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.

A recent re-examination of the role of the helices surrounding the conserved core of the hammerhead ribozyme has identified putative loop-loop interactions between stems I and II in native hammerhead sequences. These extended hammerhead sequences are more active at low concentrations of divalent cations than are minimal hammerheads. The loop-loop interactions are proposed to stabilize a more active conformation of the conserved core. Here, a kinetic and thermodynamic characterization of an extended hammerhead sequence derived from Schistosoma mansoni is performed. Biphasic kinetics are observed, suggesting the presence of at least two conformers, one cleaving with a fast rate and the other with a slow rate. Replacing loop II with a poly(U) sequence designed to eliminate the interaction between the two loops results in greatly diminished activity, suggesting that the loop-loop interactions do aid in forming a more active conformation. Previous studies with minimal hammerheads have shown deleterious effects of R p -phosphorothioate substitutions at the cleavage site and 5 to A9, both of which could be rescued with Cd 2+ . Here, phosphorothioate modifications at the cleavage site and 5 to A9 were made in the schistosome-derived sequence. In Mg 2+ , both phosphorothioate substitutions decreased the overall fraction cleaved without significantly affecting the observed rate of cleavage. The addition of Cd 2+ rescued cleavage in both cases, suggesting that these are still putative metal binding sites in this native sequence.

Magnetically separable hydrogenation catalysts serve as an example of a multifunctional M13 bacteriophage nanocomposite made by (a) growing Rh nanoparticles lengthwise along the pVIII region via nonspecific electrostatic interactions and (b) anchoring Fe3O4 nanoparticles on the pIII tip via materials-specific interactions identified through phage display methods

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