Toxic component of a type II toxin-antitoxin (TA) system, functioning in plasmid maintainence. Responsible for the post-segregational killing (PSK) of plasmid-free cells, also referred to as a plasmid addiction system. Half-life of over 2 hours. Interferes with the activity of DNA gyrase, inducing it to form a covalent GyrA-DNA complex that cannot be resolved, thus promoting breakage of plasmid and chromosomal DNA. Toxicity is inhibited by labile antitoxin CcdA, which blocks the activity of CcdB; CcdA also removes bound CcdB protein from the CcdB-GyrA complex by forming a CcdA-CcdB complex, a process termed rejuvenation. Functions as a transcriptional corepressor for the ccdAB operon, repression also requires CcdA (By similarity). Homodimer. Forms a complex with GyrA, probably a tetramer GyrA(2)CcdB(2), in which GyrA is inactive. Forms a complex with toxin CcdB; the CcdA-CcdB(2) trimer is sufficient for rejuvenation, whereas maximal operon repression occurs with CcdA(2)CcdB(2) (By similarity). Belongs to the CcdB toxin family.