Type II Toxin-Antitoxin Loci Encoded by Plasmids

Toxin–antitoxin (TA) loci were initially identified as auxiliary plasmid maintenance modules (Gerdes 2000). We review a few type II TA systems of the early list that are found in plasmids of Gram-negative bacteria and that have been more extensively characterized. These include the ccd system of plasmid F, the kis-kid system of plasmid R1, the higBA system of plasmid Rts1, and the parDE system of plasmid RK2. We also review two systems found in plasmids of Gram-positive bacteria: the ω-e-ζ system found in a plasmid pSM19035 of Streptococcus pyogenes and the axe-txe system found in pRUM, a multidrug resistant factor of Enterococcus faecium. Most of these systems have been analyzed both at the functional and structural levels and on the whole they provide an insight on the essential features, diversity, relationships, and relevance of type II TA loci.