 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.The target of CcdB is the GyrA subunit of DNA gyrase, an essential type II topoisomerase in "Escherichia coli".