THE STRUCTURE OF R(UUCGCG) HAS A 5'-UU-OVERHANG EXHIBITING HOOGSTEEN-LIKE TRANS U-CENTER-DOT-U BASE-PAIRS

The crystal structure of the RNA fragment, 5'-r(UUCGCG)-3', has been d etermined at 1.4 Angstrom resolution by a combination of single isomor phous replacement and molecular search methods. The 3'-terminal CGCG p ortion of the hexamer engages in Watson-Crick hydrogen bonding while t he 5'-terminal UU-overhang forms novel Hoogsteen-like U . U self-base pairs with the overhang of an adjacent duplex. The U . U pairs display a single conventional hydrogen bond between O4(U1) and N3 (U8) and a CH-O hydrogen bond between C5-H (U1) and O4 (U8), through the Hoogstee n face of the pyrimidine base U1. This unusual arrangement of one of t he bases results in a trans U . U pair on antiparallel strands in cont rast to the usual cis base pairs. The structure emphasizes the pronoun ced polymorphism of U . U pairs and has implications for folding of RN A molecules.