Antitoxin component of a type II toxin-antitoxin (TA) system which inhibits the post-segregational killing (PSK) of plasmid-free cells, also referred to as a plasmid addiction system. Labile antitoxin with a half-life of about 1 hour in the presence of CcdB. Binds to and blocks the activity of CcdB; will also remove bound CcdB protein from the CcdB-GyrA complex by forming a CcdA-CcdB complex, a process termed rejuvenation. The N-terminal 36 residues are not required for rejuventation. Functions as a transcriptional corepressor for the ccdAB operon, repression also requires CcdB. Homodimer in solution and when bound to DNA. Three CcdA homodimers bind to the promoter region and contact each other via Val-22. Forms a complex with toxin CcdB; there are both high- and low-affinity binding sites for CcdA such that both CcdA-CcdB(2) and CcdA(2)CcdB(2) complexes can form. The CcdA-CcdB(2) trimer is sufficient for rejuvenation, whereas maximal operon repression occurs with CcdA(2)CcdB(2). When the CcdA:CcdB ratio is lower than 1, a CcdA(2)-CcdB(4) complex is formed which is devoid of repression activity. In this case repression is alleviated and both proteins are produced. Degraded by the Lon protease. Leads to cell death and induction of the SOS pathway. Additional disruption of ccdB reverses this effect, i.e. no cell death nor induction of the SOS response. The double disruption leads to increased plasmid loss. Belongs to the CcdA antitoxin family.