UC Berkeley

ICP1, a lytic bacteriophage of Vibrio cholerae, is parasitized by phage satellites, PLEs, which hijack ICP1 proteins for their own horizontal spread. PLEs dependence on ICP1s DNA replication machinery and virion components results in inhibition of ICP1s lifecycle. PLEs are expected to depend on ICP1 factors for genome packaging, but the mechanism(s) PLEs use to inhibit ICP1 genome packaging is currently unknown. Here, we identify and characterize Gpi, PLEs indiscriminate genome packaging inhibitor. Gpi binds to ICP1s large terminase (TerL), the packaging motor, and blocks genome packaging. To overcome Gpis negative effect on TerL, a component PLE also requires, PLE uses two genome packaging specifiers, GpsA and GpsB, that specifically allow packaging of PLE genomes. Surprisingly, PLE also uses mimicry of ICP1s pac site as a backup strategy to ensure genome packaging. PLEs pac site mimicry, however, is only sufficient if PLE can inhibit ICP1 at other stages of its lifecycle, suggesting an advantage to maintaining Gpi, GpsA and GpsB. Collectively, these results provide mechanistic insights into another stage of ICP1s lifecycle that is inhibited by PLE, which is currently the most inhibitory of the documented phage satellites. More broadly, Gpi represents the first satellite-encoded inhibitor of a phage TerL.