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 essenti al gyrase of Escherichia coli, and CcdA, the unstable antidote that interac ts with CcdB to neutralize its toxicity. Although work from our group and o thers has established that CcdA and CcdB are required for transcriptional r epression of the operon, the underlying mechanism remains unclear. The resu lts 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 comp lex binds DNA. The addition of extra CcdB to that protein-DNA complex compl etely 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 Ccd B, repression results. In contrast, derepression occurs when CcdB is in exc ess of CcdA, By ensuring an antidote-toxin ratio greater than one, this mec hanism could prevent the harmful effect of CcdB in plasmid-containing bacte ria.