.5 Å resolution results in an electron density map that is a striking improvement over either of the two MIR maps or the map obtained by simply averaging the MIR maps in direct space. The background is lower, the apparent resolution is higher and most importantly, side chains are visible in this new hybrid map where none exist in either of the MIR maps. The method consists of determining a linear transformation matrix and vector relating the subunits of the two crystals, orienting the electron density map of one crystal form in the unit cell of the other and transforming this map to produce a set of calculated phases and structure-factor amplitudes. These calculated phases from the first crystal are then combined with the MIR phases of the second to produce a hybrid phase set which is used to calculate a new electron density map.