Abstract
BackgroundThe prokaryotic toxin-antitoxin systems (TAS, also referred to as TA loci) are widespread, mobile two-gene modules that can be viewed as selfish genetic elements because they evolved mechanisms to become addictive for replicons and cells in which they reside, but also possess "normal" cellular functions in various forms of stress response and management of prokaryotic population. Several distinct TAS of type 1, where the toxin is a protein and the antitoxin is an antisense RNA, and numerous, unrelated TAS of type 2, in which both the toxin and the antitoxin are proteins, have been experimentally characterized, and it is suspected that many more remain to be identified.ResultsWe report a comprehensive comparative-genomic analysis of Type 2 toxin-antitoxin systems in prokaryotes. Using sensitive methods for distant sequence similarity search, genome context analysis and a new approach for the identification of mobile two-component systems, we identified numerous, previously unnoticed protein families that are homologous to toxins and antitoxins of known type 2 TAS. In addition, we predict 12 new families of toxins and 13 families of antitoxins, and also, predict a TAS or TAS-like activity for several gene modules that were not previously suspected to function in that capacity. In particular, we present indications that the two-gene module that encodes a minimal nucleotidyl transferase and the accompanying HEPN protein, and is extremely abundant in many archaea and bacteria, especially, thermophiles might comprise a novel TAS. We present a survey of previously known and newly predicted TAS in 750 complete genomes of archaea and bacteria, quantitatively demonstrate the exceptional mobility of the TAS, and explore the network of toxin-antitoxin pairings that combines plasticity with selectivity.ConclusionThe defining properties of the TAS, namely, the typically small size of the toxin and antitoxin genes, fast evolution, and extensive horizontal mobility, make the task of comprehensive identification of these systems particularly challenging. However, these same properties can be exploited to develop context-based computational approaches which, combined with exhaustive analysis of subtle sequence similarities were employed in this work to substantially expand the current collection of TAS by predicting both previously unnoticed, derived versions of known toxins and antitoxins, and putative novel TAS-like systems. In a broader context, the TAS belong to the resistome domain of the prokaryotic mobilome which includes partially selfish, addictive gene cassettes involved in various aspects of stress response and organized under the same general principles as the TAS. The "selfish altruism", or "responsible selfishness", of TAS-like systems appears to be a defining feature of the resistome and an important characteristic of the entire prokaryotic pan-genome given that in the prokaryotic world the mobilome and the "stable" chromosomes form a dynamic continuum.ReviewersThis paper was reviewed by Kenn Gerdes (nominated by Arcady Mushegian), Daniel Haft, Arcady Mushegian, and Andrei Osterman. For full reviews, go to the Reviewers' Reports section.
Highlights
The prokaryotic toxin-antitoxin systems (TAS, referred to as TA loci) are widespread, mobile two-gene modules that can be viewed as selfish genetic elements because they evolved mechanisms to become addictive for replicons and cells in which they reside, and possess "normal" cellular functions in various forms of stress response and management of prokaryotic population
Bacterial toxin-antitoxin systems (TAS, referred to as TA loci) originally have been characterized in the 1980s as molecular systems encoded in plasmids and ensuring the persistence of a plasmid in a host lineage during replication by making the cells "addicted" to the plasmid so that only plasmid-containing daughter bacteria survived after a cell division [1,2]
Two complementary approaches for the prediction of new TAS The TAS are prone to frequent horizontal gene transfer (HGT) and intragenomic recombination, so they are commonly described as mobile genetic elements [26,27,32]
Summary
The prokaryotic toxin-antitoxin systems (TAS, referred to as TA loci) are widespread, mobile two-gene modules that can be viewed as selfish genetic elements because they evolved mechanisms to become addictive for replicons and cells in which they reside, and possess "normal" cellular functions in various forms of stress response and management of prokaryotic population. Bacterial toxin-antitoxin systems (TAS, referred to as TA loci) originally have been characterized in the 1980s as molecular systems encoded in plasmids and ensuring the persistence of a plasmid in a host lineage during replication by making the cells "addicted" to the plasmid so that only plasmid-containing daughter bacteria survived after a cell division [1,2]. As implied by their name, the overwhelming majority of TAS consist of two components encoded in an operon [3]. Unless the antitoxin is continuously replenished through gene expression, the free toxin accumulates in amounts sufficient to kill a cell, which is what occurs after cell division if a daughter cell does not receive the TAS-encoding plasmid [3,4,5]
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