The key to TOPO cloning is the enzyme DNA topoisomerase I, which functions both as a restriction enzyme and as a ligase. Its biological role is to cleave and rejoin DNA during replication. Vaccinia virus topoisomerase I specifically recognizes the pentameric sequence 5´-(C/T)CCTT-3´ and forms a covalent bond with the phosphate group attached to the 3´ thymidine. It cleaves one DNA strand, enabling the DNA to unwind. The enzyme then relegates the ends of the cleaved strand and releases itself from the DNA. To harness the religating activity of topoisomerase, TOPO vectors are provided linearized with topoisomerase I covalently bound to each 3´ phosphate. This enables the vectors to readily ligate DNA sequences with compatible ends (Figures 1, 2, and 3). The ligation is complete in only 5 minutes at room temperature.

Figure 1. TOPO TA cloning of Taq-amplified DNA.

Figure 2 .  Zero Blunt TOPO cloning of blunt-end DNA.

Figure 3.  Directional TOPO cloning of blunt-end DNA.