1.K.5. The Marine Podoiurus Phage P-SSP7 Nozzel Complex (P-SSP7 Nozzel) Family
Podovirus P-SSP7 infects Prochlorococcus marinus, the most abundant oceanic photosynthetic microorganism. Single-particle cryo-EM yielded icosahedral and asymmetrical structures of infectious P-SSP7 with 4.6-Å and 9-Å resolution, respectively (Liu et al. 2010). The asymmetric reconstruction revealed how symmetry mismatches are accommodated among five of the gene products at the portal vertex. Reconstructions of infectious and empty particles showed 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-EM of P-SSP7 infecting Prochlorococcus showed the same tail-fiber conformation as that in empty particles. These 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 (Liu et al. 2010).