CcdA/CcdB Type II Toxin-antitoxin system

Mechanism of action

The target of CcdB is the GyrA subunit of DNA gyrase, an essential type II topoisomerase in Escherichia coli. Gyrase alters DNA topology by effecting a transient double-strand break in the DNA backbone, passing the double helix through the gate and resealing the gaps. The CcdB poison acts by trapping DNA gyrase in a cleaved complex with the gyrase A subunit covalently closed to the cleaved DNA, causing DNA breakage and cell death in a way closely related to quinolone antibiotics.

In absence of the antitoxin, the CcdB poison traps DNA-gyrase cleavable complexes, inducing breaks into DNA and cell death.

Regulation of the ccd operon by the CCdA/CCdB complex is dependent upon the ratio of the two molecules to each other in the complex: a (CcdA)2–(CcdB)2 complex binds the DNA of the operon thus repressing transcription, but when CcdB is in excess of CcdA de-repression occurs, whereas repression will occur when CcdA levels are greater than or equal to that of CcdB. As a model system, by ensuring an antidote–toxin ratio greater than one, this mechanism might prevent the harmful effect of CcdB in plasmid-containing bacteria.

Comparison with parD

The Ccd and parD systems are found to be strikingly similar in terms of their structures and actions. The antitoxin protein of each system interacts with its cognate toxin to neutralise the activity of the toxin and in the process the complex of the two becomes an efficient transcription repressor.

Use and availability

In recombinant DNA technology, the ccdB gene is widely used as a positive selection marker (e.g. the Invitrogen's Zero Background and Gateway cloning vectors). In August 2016, the CcdB positive selection technology falls completely within the public domain and is now fully free for personal or commercial use. Ccd operon was also used to stabilize plasmid for industrial use in the Staby(r) technology developed and commercialized by Delphi Genetics. In this technology, conventional antibiotic resistance gene is replaced by ccdA in the plasmid while ccdB gene is introduced into the chromosome of the bacteria. This technology allows to remove antibiotic resistance gene but is also able to reach higher yields in recombinant protein production and plasmid DNA. Some applications of this technology are patented and could need a license for commercial exploitation.

References

References

1. (April 1999). "Interactions of CcdB with DNA gyrase. Inactivation of Gyra, poisoning of the gyrase-DNA complex, and the antidote action of CcdA". The Journal of Biological Chemistry.
2. (November 2006). "Structural basis for nucleic acid and toxin recognition of the bacterial antitoxin CcdA". Journal of Molecular Biology.
3. (December 1993). "The F plasmid CcdB protein induces efficient ATP-dependent DNA cleavage by gyrase". Journal of Molecular Biology.
4. (August 1992). "Cell killing by the F plasmid CcdB protein involves poisoning of DNA-topoisomerase II complexes". Journal of Molecular Biology.
5. (July 2001). "The ratio between CcdA and CcdB modulates the transcriptional repression of the ccd poison-antidote system". Molecular Microbiology.
6. (2012). "A common origin for the bacterial toxin-antitoxin systems parD and ccd, suggested by analyses of toxin/target and toxin/antitoxin interactions". PLOS ONE.
7. (August 1996). "Positive selection of recombinant DNA by CcdB". BioTechniques.
8. (2005). "Separate-component-stabilization system for protein and DNA production without the use of antibiotics". BioTechniques.