Effects of crystal twinning on the ability to solve a macromolecular structure using multiwavelength anomalous diffraction

The crystal structure of gpD, the capsid-stabilizing protein of bacteriopha ge lambda, was solved by multiwavelength anomalous diffraction (MAD) for a selenomethionine (SeMet) derivative of the protein at 1.8 Angstrom resoluti on, using crystals in space group P2(1) [Yang et al. (2000), Nature Struct. Biol. 7, 230-237]. Subsequent analysis showed that the crystals of both th e original protein and the SeMet derivative were pseudomerohedrally twinned with a twinning fraction similar or equal to 0.36, owing to the near-ident ity of the a and c axes. An analysis of the crystal structure solution is p resented and the utility of twinned crystals for solving the structure usin g MAD and of different phasing strategies is discussed; the results obtaine d with several software packages are compared.