Background: Although often viewed as elements "at the service of" bacteria, plasmids exhibit replication and maintenance mechanisms that make them purely "selfish DNA" candidates. Toxin-antitoxin (TA) systems are a spectacular example of such mechanisms: a gene coding for a cytotoxic stable protein is preceded by a gene coding for an unstable antitoxin. The toxin being more stable than the antitoxin, absence of the operon causes a reduction of the amount of the latter relative to the amount of the former. Thus, a cell exhibiting a TA system on a plasmid is 'condemned' either not to loose it or to die.
Results: Different TA systems have been described and classified in several families, according to similarity and functional parameters. However, given the small size and large divergence among TA system sequences, it is likely that many TA systems are not annotated as such in the rapidly accumulating NCBI database. To detect these putative TA systems, we developed an algorithm that searches public databases on the basis of predefined similarity and TA-specific structural constraints. This approach, using a single starting query sequence for each of the ParE, Doc, and VapC families, and two starting sequences for the MazF/CcdB family, identified over 1,500 putative TA systems. These groups of sequences were analyzed phylogenetically for a better classification and understanding of TA systems evolution.
Conclusion: The phylogenetic distributions of the newly uncovered TA systems are very different within the investigated families. The resulting phylogenetic trees are available for browsing and searching through a java program available at http://ueg.ulb.ac.be/tiq/.