The ratio between CcdA and CcdB modulates the transcriptional repression of the ccd poison–antidote system

The ccd operon of the F plasmid encodes CcdB, a toxin targeting the essential gyrase of Escherichia coli, and CcdA, the unstable antidote that interacts with CcdB to neutralize its toxicity. Although work from our group and others has established that CcdA and CcdB are required for transcriptional r...

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Bibliographic Details
Published in:Molecular microbiology 2001-07, Vol.41 (1), p.73-82
Main Authors: Afif, Hassan, Allali, Noureddine, Couturier, Martine, Van Melderen, Laurence
Format: Article
Language:English
Subjects:
Bacterial Proteins - genetics
Bacterial Proteins - metabolism
Bacterial Toxins - genetics
Bacterial Toxins - metabolism
ccd gene
CcdA protein
CcdB protein
DNA, Bacterial - metabolism
Electrophoresis, Polyacrylamide Gel
Escherichia coli
Escherichia coli - genetics
Gene Expression Regulation, Bacterial
Mutation
Operator Regions, Genetic - genetics
Operator Regions, Genetic - physiology
Operon - genetics
Plasmids - genetics
Promoter Regions, Genetic - genetics
Promoter Regions, Genetic - physiology
Transcription, Genetic
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Summary:The ccd operon of the F plasmid encodes CcdB, a toxin targeting the essential gyrase of Escherichia coli, and CcdA, the unstable antidote that interacts with CcdB to neutralize its toxicity. Although work from our group and others has established that CcdA and CcdB are required for transcriptional repression of the operon, the underlying mechanism remains unclear. The results presented here indicate that, although CcdA is the DNA‐binding element of the CcdA–CcdB complex, the stoichiometry of the two proteins determines whether or not the complex binds to the ccd operator–promoter region. Using electrophoretic mobility shift assays, we show that a (CcdA)2–(CcdB)2 complex binds DNA. The addition of extra CcdB to that protein–DNA complex completely abolishes DNA retardation. Based on these results, we propose a model in which the ratio between CcdA and CcdB regulates the repression state of the ccd operon. When the level of CcdA is superior or equal to that of CcdB, repression results. In contrast, derepression occurs when CcdB is in excess of CcdA. By ensuring an antidote–toxin ratio greater than one, this mechanism could prevent the harmful effect of CcdB in plasmid‐containing bacteria.
ISSN:0950-382X
1365-2958
DOI:10.1046/j.1365-2958.2001.02492.x