RNA is known to fold into a variety of structural elements, many of which have sufficient sequence complexity to make the thermodynamic study of each possible variant impractical. We previously reported a method for isolating stable and unstable RNA sequences from combinatorial libraries using temperature gradient gel electrophoresis (TGGE). This method was used herein to analyze a six-nucleotide RNA hairpin loop library. Three rounds of in vitro selection were performed using TGGE, and unusually stable RNAs were identified by cloning and sequencing. Known stable tetraloops were found, including sequences belonging to the UNCG motif closed by a CG base pair, and the CUUG motif closed by a GC base pair. In addition, unknown tetraloops were found that were nearly as stable as cUNCGg, including sequences related through substitution of the U with a C (Y), the C with an A (M), or both. These substitutions allow hydrogen bonding and stacking interactions in the UNCG loop to be maintained. Thermodynamic analysis of YNMG and variant loops confirmed optimal stability with Y at position 1 and M at position 3. Similarity in structure and stability among YNMG loops was further supported by deoxyribose substitution, CD, and NMR experiments. A conserved tertiary interaction in 16S rRNA exists between a YAMG loop at position 343 and two adenines in the loop at position 159 (Escherichia coli numbering). NMR and functional group substitution experiments suggest that YNAG loops in particular have enhanced flexibility, which allows the tertiary interaction to be maintained with diverse loop sequences at position 159. Taken together, these results support the existence of an extended family of UNCG-like tetraloops with the motif cYNMGg that are thermodynamically stable and structurally similar and can engage in tertiary interactions in large RNA molecules.
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