Reverse transcription as key step in RNA in vitro evolution with unnatural base pairs†

Unnatural base pairs (UBPs) augment the chemical diversity of artificial nucleic acids and can thus enable the generation of new aptamers and catalytic nucleic acids by in vitro selection. However, owing to a lack of methodologies, the reverse transcription of UBPs, a key step in RNA aptamer selection, has not been sufficiently characterized. Here, we present a series of versatile assays to investigate the reverse transcription of the TPT3:NaM base pair as a representative for hydrophobic unnatural base pairs. We determine the fidelity and retention of the UBP for four different reverse transcriptases (RT) in the context of RNA in vitro evolution. The retention of the TPT3:NaM pair during the RNA in vitro selection process was investigated using a novel click-chemistry based electromobility shift assay. Real-time monitoring of reverse transcription kinetics revealed considerable differences in polymerase activity processing the TPT3:NaM base pair. Our findings identified SuperScript IV RT as the most efficient RT for processing the TPT3:NaM pair. Our approach can be applied universally to study newly developed UBPs, not only at the reverse transcription level, but also during PCR and in vitro transcription.