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UniProt release 2017_03

Published March 15, 2017


Viral Short Message Service: peptide texting guides the outcome of infection

Communication is not simply a dispensable tool invented by Homo sapiens to do business and to have an enjoyable social life. Long before the advent of cell phones, most living organisms, from animals and plants to bacteria, were communicating with each other in order to ensure species survival. The recent discovery of a peptide-based communication system in some bacterial viruses extends this observation far beyond our wildest imaginings.

Some bacterial viruses, called temperate bacteriophages, have the ability to infect their host through a lytic (productive infection) or a lysogenic (latent) cycle. The lytic cycle leads to the lysis of the host bacterial cell and release of progeny virions. In the lysogenic cycle, on the other hand, the bacteriophage genome becomes integrated into the host genome as a prophage without any virion production. The decision between lysis and lysogeny is probabilistic in nature, but usually depends on the number of co-infecting viruses and the bacterial nutritional state. When uninfected bacteria are abundant and healthy, the lytic pathway is preferred. In later stages of infection, when the number of uninfected bacteria is reduced, progeny phages are at risk of no longer having a new host to infect. At this point, lysogeny is favoured. Although the molecular mechanism undelying the phage lytic or lysogenic decision is still largely unknown, even in well-studied bacteriophages like Lambda or Mu, a substantial leap forward was made earlier this year.

Erez et al. were investigating whether phage-infected bacteria may produce molecules to alert other bacterial cells of their infection, when they made an amazing discovery. A screening of the culture medium of Bacillus subtilis infected by Phi3T bacteriophages led to the identification not of a bacterial, but of a... viral hexapeptide! This peptide was called AimP. The bacteriophage also encodes a cytoplasmic receptor for AimP, called AimR. In the absence of AimP, the AimR receptor behaves as a DNA-binding homodimer which activates the transcription of a third phage component of the system, AimX. AimX is a regulatory non-coding RNA which favors lysis, either by inhibiting lysogeny or by promoting lysis, in an as yet undefined manner. In the presence of AimP, the AimR receptor becomes a peptide-bound, transcriptionally inactive monomer. As a result, the expression of AimX drops and lysogeny is promoted.

The current experimental data suggest the following model. AimP is synthesized in infected bacteria as a pre-pro-peptide. Its N-terminal signal sequence is recognized by the host secretion system and cleaved off upon secretion. Once released in the extracellular milieu, the inactive pro-peptide is further processed by bacterial extracellular proteases to yield the mature active 6 amino-acid long AimP peptide, which is internalized by surrounding bacteria through the oligopeptide permease transporter (OPP). AimP accumulates in the bacterial cytoplasm. When a phage infects an ‘AimP-rich’ bacterium, the expressed AimR receptor binds AimP and cannot activate the expression of AimX, leading to preferential lysogeny. In other words, a phage can “sense” the level of global infection in the environment and adapt to preserve chances for viable reproduction.

This viral mode of 3-membered communication has been called ‘arbitrium’ (after the Latin word meaning ‘decision’). It may not be restricted to Phi3T bacteriophages. Indeed, Erez et al. found 112 instances of AimR homologues in Bacillus phages and, in all cases, aimR homologues were found upstream of aimP candidate genes.

As of this release, Bacillus phage Phi3T AimP and AimR entries have been updated and are publicly available.

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