Crystal structure of rGd(CGCGCG): a Z-DNA hexamer duplex with a 5'-(rG) overhang

The crystal structure of the heptamer rGd(CGCGCG) has been determined at 1.54 A resolution with R(work) and R(free) of 0.191 and 0.235, respectively. The crystal belongs to space group P2(1)2(1)2(1), with unit-cell parameters a = 17.96, b = 31.47, c = 44.73 A and two independent strands in the asymmetric unit. The chimera forms a Z-DNA hexamer duplex d(CGCGCG)(2), with the 5'-overhang rG invisible in the density. The replacement of rG in the 5' terminus of d(GCGCGCG) changes the reverse Hoogsteen G.G base pairing of the 5'-overhang dG (Pan et al., 1997). However, the replacement does not change the helix from Z-form to A-form or the water-structure motifs in the Z-DNA double-helical structure.