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Bacteriophages Display 'Infectious Altruism' through Antitoxin Mimicry and Horizontal Transfer

A toxin-antitoxin system is a set of two or more closely linked genes that together encode both a protein 'poison' and a corresponding 'antidote'. There are 3 types of these systems: Type I toxin-antitoxin systems rely on the base-pairing of complementary antitoxin RNA with the toxin's mRNA, Type 2 where a protein antitoxin tightly binds and inhibits the activity of a stable toxin, and finally Type 3 which rely on direct interaction between a toxic protein and an RNA antitoxin. In type 3 systems, the toxic effects of the protein are neutralised by RNA molecules which interact and inhibit the activity of the toxic protein.
These systems can be used for plasmid maintanence, formation of persistor cells, and defense systems for bacteriophage infection. This study identified phiTE phage mutants that could escape a Type 3 toxin which normally activates a suicide mechanism in bacteria in order to altruistically prevent a productive bacteriophage infection. Genome sequencing of these phage escape artists revealed an expansion of repeats which were similar in sequence to an antitoxic non-coding RNA. It was found that this expansion was necessary so that the RNA molecule could properly fold in order to mimic the host encoded ToxI RNA molecule, which normally inhibits ToxN, the toxic protein.
Since the ToxI RNA molecule is usually expressed from a plasmid, the authors tested if this phage could also escape the toxic system by horizontally transferring this plasmid. Indeed, this horizontal transmission, presumably a result of co-packaging, was demonstrated.
Collectively, this study found that phage could either evolve sequences mimicking an anti-toxin system or copackage a plasmid already containing this defensive system. In doing so it may have indirectly created populations of host cells that the phage can successfully replicate within while potentially providing the new host with better protection from competing viral predators. An example of 'infectious altruism'
 
 

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