Structure Of The Dodecamer R(Gaucacuucggu) With Four 5 '-Overhang Nucleotides

The crystal structure of an RNA dodecamer, r( GAUCA-CUUCGGU), was solved at 2.6 Angstrom resolution by the molecular- replacement method and refined to an R-work of 18.8% ( R-free = 22.8%) using 2494 reflections. The dodecamer crystallized in the monoclinic space group C2, with unit- cell parameters a = 71.34, b = 39.98, c = 32.47 Angstrom, beta = 104.7degrees and two independent strands in the asymmetric unit. The dodecamer adopts an octamer duplex structure with four 5'- overhang residues ( G1A2U3C4), which form Watson - Crick base pairs with another four 5'- overhang residues of a symmetry- related duplex. The octamer duplex ( ACUUCGGU) contains at its center four mismatched base pairs flanked by two Watson - Crick base pairs. The mismatched bases form two G . U wobble base pairs at the ends and two U . C base pairs at the center, with one base - base hydrogen bond N4( C) .... O4( U) and a water bridge connecting the N( 3) of the cytosine and uridine. The present study reinforces the concept of the stability of the conformation of UUCG in RNA double- helical structures.