Expression of the F plasmid ccd toxin-antitoxin system in Escherichia coli cells under nutritional stress.

The ccd system of the F plasmid encodes CcdB, a protein toxic to DNA-gyrase, and CcdA, its antitoxin. The function attributed to this system is to contribute to plasmid stability by killing bacteria that lose the plasmid during cell division. However, the function of ccd in resting bacteria is not clear. Results presented show that ccd transcription increases as bacteria enter stationary phase and that the amount of the Ccd proteins is higher in bacteria under nutritional stress than in growing bacteria. Moreover, an increase in the frequency of Lac+ "adaptive" mutations was observed in stationary-phase bacteria that over-express the Ccd proteins.