From 2007.igem.org

TOPO cloning TOPO overview: The pTrcHis TOPO TA Expression Kit provides an efficient cloning strategy for the direct insertion of Taq polymerase-amplified PCR products into a plasmid vector for expression in E. coli. No ligase, post-PCR procedures, or PCR primers containing specific sequences are required. Once the PCR product of interest is TOPO Cloned into pTrcHis-TOPO, the construct is transformed into E. coli and expression is induced with isopropyl β-thiogalactoside (IPTG).