A simple modification was developed for construction of genomic/metagenomic libraries by T-vectors. Topoisomerase II was employed to introduce superhelical turns into relaxed circular T-A recombinant molecules, which greatly increased the transformation efficiency. By using this technique, a metagenomic library was constructed and a γ-hexachlorocyclohexane dehydrochlorinase gene linA-like was screened from it.