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).
- Austin 16:35, 7 Oct 2005 (EDT): I observed a strange result where when DB3.1 and non-DB3.1 strains were transformed with non-ccdB plasmids (three different plasmids), the transformation efficiency of DB3.1 was markedly lower. When the same strains were transformed with ccdB plasmids, as expected, the DB3.1 strain had many colonies and non-DB3.1 had zero or close to zero. I'm not sure why the DB3.1 transformation appears to need ccdB as the strain is propagated fine without any plasmid.
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