Oligonucleotides labeled with single fluorophores as sensors for deoxynucleotide triphosphate binding by DNA polymerases

Primer design strategy for denaturation bubble-mediated strand exchange amplification

Denaturation bubble-mediated strand exchange amplification (SEA) is a novel, rapid isothermal nucleic acid amplification has been applied for point-of-care molecular diagnostic in food safety, meat adulteration, forest disease and animal disease. Nevertheless, the absence of specialized strategy for SEA primers design led to long-time of primer screening progress before SEA reaction execution, which would largely increase the time consuming when SEA is utilized for detecting other new targets. In this present work, we investigated the impact of the following primers' attributes on SEA efficiency, including Tm value, 3' end G/C content, self-complementary and 3' complementary, according to which we demonstrated that optimal Tm value and reaction temperature were all 61 °C, while 3'-terminal nucleotide should be G/C, as the SEA reaction induced by the primers possessing these attributes exhibited significantly lower threshold time (Tt) value. Moreover, self-complementary and 3' complementary of primers should be avoided. Besides, we also discussed the consideration priority order of these factors, which was self-complementary and 3' complementary, Tm value and 3' end G/C content in turn. Because the SEA primer design strategy is first presented, our work will greatly promote the application of SEA in point-of-care test.

Fluorogenic DNA ligase and base excision repair enzyme assays using substrates labeled with single fluorophores

Continuing our work on fluorogenic substrates labeled with single fluorophores for nucleic acid modifying enzymes, here we describe the development of such substrates for DNA ligases and some base excision repair enzymes. These substrates are hairpin-type synthetic DNA molecules with a single fluorophore located on a base close to the 3' ends, an arrangement that results in strong fluorescence quenching. When such substrates are subjected to an enzymatic reaction, the position of the dyes relative to that end of the molecules is altered, resulting in significant fluorescence intensity changes. The ligase substrates described here were 5' phosphorylated and either blunt-ended or carrying short, self-complementary single-stranded 5' extensions. The ligation reactions resulted in the covalent joining of the ends of the molecules, decreasing the quenching effect of the terminal bases on the dyes. To generate fluorogenic substrates for the base excision repair enzymes formamido-pyrimidine-DNA glycosylase (FPG), human 8-oxo-G DNA glycosylase/AP lyase (hOGG1), endonuclease IV (EndoIV), and apurinic/apyrimidinic endonuclease (APE1), we introduced abasic sites or a modified nucleotide, 8-oxo-dG, at such positions that their enzymatic excision would result in the release of a short fluorescent fragment. This was also accompanied by strong fluorescence increases. Overall fluorescence changes ranged from approximately 4-fold (ligase reactions) to more than 20-fold (base excision repair reactions).

Generic assay format for endo- and exonucleases based on fluorogenic substrates labeled with single fluorophores

We previously described the development of fluorogenic assays for nucleic acid-modifying enzymes based on synthetic oligonucleotides labeled with a single fluorophore. In the current work, we studied the performance of such singly labeled substrates as a function of the nucleotide sequence in the vicinity of the fluorophore and the nature of the fluorophore itself. In agreement with published studies, we found that a 3' end of the primer terminating in a dC residue opposite a 5' dG provides the greatest degree of fluorophore quenching. Adding a second dC residue at the 3' penultimate position opposite another dG increased the quenching further. Among the various dyes tested, the difluoro substituted fluorescein derivative Oregon Green emerged as a superior fluorophore for this assay format. We have now combined these findings into a new generic format for endonuclease assays. This format allows a substrate for any endonuclease to be obtained rapidly by simply replacing the enzyme's recognition sequence within the generic labeled molecule. Compared with our previous format, the new assays show greatly expanded signal dynamic ranges. The format is applicable to other nucleic acid-modifying enzymes such as exonucleases (e.g., T7 gene 6 exonuclease) and DNA repair enzymes (e.g., uracil-DNA glycosylase).