CRYSTAL-STRUCTURE OF A Z-DNA HEXAMER D(CGCICG) AT 1.7-ANGSTROM RESOLUTION - INOSINE CYTIDINE BASE-PAIRING, AND COMPARISON WITH OTHER Z-DNA STRUCTURES

The crystal structure of the deoxyhexamer, d(CGCICG), has been determi ned and refined to a resolution of 1.7 angstrom. The DNA hexamer cryst allises in space group P2(1)2(1)2(1) with unit cell dimensions of a = 18.412 +/- .01 7 angstrom, b=30.485 +/- .036 angstrom, and c=43.318 +/ - .024 angstrom. The structure has been solved by rotation and transla tion searches and refined to an R-factor of 0.148 using 2678 unique re flections greater than 1.0 sigma (F) between 10.0-1.7 angstrom resolut ion. Although the crystal parameters are similar to several previously reported Z-DNA hexamers, this inosine containing Z-DNA differs in the relative orientation, position, and crystal packing interactions comp ared to d(CGCGCG) DNA. Many of these differences in the inosine form o f Z-DNA can be explained by crystal packing interactions, which are re sponsible for distortions of the duplex at different locations. The mo st noteworthy features of the inosine form of Z-DNA as a result of suc h distortions are: (1) sugar puckers for the inosines are of C4'-exo t ype, (2) all phosphates have the Z(I) conformation, and (3) narrower m inor grove and compression along the helical axis compared to d(CGCGCG ) DNA. In addition, the substitution of guanosine by inosine appears t o have resulted in Watson-Crick type base-pairing between inosine and cytidine with a potential bifurcated hydrogen bond between inosine Nl and cytidine N3 (2.9 angstrom) and O2 (3.3 - 3. angstrom).