Biosensing by Tandem Reactions of Structure Switching, Nucleolytic Digestion, and DNA Amplification of a DNA Assembly

phi 29 DNA polymerase (phi 29DP) is able to carry out repetitive rounds of DNA synthesis using a circular DNA template by rolling circle amplification (RCA). It also has the ability to execute 3-5 digestion of single-stranded but not double-stranded DNA. A biosensor engineering strategy is presented that takes advantage of these two properties of phi 29DP coupled with structure-switching DNA aptamers. The design employs a DNA assembly made of a circular DNA template, a DNA aptamer, and a pre-primer. The DNA assembly is unable to undergo RCA in the absence of cognate target owing to the formation of duplex structures. The presence of the target, however, triggers a structure-switching event that causes nucleolytic conversion of the pre-primer by phi 29DP into a mature primer to facilitate RCA. This method relays target detection by the aptamer to the production of massive DNA amplicons, giving rise to dramatically enhanced detection sensitivity.