Reductive Charge Transfer through an RNA Aptamer

The transfer of charges through double helical DNA is a very well investigated bioelectric phenomenon. RNA, on the contrary, has been less studied in this regard. The few available data report on charge transfer through RNA duplex structures mainly composed of homonucleotide sequences. In the light of the RNA world scenarios, it is an interesting question, if charge transfer can be coupled with RNA function. Functional RNAs however, contain versatile structural motifs. Therefore, electron transport also through non‐Watson–Crick base‐paired regions might be required. We here demonstrate distance‐dependent reductive charge transfer through RNA duplexes and through the non‐Watson–Crick base‐paired region of an RNA aptamer.

Preparation of modified long-mer RNAs and analysis of FMN binding to the ypaA aptamer from B. subtilis

In recent years, RNA has been shown to fulfil a number of cellular functions. This has led to much interest in elucidation of the structure of functional RNA molecules, and thus, in the preparation of suitably functionalized RNAs. The chemical synthesis of RNAs allows for the site-specific modification; however, is limited to sequences of about 60-70 nucleotides in length. At the example of the flavine mononucleotide (FMN) responsive aptamer of the ypaA riboswitch from B. subtilis, we demonstrate the highly efficient preparation of site-specifically modified long-mer RNAs. Our strategy consists of the chemical synthesis of fragments followed by enzymatic or chemical ligation. Splint ligation with T4 RNA ligase turned out to be most successful among the enyzymatic protocols. Highly efficient chemical ligation was performed by azide-alkyne cycloaddition of suitably modified RNA fragments. Wild-type and 2-aminopurine (2-AP)-modified variants of the ypaA aptamer were prepared. FMN binding to all synthesized ypaA aptamer variants is demonstrated. However, dissociation of FMN from its binding site by reduction of the isoalloxazin unit as demonstrated before for a small-hairpin-derived aptazyme could not be shown. This implies that either FMN is less accessible to reduction when it is bound to its natural aptamer; that reduced FMN remains bound to the aptamer; or that FMN upon reduction indeed is released from its binding site, without the aptamer folding back in the natural ligand-free state. The results of this study are of general interest to the preparation of site-specifically modified RNAs for investigation into structure and function.

RNA self-processing: formation of cyclic species and concatemers from a small engineered RNA

We have engineered a self-processing RNA, derived from the hairpin ribozyme that runs through a cascade of cleavage and ligation reactions thereby changing its topology. The first two cleavage events leave the resulting RNA with a 5'-OH group and a 2',3'-cyclic phosphate. Thus, upon refolding, intramolecular ligation delivers a cyclic species. In addition, we demonstrate formation of concatemers resulting from multiple intermolecular ligations. Our results demonstrate the potential of RNA for self-supported topology changes and support the suggestion of 2',3'-cyclic phosphates being suitable activated building blocks for reversible phosphodiester bond formation in the RNA world.

Kinetic characterization of hairpin ribozyme variants

Kinetic analysis of ribozyme reactions is a common method to evaluate and compare activities of catalytic RNAs. The hairpin ribozyme catalyzes the reversible cleavage of a suitable RNA substrate at a specific site. Hairpin ribozyme variants as an allosteric ribozyme responsive to flavine mononucleotide and a hairpin-derived twin ribozyme that catalyzes two cleavage reactions and two ligation events with the result of a fragment exchange have been developed by rational design and were kinetically characterized. Herein, protocols for preparation of ribozymes and dye-labeled substrates as well as for analysis of cleavage, ligation, and fragment exchange reactions are provided.