1.76 å structure of a pyrimidine start alternating A-RNA hexamer r(CGUAC)dG

The crystal structure of the alternating RNA r(CGUAC)dG with a 3′-terminal deoxy G residue has been determined at 1.76 Å resolution. The crystal belongs to the orthorhombic space group C222₁, unitcell dimensions a = 29.53, b = 44.61 and c = 94.18 Å, with two independent duplexes (I and II) per asymmetric unit. The structure was solved by the molecularreplacement method. The final R factor was 18.8% using 4757 reflections in the resolution range 8.0-1.76 Å. The model contains a total of 496 atoms and 85 solvent molecules. The two duplexes form the repeating unit and stack in the usual head-to-tail (5′ ,3′/5′ ,3′) fashion into a pseudocontinuous helical column. Almost all of the 2′-hydroxyl groups are engaged in the three modes of water-mediated interactions to the base N3/O2 atoms, the sugar O4′ atoms and the backbone phosphates. Thus, the 2′-hydroxyl group of RNA is probably contributing to the stability of the duplexes.