Isolation and characterization of hyperthermophilic archaeal viruses

Overview & Synopsis

Extreme geothermal environments are inhabited by archaeal viruses with unique genome contents and remarkable morphologies, many of which have not been described among viruses infecting bacteria or eukaryotes. However, the number of known species of viruses infecting archaea remains low compared to the eukaryotic or bacterial viruses. Moreover, the lack of relationships to other known viruses and distinctiveness of their genomes suggest that the mechanisms of virus-host interaction are likely to be also novel. Therefore, in the framework of my studies, I have focused on two major lines of research: (i) isolation and characterization of new hyperthermophilic archaeal viruses and (ii) molecular mechanisms of virus-host interactions in Archaea. We investigated the virus diversity in the sulfurous fields of the Campi Flegrei volcano in Pozzuoli, Italy. Five new archaeal viruses infecting neutrophilic hyperthermophiles of the genus Pyrobaculum and acidophilic hyperthermophiles belonging to three different genera of the order Sulfolobales, namely, Saccharolobus, Acidianus, and Metallosphaera were isolated. The newly characterized viruses belong to the families Rudiviridae, Globuloviridae and Tristromaviridae. Notably, phylogenomic analysis of the newly isolated and previously sequenced rudiviruses revealed a clear biogeographic pattern, with all Italian rudiviruses forming a monophyletic clade, suggesting geographical structuring of virus communities in extreme geothermal environments. One of the non-enveloped rudiviruses isolated in Pozzuoli, namely, Saccharolobus solfataricus rod-shaped virus 1 (SSRV1), and the enveloped Sulfolobus islandicus filamentous virus (SIFV) were structurally and biochemically characterized. The study has revealed conserved structural features shared by these viruses and clarified the evolutionary relationship between non-enveloped and enveloped filamentous viruses. The second line of research focused on understanding the mechanisms of virion assembly and release on the example of the enveloped filamentous virus SIFV, which infects the hyperthermophilic and acidophilic archaeon Sulfolobus islandicus. Our results showed that SIFV is a lytic virus, which is released through pyramidal portals formed in the host cell membrane, a highly unexpected egress mechanism for an enveloped virus. Interestingly, dual-axis electron tomography revealed that SIFV virions acquire their lipid envelopes inside the host through an unknown mechanism, involving either de novo membrane formation or trafficking of lipids from the cytoplasmic membrane to virion assembly centers.