Preparation of DNA backbone

The DNA backbone was prepared by RCA⁵². A volume of 10 μl of 3 μM circular DNA template and 0.5 μl of 100 μM DNA primer was mixed and annealed at 95 °C for 4 min. Then, the mixture was cooled to room temperature over 2 h and incubated with phi29 DNA polymerase (0.2 U μl⁻¹), bovine serum albumin (0.4 μg μl⁻¹) and dNTPs (0.1 mM) at 37 °C for 5 h in 150 μl of 1 × phi29 reaction buffer. After reaction, the mixture was incubated at 65 °C for 10 min to denature the phi29 DNA polymerase, and then purified by ultrafiltration (100,000 molecular weight cutoff membrane, Millipore) for three times to obtain the DNA backbone. DNA backbone has repeating strand segments with sequences complementary to TRU. The concentration of the repeat unit in DNA backbone was obtained by measuring ultraviolet absorbance at 260 nm and used as the concentration of DNA backbone²⁹.