The crystal and molecular structure of d(CGUAʹCG)2(where Aʹis 2-aminoadenine) has been determined and refined to an R factor of 13.8% for data 8.0-1.3 Å. The structure is very similar to the original Z-DNA structures with the sequence d(CGCGCG)2[Gessner, R.V., Frederick, C.A., Quigley, G.J., Rich, A., & Wang, A.H.-J. (1989) J. Biol. Chem. 264, 7921] and shows that the substitution of 2-aminoadenine-uracil base pairs in the two central steps is consistent with Z-DNA formation. In addition, we show how waters mediating intermolecular interactions may help to explain the ZI-ZII conformational pattern found in many Z-DNA structures.
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