Conformation and dynamics of an RNA internal loop.

The conformation and the dynamics of an RNA oligonucleotide (26 nucleotides) which is a model for loop E in eukaryotic 5S RNA have been investigated by one- and two-dimensional NMR. The central portion of the oligonucleotide contains two G A oppositions, a common feature of ribosomal RNAs. The exchangeable proton spectrum indicates that an internal loop separates two stems of four and five base pairs. This observation is not consistent with structures for loop E containing mismatched G.A base pairs proposed from chemical and enzymatic studies on Xenopus laevis 5S RNA. The nonexchangeable proton spectrum has been assigned by two-dimensional NMR. Scalar couplings from correlated experiments and interproton distances from NOESY experiments at short mixing times have been used to determine glycosidic angles, sugar puckers, and other conformational features. The conformation of the stems is very close to standard A-form RNA, and extensive base stacking continues into the internal loop. This result provides a structural basis for the large favorable enthalpy of duplex formation determined in thermodynamic studies. Unusual structural and dynamic features are localized in the nucleotides connecting the loop to the stems.