Structural changes in a marine podovirus associated with release of its genome into Prochlorococcus

Single particle cryo-electron microscopy reconstructions of marine podovirus particles, together with cryo-electron tomography studies during the host infection process, have revealed structural features that suggest a mechanism whereby, upon binding to the Prochlorococcus host cell, the tail fibers induce a cascade of structural alterations of the portal vertex complex that triggers release of its genome. Podovirus P-SSP7 infects Prochlorococcus marinus , the most abundant oceanic photosynthetic microorganism. Single-particle cryo–electron microscopy yields icosahedral and asymmetrical structures of infectious P-SSP7 with 4.6-Å and 9-Å resolution, respectively. The asymmetric reconstruction reveals how symmetry mismatches are accommodated among five of the gene products at the portal vertex. Reconstructions of infectious and empty particles show a conformational change of the 'valve' density in the nozzle, an orientation difference in the tail fibers, a disordering of the C terminus of the portal protein and the disappearance of the core proteins. In addition, cryo–electron tomography of P-SSP7 infecting Prochlorococcus showed the same tail-fiber conformation as that in empty particles. Our observations suggest a mechanism whereby, upon binding to the host cell, the tail fibers induce a cascade of structural alterations of the portal vertex complex that triggers DNA release.