DATA REDUCTION FROM TWINNED RNA CRYSTALS

Methods were developed to process diffraction data from epitaxially tw inned crystals. Four programs for data reduction and two display progr ams were developed to augment the data-reduction program XDS [Kabsch ( 1988). J. Appl. Cryst. 21, 916-924]. The programs can be generalized f or use with other data-reduction software that provides the user with a list of the reflections used to determine lattice constants and crys tal orientation. LATTICE_VIEW generates a PDB file containing 'water m olecules' at the reciprocal-space coordinates of the strong spots foun d in the initial data frames. The PDB file is visualized to identify s pots that belong to the same lattice, obtain unit-cell dimensions for a lattice, and assess data quality. VECTOR_MATCH is used to find addit ional spots belonging to a lattice. ACCOUNT4 determines which spots ha ve been processed by XDS. COMFORT discards reflections that are too cl ose to a reflection in another lattice. The display programs provide u seful visual information on the quality of the crystal orientations us ed. Data with an R(merge) of 7.1% at 2.4 Angstrom resolution were obta ined from epitaxially twinned crystals of an RNA dodecamer. The data w ere of sufficient quality to solve the structure with a combination of molecular replacement and single isomorphous replacement methods.