Sequence-Specific Fluorescence Turn-On Sensing of RNA by DNA Probes Incorporating the Tricyclic Cytidine Analogue tC.

Sequence-specific fluorescent probes for RNA are widely used in microscopy applications such as fluorescence in situ hybridization and a growing number of newer approaches to live-cell RNA imaging. The sequence specificity of most of these approaches relies on differential hybridization of the probe to the correct target. Competing sequences with only one or two base mismatches are prone to causing off-target recognition. Here, we report the sequence-specific fluorescent detection of model RNA targets using a tricyclic cytidine analogue tC that is included as a surrogate for natural cytidine in DNA probe strands and that reports directly on Watson-Crick base pairing. The tC-containing DNA oligonucleotide probes exhibit an average 8-fold increase in fluorescence intensity when hybridized to matched RNA with tC base paired with G and little fluorescence turn-on when tC is base paired with A. Duplex structure determination by NMR, time-resolved fluorescence studies, and Stern-Volmer quenching experiments suggest that the combination of greater π stacking and narrower grooves in the A-form DNA-RNA heteroduplex provides additional shielding and favorable electronic interactions between bases, explaining why tC's fluorescence turn-on response to RNA targets is typically 3-fold greater than for DNA targets.