title: An unusual case of pseudo-merohedral twinning in orthorhombic crystals of Dicer.
abstract: The crystal structure of the enzyme Dicer from Giardia intestinalis was solved to 3.3 A resolution by MAD using crystals belonging to space group P2(1)2(1)2 [Macrae et al. (2006), Science, 311, 195-198]. These crystals were derived from crystals that diffracted X-rays to 3.0 A resolution but were refractory to structure determination because they were twinned. It is shown here that the original Dicer crystals represent an unusual case of perfect pseudo-merohedral twinning of orthorhombic crystals. Before the twinning problem was overcome, it was possible to calculate a low-resolution electron-density map in space group P4(1) that was used to build a partial molecular model. Experimental phases were sufficient to identify heavy-atom sites that indicated space-group inconsistency, leading to identification of the true space group. This information guided the search for different crystallization conditions that yielded untwinned crystals and ultimately a fully interpretable electron-density map.
authors: Howard Hughes Medical Institute, Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720, USA.
authors: Ian J
authors: IJ
authors: MacRae
authors: Jennifer A
authors: JA
authors: Doudna
mesh_terms: D000818:Animals; D003460:Crystallization; D018360:Crystallography, X-Ray; D016829:Giardia lamblia / Q000201:enzymology*; D008958:Models, Molecular; D011487:Protein Conformation; D017434:Protein Structure, Tertiary; D043244:Ribonuclease III / Q000737:chemistry* / Q000378:metabolism
chemical_list: C523566:Dicer protein, Giardia intestinalis; D043244:Ribonuclease III