Bacteriophages are natural predators of bacteria. Following the injection of their genetic material into the host cell, they highjack the host cell machinery for their replication and assembly. Early mechanisms of host takeover by phages remain poorly understood.
The virulent phage T5 constitutes an original model to uncover early viral effectors targeting host cell functions : it injects its 122-kb linear dsDNA in two steps. Eight percent of the genome are first transferred into the cell and the injection pauses. Following expression of 17 pre-early genes, the host DNA is degraded and host defense systems are inactivated. Viral DNA transfer resumes after 3-5 minutes to allow entry of the rest of the genome (that encodes proteins necessary for viral DNA replication, virion assembly and release).
Among the pre-early genes injected first, thirteen escape functional prediction. To assess their significance for T5 infection, we developed several genome engineering strategies to generate phages carrying deletions or conditional mutations. Characterization of the mutants showed that at least four pre-early genes play a major role in infection. In parallel, ectopic expression revealed that some pre-early genes are toxic to E. coli. Exploration of the early events of phage infection now calls for further functional and structural investigations of the pre-early proteins, which are crucial for host neutralization and thus might eventually inspire the design of new antimicrobial drugs.