Cell killing by the F plasmid CcdB protein involves poisoning of DNA-topoisomerase II complexes

J Mol Biol. 1992 Aug 5;226(3):735-45. doi: 10.1016/0022-2836(92)90629-x.


In Escherichia coli, the miniF plasmid CcdB protein is responsible for cell death when its action is not prevented by polypeptide CcdA. We report the isolation, localization, sequencing and properties of a bacterial mutant resistant to the cytotoxic activity of the CcdB protein. This mutation is located in the gene encoding the A subunit of topoisomerase II and produces an Arg462----Cys substitution in the amino acid sequence of the GyrA polypeptide. Hence, the mutation was called gyrA462. We show that in the wild-type strain, the CcdB protein promotes plasmid linearization; in the gyrA462 strain, this double-stranded DNA cleavage is suppressed. This indicates that the CcdB protein is responsible for gyrase-mediated double-stranded DNA breakage. CcdB, in the absence of CcdA, induces the SOS pathway. SOS induction is a biological response to DNA-damaging agents. We show that the gyrA462 mutation suppresses this SOS activation, indicating that SOS induction is a consequence of DNA damages promoted by the CcdB protein on gyrase-DNA complexes. In addition, we observe that the CcdBS sensitive phenotype dominates over the resistant phenotype. This is better explained by the conversion, in gyrA+/gyrA462 merodiploid strains, of the wild-type gyrase into a DNA-damaging agent. These results strongly suggest that the CcdB protein, like quinolone antibiotics and a variety of antitumoral drugs, is a DNA topoisomerase II poison. This is the first proteinic poison-antipoison mechanism that has been found to act via the DNA topoisomerase II.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Sequence
  • Bacterial Proteins / metabolism*
  • Bacterial Toxins / metabolism*
  • Cell Death
  • Cytotoxins / metabolism
  • DNA Damage
  • DNA Gyrase
  • DNA Topoisomerases, Type II / genetics
  • DNA Topoisomerases, Type II / metabolism*
  • DNA, Bacterial / genetics
  • DNA, Bacterial / metabolism*
  • Escherichia coli / cytology
  • Escherichia coli / genetics
  • Escherichia coli / metabolism*
  • F Factor*
  • Genes, Bacterial
  • Macromolecular Substances
  • Mutation
  • Plasmids
  • Protein Binding
  • Recombination, Genetic
  • Restriction Mapping
  • SOS Response, Genetics


  • Bacterial Proteins
  • Bacterial Toxins
  • CcdB protein, Plasmid F
  • Cytotoxins
  • DNA, Bacterial
  • Macromolecular Substances
  • DNA Gyrase
  • DNA Topoisomerases, Type II