Crystallization and preliminary crystallographic study of triple-helical DNA

Single crystals of d(CTCCT(S)CCGCGCG).d(CGCGCGGAG) have been grown by the v apor-diffusion method using 2-methyl-2,4-pentanediol as a precipitant. The crystals are tetragonal, space group P4(2), with unit-cell parameters a = b = 53.8, c = 43.1 Angstrom, and diffract to 1.8 Angstrom resolution at a sy nchrotron X-ray beamline. In the crystal, the asymmetric unit contains one copy of the construct. The two halves of the structure are related by non-c rystallographic twofold symmetry. These observations are consistent with th e conclusion that the sequences of the 12-mer and 9-mer oligonucleotides fo rm a duplex DNA at one end and a tripler DNA at the other end.