CcdA/CcdB Type II Toxin-antitoxin system

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CcdB Toxin of Type II Toxin-antitoxin system
PDB 2vub EBI.jpg
CcdB, a topoisomerase toxin from E. coli
CcdA Antitoxin of Type II Toxin-antitoxin system
Pfam clanCL0057

The CcdA/CcdB Type II Toxin-antitoxin system is one example of the bacterial toxin-antitoxin (TA) systems that encode two proteins, one a potent inhibitor of cell proliferation (toxin) and the other its specific antidote (antitoxin). These systems preferentially guarantee growth of plasmid-carrying daughter cells in a bacterial population by killing newborn bacteria that have not inherited a plasmid copy at cell division (post-segregational killing).[1]

The ccd system (control of cell death) of the F plasmid encodes two proteins, the CcdB protein (101 amino acids; toxin) and the CcdA antidote (72 amino acids). The antidote prevents CcdB toxicity by forming a tight CcdA–CcdB complex.[2]

Mechanism of action[edit]

The target of CcdB is the GyrA subunit of DNA gyrase, an essential type II topoisomerase in Escherichia coli.[3] 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 quinolones antibiotics.[4]

In absence of the antitoxin, the CcdB poison traps DNA-gyrase cleavable complexes, inducing breaks into DNA and cell death.[3]

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.[5]

Comparison with parD[edit]

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.[6]

Use and availability[edit]

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).[7] In August 2016, the CcdB positive selection technology falls completely within the public domain and is now fully free for personal or commercial use.


  1. ^ Bahassi EM, O'Dea MH, Allali N, Messens J, Gellert M, Couturier M (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. 274 (16): 10936–44. doi:10.1074/jbc.274.16.10936. PMID 10196173.
  2. ^ Madl T, Van Melderen L, Mine N, Respondek M, Oberer M, Keller W, Khatai L, Zangger K (November 2006). "Structural basis for nucleic acid and toxin recognition of the bacterial antitoxin CcdA". Journal of Molecular Biology. 364 (2): 170–85. doi:10.1016/j.jmb.2006.08.082. PMID 17007877.
  3. ^ a b Bernard P, Couturier M (August 1992). "Cell killing by the F plasmid CcdB protein involves poisoning of DNA-topoisomerase II complexes". Journal of Molecular Biology. 226 (3): 735–45. doi:10.1016/0022-2836(92)90629-X. PMID 1324324.
  4. ^ Bernard P, Kézdy KE, Van Melderen L, Steyaert J, Wyns L, Pato ML, Higgins PN, Couturier M (December 1993). "The F plasmid CcdB protein induces efficient ATP-dependent DNA cleavage by gyrase". Journal of Molecular Biology. 234 (3): 534–41. doi:10.1006/jmbi.1993.1609. PMID 8254658.
  5. ^ Afif H, Allali N, Couturier M, Van Melderen L (July 2001). "The ratio between CcdA and CcdB modulates the transcriptional repression of the ccd poison-antidote system". Molecular Microbiology. 41 (1): 73–82. doi:10.1046/j.1365-2958.2001.02492.x. PMID 11454201.
  6. ^ Smith AB, López-Villarejo J, Diago-Navarro E, Mitchenall LA, Barendregt A, Heck AJ, Lemonnier M, Maxwell A, Díaz-Orejas R (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 (9): e46499. doi:10.1371/journal.pone.0046499. PMC 3460896. PMID 23029540.
  7. ^ Bernard P (August 1996). "Positive selection of recombinant DNA by CcdB". BioTechniques. 21 (2): 320–3. PMID 8862819.

External links[edit]

This article incorporates text from the public domain Pfam and InterPro: IPR009956