The small peptide based signalling system of SPbeta group bacteriophages, also called the “Arbitrium system” , is used by phages to regulate their lytic and lysogenic behaviour. In fact, a phage that enters a bacterium early in the infection will follow a lytic cycle by default. This means that the phage will replicate itself inside the cell and then it will lyse the cell to be released back into the environment to infect other bacteria.
After a while, a lot of phages will be in the bacterial environment. That represents a risk for the persistence of the bacterial culture, and consequently for the phages’ survival.
That is why bacteriophages have developed a mechanism to prevent the total exhaustion of their hosts, based on a peptide based signal. The peptide is produced in the cell after an infection and released to the outside. When the extracellular concentration of the peptide reaches high levels, the phages’ behaviour will change and they will be more likely to follow a lysogenic cycle.
This communication system is made up of three main components: the peptide which is the communication molecule, the receptor which is a transcription factor that is inhibited by the peptide, and the promoter which is activated by the receptor.
In the paper which initially described this communication system , the peptide is six amino acids long and is expressed from a gene called aimP, the receptor is called AimR, and the promoter controls a gene called aimX (so we called it pAimX) which regulates the switch to lysogeny.
When a phage infects a bacterium, it will express both aimR and aimP. A dimer of AimR then activates aimX which represses lysogeny, resulting in the lytic cycle of the phage. The AimP pre-pro-peptide is secreted to the outside of the cell and afterwards is processed by an extracellular protease to its mature form: SAIRGA. As SAIRGA reaches high concentration levels outside the bacteria it enters bacteria through the OPP transporter. When the bacteriophage infects a bacterium which already contains SAIRGA, AimR binds to the peptide and cannot activate aimX, which results in the activation of lysogeny.
As explained previously, the aim of our project is to develop a new system for cell-to-cell communication in E. coli. We chose the Arbitrium system to reach this goal for several reasons. First, the orthogonality of these systems would be better since we are using a bacteriophage system non-native to our bacterium of choice: E. coli. Second, unlike small molecules and their receptors, peptides and their receptor proteins are easier to diversify by directed evolution which is useful for generating new variants of the communication system. More details on the reasons for our choice are presented in the Integrated Human Practices.
 Erez Z, Steinberger-Levy I, Shamir M, Doron S, Stokar-Avihail A, Peleg Y, Melamed S, Leavitt A, Savidor A, Albeck S, Amitai G, Sorek R. Communication between viruses guides lysis-lysogeny decisions. Nature (2017) 541, 488-493.