Probing the Mechanism of Structure-Switching Aptamer Assembly by Super-Resolution Localization of Individual DNA Molecules.

Oligonucleotide aptamers can be converted into structure-switching biosensors by incorporating a short, typically labeled oligonucleotide that is complementary to the analyte-binding region. Binding of a target analyte can disrupt the hybridization equilibrium between the aptamer and the labeled-complementary oligo producing a concentration-dependent signal for target-analyte sensing. Despite its importance in the performance of a biosensor, the mechanism of analyte-response of most structure-switching aptamers is not well understood. In this work, we employ single-molecule fluorescence imaging to investigate the competitive kinetics of association of a labeled complementary oligonucleotide and a target analyte, l-tyrosinamide (L-Tym), interacting with an L-Tym-binding aptamer. The complementary readout strand is fluorescently labeled, allowing us to measure its hybridization kinetics with individual aptamers immobilized on a surface and located with super-resolution techniques; the small-molecule L-Tym analyte is not labeled in order to avoid having an attached dye molecule impact its interactions with the aptamer. We measure the association kinetics of unlabeled L-Tym by detecting its influence on the hybridization of the labeled complementary strand. We find that L-Tym slows the association rate of the complementary strand with the aptamer but does not impact its dissociation rate, suggesting an SN1-like mechanism where the complementary strand must dissociate before L-Tym can bind. The competitive model revealed a slow association rate between L-Tym and the aptamer, producing a long-lived L-Tym-aptamer complex that blocks hybridization with the labeled complementary strand. These results provide insight about the kinetics and mechanism of analyte recognition in this structure-switching aptamer, and the methodology provides a general means of measuring the rates of unlabeled-analyte binding kinetics in aptamer-based biosensors.