When pseudosymmetry and merohedral twinning come across: the case of the d(ApTpApTpApT) oligonucleotide in a hexagonal lattice

The d(ApTpApTpApT) deoxyhexanucleotide packs in two different lattice systems: monoclinic and hexagonal. In this study, it is shown that in the hexagonal crystals twinning and pseudosymmetry coexist. The presence of both components resulted in a puzzling space-group determination. Only when a correct model became available [Hoogsteen-DNA; Abrescia et al. (2002), Proc. Natl Acad. Sci. USA, 99, 2806-2811] was it possible to discriminate between twinning and pseudosymmetry. Here, the steps that led to a correct space-group assignment and to the solution of the oligonucleotide structure in the hexagonal lattice by molecular replacement are described.