THE LOW-TEMPERATURE CRYSTAL-STRUCTURE OF THE PURE-SPERMINE FORM OF Z-DNA REVEALS BINDING OF A SPERMINE MOLECULE IN THE MINOR-GROOVE

The X-ray crystal structure of the pure-spermine form of the left-hand ed Z-DNA duplex [d(CGCGCG)](2) has been determined at a temperature of -110 degrees C. Whereas the previously described room temperature str ucture of the pure-spermine form showed only the presence of a single ''interhelix'' spermine molecule, mediating contacts between neighbori ng duplexes (Egli et al., 1991), a second ''intrahelix'' spermine mole cule as well as two hydrated sodium ions were found in the structure d etermined at low temperature. This second spermine molecule binds prim arily within the minor groove of two hexamer duplexes that are stacked in an end-to-end fashion in the crystal lattice. Thus, the intrahelix spermine molecule interacts with a single infinite helix. The spine o f hydration observed in other structures of Z-DNA hexamers is partiall y replaced and partially displaced by the intrahelix spermine molecule . In Z-DNA, phosphate groups are relatively closely spaced across the minor groove compared to the right-handed double-helical conformation of B-DNA. The intrahelix spermine molecule decreases cross-groove elec trostatic repulsion within the Z-DNA helix, thereby increasing its rel ative stability. This structure may therefore provide an explanation f or the role of spermine as a very effective inducer of the conformatio nal B-DNA to Z-DNA transition with alternating dG-dC sequences in solu tion.