Data reduction from twinned RNA crystals.

Methods were developed to process diffraction data from epitaxically twinned crystals. Four programs for data reduction and two display programs were developed to augment the data-reduction program XDS [Kabsch (1988). J. Appl. Cryst. 21, 916-924]. The programs can be generalized for use with other data-reduction software that provides the user with a list of the reflections used to determine lattice constants and crystal orientation. LATTICE_VIEW generates a PDB file containing 'water molecules' at the reciprocal-space coordinates of the strong spots found in the initial data frames. The PDB file is visualized to identify spots that belong to the same lattice, obtain unit-cell dimensions for a lattice, and assess data quality. VECTOR_MATCH is used to find additional spots belonging to a lattice. ACCOUNT4 determines which spots have been processed by XDS. COMFORT discards reflections that are too close to a reflection in another lattice. The display programs provide useful visual information on the quality of the crystal orientations used. Data with an R(merge) of 7.1% at 2.4 A resolution were obtained from epitaxically twinned crystals of an RNA dodecamer. The data were of sufficient quality to solve the structure with a combination of molecular replacement and single isomorphous replacement methods.