Type II toxin-antitoxin loci: the ccdAB and ParDE families

The ccd F and parDE RK2 systems were originally identified on the E. coli F plasmid and on the RK2 broad-host-range plasmid, respectively. Early reports indicated that these two loci promote plasmid maintenance in growing population by killing daughter bacteria that did not inherit a plasmid copy at cell division by a mechanism termed ‘post-segregational killing’. The toxins encoded by these two loci target DNA-gyrase, an essential topoisomerase II, although the molecular details appear to be different. Interestingly, CcdBF and ParERK2 do not share a common ancestor, and therefore appear to have evolved independently. CcdB and ParE protein families share structural similarity with toxins belonging to the MazF and RelE toxin families, respectively, members of which inhibit translation by different mechanisms. In addition to focus on the molecular mechanism of poisoning of DNA-gyrase by CcdBF and ParERK2 as well as structural data, we describe here the different systems encoding CcdB-like and ParE-like toxins and highlight the specific features of those systems in terms of antitoxin structure and properties, autoregulation, biological functions, and evolution.