page_title: Toxin-antitoxin system
section_title: Genome stabilization
hierarchical_section_title: Toxin-antitoxin system / Biological functions / Genome stabilization
caption_attribution_description: TA system map of Sinorhizobium meliloti strain 1021
context_page_description: A toxin-antitoxin system is a set of two or more closely linked genes that together encode both a "toxin" protein and a corresponding "antitoxin". Toxin-antitoxin systems are widely distributed in prokaryotes, and organisms often have them in multiple copies. When these systems are contained on plasmids – transferable genetic elements – they ensure that only the daughter cells that inherit the plasmid survive after cell division. If the plasmid is absent in a daughter cell, the unstable antitoxin is degraded and the stable toxic protein kills the new cell; this is known as 'post-segregational killing'.
  Toxin-antitoxin systems are typically classified according to how the antitoxin neutralises the toxin. In a type I toxin-antitoxin system, the translation of messenger RNA that encodes the toxin is inhibited by the binding of a small non-coding RNA antitoxin that binds the toxin mRNA. The toxic protein in a type II system is inhibited post-translationally by the binding of an antitoxin protein. Type III toxin-antitoxin systems consist of a small RNA that binds directly to the toxin protein and inhibits its activity. There are also types IV-VI, which are less common.
context_section_description: Toxin-antitoxin systems could prevent harmful large deletions in a bacterial genome, though arguably deletions of large coding regions are fatal to a daughter cell regardless. In Vibrio cholerae, multiple type II toxin-antitoxin systems located in a super-integron were shown to prevent the loss of gene cassettes.
caption: A chromosome map of Sinorhizobium meliloti, with its 25 chromosomal toxin-antitoxin systems. Orange-labelled loci are confirmed TA systems[16] and green labels show putative systems.[17]
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