CcdB

Overview
ccdB is a lethal gene that targets DNA gyrase. A strain of E. coli with a specific mutation in the gyrase (such as DB3.1, see E. coli genotypes) can be used to propagate ccdB. The lethal gene is useful for ensuring that the plasmid containing it cannot be propagated in standard E. coli strains (for cloning purposes). The ccdB positive-selection marker acts by killing the background of cells with no cloned DNA, only cells containing a recombinant DNA giving rise to viable clones (insertional inactivation of ccdB).