Genomic Transfer via Membrane Vesicle: A Strategy of Giant Phage phiKZ for Early Infection

During infection, the giant phiKZ phage forms a specialized structure at the center of the host cell called the phage nucleus. This structure is crucial for safeguarding viral DNA against bacterial nucleases and for segregating the transcriptional activities of late genes. Here, we describe a morphological entity, the early phage infection vesicle (EPI vesicle), which appears to be responsible for earlier gene segregation at the beginning of the infection process. Using cryo-electron microscopy, electron tomography, and fluorescence microscopy with membrane-specific dyes, we found that the EPI vesicle is enclosed in a lipid bilayer originating, apparently, from the inner membrane of the bacterial cell. Our investigations further disclose that the phiKZ EPI vesicle contains both viral DNA and viral RNA polymerase (vRNAP). We have observed that the EPI vesicle migrates from the cell pole to the center, displaying co-localization with ChmA, the primary protein of the phage nucleus. While phage DNA is transported into the phage nucleus after phage maturation, the EPI vesicle remains outside. We hypothesized that the EPI vesicle acts as a membrane transport agent, efficiently delivering phage DNA to the phage nucleus while protecting it from the nucleases of the bacterium.