Many viruses face a choice after they have infected their hosts: to replicate quickly, killing the cell in the process, or to become dormant and lie in wait. HIV, herpes, and a number of other human viruses behave this way and, in fact, even the viruses that attack bacteria – phages – face similar decisions when invading a cell.

What causes a virus to choose dormancy over immediate gratification? Prof. Rotem Sorek and his group in the Weizmann Institute’s Department of Molecular Genetics have now discovered that, during infection, viruses secrete small molecules into their environment that other viruses can pick up and “read.” In this way, they can actually coordinate their attack, turning simple messages into a fairly sophisticated strategy.

To find evidence for this communication, the team grew bacteria in culture and infected them with phages; they then filtered the bacteria and phages out of the culture, leaving only the smallest molecules that had been released to the medium. When they grew more bacteria on the filtered medium, infecting them with the same phages, the new phages became dormant rather than killing the bacteria.

The team worked to isolate the communication molecule, eventually discovering that it is a small piece of protein called a peptide; they also worked to identify the gene encoding it and to unravel the way it functions. They found that in the presence of high concentrations of this peptide, phages choose the dormancy strategy, so they named it arbitrium, the Latin word for decision.

Once they had identified this communication molecule in one phage, the researchers were able to find similar molecules in dozens of related phages – each phage encoding a slightly different communication molecule.

“We deciphered a phage-specific communication code. It is as if each phage species broadcasts on a specific molecular ‘frequency’ that can be ‘read’ by phages of its own kind, but not by other phages,” says Prof. Sorek.