(A) Univector plasmid fusion system. Cre-loxP–mediated site-specific recombination fuses the pUNI and pHOST plasmids at the loxP site. As a result, the gene of interest is placed under the control of the pHOST promoter and fused to any tag sequences present in the pHOST plasmid. Figure adapted from Liu et al. (1998). (B) Schematic illustration of directional topoisomerase cloning into the pUNI-D vector. The sequence 5′-CACC is required at the 5′ end of the PCR product for directional topoisomerase-mediated cloning. The oval represents vaccinia virus topoisomerase I. In the example shown, the 5′-CACC sequence is appended immediately 5′ of the ATG start codon of the gene to be inserted.
