Molecular beacon-style hybridization assay for quantitative analysis of surface invasive cleavage reactions.

A hybridization-based FRET format for the scoring of SNPs in surface invasive cleavage reactions is described. In early versions of the surface invasive cleavage reaction, dual-labeled oligonucleotides, containing both a quencher moiety and a fluorophore, were attached to the substrate. The invasive cleavage reaction cleaved the DNA strand between the two, resulting in an increase in fluorescence signal due to the separation of the quencher from the fluorophore. A limitation of this assay format was the relatively low quenching efficiency of 84% obtained, as well as the complexity of synthesis for these dual-labeled probes. In the assay format presented here, singly labeled oligonucleotides are employed, with the quencher and fluorophore placed on separate complementary oligonucleotides. The surface-bound probe is terminated at the 5' end with the quencher and the complement is terminated at its 3' end with a fluorophore, such that upon hybridization the two are positioned directly across from one another in the duplex. Quenching efficiency in this "molecular beacon" format is increased to 88%, much closer to the 91% level that has been reported for molecular beacon assays. A second benefit of the approach described here is that the portion of probe oligonucleotide that is removed by the enzyme is shorter, thus increasing the rate of probe cleavage. The improved quenching efficiency and increased probe cleavage rate result in a lower detection limit for the assay. A theoretical model of the FRET process occurring on the surfaces was used to relate the observed surface fluorescence intensity to the progress of the invasive cleavage reaction.