Same players, new rules: Redefining the roles of aimP, AimR, and AimX in the arbitrium communication system

Phages can use a small-molecule communication system, termed arbitrium, to coordinate lysis-lysogeny decisions. It was proposed that this system contains three main players. AimR is a transcriptional activator that promotes expression of aimX, a small RNA which favours the lytic cycle of the phage by inhibiting the phage master repressor using an uncharacterised cis-antisense mechanism. The third player, AimP, is a peptide that accumulates after several rounds of phage infection. When this occurs, AimP binds to AimR inhibiting its function, a process that promotes lysogeny, since aimX will not be transcribed. Here we redefined the role for the main players and identified new key ones, explaining the molecular basis of the arbitrium system. Our results demonstrate that AimR is an antiterminator that eliminates the function of a terminator located between aimP and aimX, which are expressed then as part of a bicistronic operon that initiates in the aimP promoter. Importantly, our genetic, biochemical, and structural analyses also demonstrate that aimX is not a regulatory small RNA but encodes a peptide, AimX, which promotes the lytic cycle of the phage by binding to the second repressor of the arbitrium system, YopN, eliminating its function. Our results reveal the mechanism by which the arbitrium system controls lysis/lysogeny.