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The use of vectors that are designed to allow positive selection of recombinants facilitates cloning experiments in E. coli. Using kid, a lethal gene of the R1 plasmid parD locus, we generated pKID vectors leading to high selective efficiency of recombinants (greater than 90%). The E. coli bacterial host used to propagate these vectors produces the Kis protein, the natural antagonist of Kid. This new positive-selection system exhibits the same efficiency as the original ccdB-based selection vectors, pKIL (4). We also show that the ccdB and kid systems are independent. This property increases the potential of plasmidic poison-antidote systems for genetic applications and opens the door to a generation of new vectors containing the two selection systems.

The use of PCR-amplified DNA-fragments is a classical approach to generate recombinant DNA. To facilitate the cloning of PCR products, we have constructed two new pKIL vectors that allow selection of recombinants. The multiple cloning sites (MCS) of these plasmids contain two adjacent Aspel sites and a unique HindII site. Cleavage of these vectors with Aspel produce linearized molecules with a single thymidine nucleotide at the 3' ends allowing TA cloning of Taq-amplified fragments. On the other hand, cleavage with HindII can be used for the cloning of blunt-ended PCR products generated by other DNA polymerases. The LacZ alpha-CcdB fusion protein produced by these plasmids has retained both the CcdB killer activity and the ability to alpha-complement the truncated LacZ delta M15. This bifunctionality allowed us to show that small PCR products (< 1000 bp) that do not disrupt lacZ alpha efficiently do inactivate CcdB, which demonstrates that the CcdB-based selection is well adapted for cloning of PCR products, especially for small size fragments.