Series or parallel toehold-mediated strand displacement and its application in circular RNA detection and logic gates

Toehold-mediated strand displacement (TMSD) is widely employed in constructing a wide range of chemical reaction networks. In TMSD, single-stranded DNA or RNA can fold back upon itself to form a local short doublestrand structure often hindering bimolecular hybridization. Here, based on series and parallel circuits, we introduce two mechanisms: series toehold-mediated strand displacement (STMSD) and parallel toehold-mediated strand displacement (PTMSD). These mechanisms can be highly effective when the target area is blocked by a secondary structure. In addition, these systems allow regulating the reaction rates spanning three to five orders of magnitude by adjusting the length of the two toeholds with the added advantage of multifunctional regulation and selectivity. To demonstrate the impressive function of this approach, a logic operation system based on STMSD was constructed to simulate the signal processing of a half-adder. We believe that the introduction of series and parallel toeholds will provide design flexibility contributing to the development of molecular computers, molecular robotics, and DNA-based biosensors.