Molecular structure of d(m5CGTAm5CG): TA pairs in Z-DNA and the consequences.

The hexamer d(m5CGTAm5CG) crystallizes as a left-handed Z-DNA molecule in a isomorphous lattice to d(m5CGm5CGm5CG). The three-dimensional structure has been determined by x-ray diffraction and refined to a final R value of 16% at 1.2A resolution. The molecular structure is similar to the Z-DNA structure seen in d(m5CGm5CGm5CG) and d(CGCGCG). The presence of TpA sequence in the middle of the molecule has resulted in slight changes in the helical twist angles, base pair stacking and twist angles of the base pair planes. The hydration structure is significantly changed from both of methylated and non-methylated d(CGCGCG). A large hole which has no water molecule tightly bound to the DNA molecule is found inside of the deep groove of the DNA helix at the TpA sequence. This deterioration of the hydration structure is associated with destabilization of the Z-DNA structure. These characteristic features of the structure are compared with another Z-DNA hexamer contains TpA sequence, d(br5CGTAbr5CG).