[Recombinant expression of truncated exonuclease Ⅷ and its application in in vitro DNA recombination]

Exonuclease Ⅷ (Exo Ⅷ), an ATP-independent dsDNA 5'-3' exonuclease, is a candidate protein with great application value for in vitro DNA recombination. However, the application of Exo Ⅷ in DNA recombination in vitro has not been reported. In this study, the recombinant expression vector of the truncated Exo Ⅷ (tExo Ⅷ) with the full exonuclease activity was built and used to achieve the overexpression of tExo Ⅷ in Escherichia coli. Based on the purified tExo Ⅷ protein with high-purity, the feasibility of tExo Ⅷ applied in vitro DNA recombination and effects of the reaction temperatures, reaction duration, and homology arm lengths were examined. The results showed that tExo Ⅷ was highly expressed in soluble form in E. coli. One liter of bacterial culture yielded 92.40 mg of purified tExo Ⅷ with the specific activity of 1.21×10⁵ U/mg. In a 10 μL recombination system containing 2.5 U tExo Ⅷ, the highest cloning efficiency was achieved in a reaction at 25 °C for 12.5 min and followed by incubation at 50 °C for 50 min. With addition of Pfu DNA polymerase, the homology arm extension strategy can effectively improve the recombination efficiency. Using competent E. coli Mach1 T1 with 2.2×10⁶ cfu/μg transformation efficiency as recipient cell, the recombination of a 1 kb fragment with a 21 bp homology arm and a 5.8 kb linearized vector can form about 1.1×10⁴ recombinant clones per μg vector, and the positive rates was over 80%. The recombination efficiency was increased with the increasing length of homology arm ranged from 8 to 21 bp. Under the optimal reaction condition, only 8 bp homology arm can still achieve valid DNA recombination. This novel in vitro DNA recombination system mediated by tExo Ⅷ was particularly characterized by its easy preparation, no limitation on restriction sites and high recombination cloning efficiency. All results revealed that the new efficient gene cloning system has potential application in the field of molecular biology.