Viruses talk to each other, and not just about the weather. They talk about whether or not they should infect or kill their host (no big deal, right?).
Rotem Zorek, lead author Zohar Erez, and colleagues at the Weizmann Institute of Science in Israel report their findings in the most recent issue of Nature. This is the first time that viruses have been shown to communicate with each other.
Bacteria talk to each other through a process called quorum sensing - that’s old news. Zorek and his team wanted to know if bacteria talked to each other about phages (viruses that infect bacteria).
They found that phi3T, a phage that infects Bacillus subtilis, produces a chemical signal that tells other phages whether they should kill the bacteria or just infect them. By “kill” or “infect”, I really mean whether the phages should undergo a lysogenic or lytic infection.
During a lysogenic infection, the phage incorporates its own genetic material into the bacteria’s genome (or it may exist as a circular replicon in the cytoplasm). The lysogen hangs out for a while, waiting for just the right conditions in which to replicate (viral replication ultimately kills the host cell). In a lytic infection, the phage enters its host cell and starts replicating immediately.
The group made their discovery when they added phi3T to a culture of B. subtilis. The phage killed a majority of the bacteria, and they collected the “conditioned medium” - they removed the phage and bacteria, leaving behind small proteins and possible signaling molecules.
They took the conditioned medium and added it to a new batch of bacteria and phage. Now, instead of producing a lytic infection and killing a majority of bacteria, most phages produced a lysogenic infection! As such, they named the chemical signal “arbitrium” - the Latin word for “decision.”
It turns out that arbitrium is a small protein that is made when the phage protein AimP is degraded. Arbitrium is then released by bacterial cells when they die - sort of like a “this phage was here” marker. Arbitrium is sucked up into neighboring bacterial cells by the OPP transporter. A little bit of arbitrium tells phages they can produce a lytic infection, but a higher level tells phages to put on the brakes and enter a lysogenic cycle. After all, you don’t want to kill all your host cells at once!
It’s a bit more complicated in practice, however. More specifically, two phage genes - aimR and aimP - are expressed when phages first infect bacteria. AimR promotes aimX expression, and AimX inhibits lysogeny. At the same time, AimP accumulates - it binds and inhibits AimR to promote lysogeny.
The next question is whether disease-causing human viruses communicate in a similar way. HIV, for example, has a similar lifestyle - perhaps viral communication could be hijacked, telling the virus to stop killing human cells.