Remarkable morphological diversity of virus-like particles was observed by transmission electron microscopy in a hypersaline water sample from Lake Retba, Senegal. The majority of particles morphologically resembled hyperthermophilic archaeal DNA viruses isolated from extreme geothermal environments. Some hypersaline viral morphotypes have not been previously observed in nature, and less than 1% of observed particles had a head-and-tail morphology, which is typical for bacterial DNA viruses. Culture-independent analysis of the microbial diversity in the sample suggested the dominance of extremely halophilic archaea. Few of the 16S sequences corresponded to known archeal genera (Haloquadratum, Halorubrum and Natronomonas), whereas the majority represented novel archaeal clades. Three sequences corresponded to a new basal lineage of the haloarchaea. Bacteria belonged to four major phyla, consistent with the known diversity in saline environments. Metagenomic sequencing of DNA from the purified virus-like particles revealed very few similarities to the NCBI non-redundant database at either the nucleotide or amino acid level. Some of the identifiable virus sequences were most similar to previously described haloarchaeal viruses, but no sequence similarities were found to archaeal viruses from extreme geothermal environments. A large proportion of the sequences had similarity to previously sequenced viral metagenomes from solar salterns.

Some viruses of Archaea use an unusual egress mechanism that involves the formation of virus-associated pyramids (VAPs) on the host cell surface. At the end of the infection cycle, these structures open outward and create apertures through which mature virions escape from the cell. Here we describe in detail the structure and composition of VAPs formed by the Sulfolobus islandicus rodshaped virus 2 (SIRV2) in cells of its hyperthermophilic archaeal host. We show that the VAPs are stable and autonomous assemblies that can be isolated from membranes of infected cells and purified without affecting their structure. The purified VAPs are heterogeneous in size, reflecting the dynamics of VAP development in a population of infected cells; however, they have a uniform geometry, consisting of seven isosceles triangular faces forming a baseless pyramid. Biochemical and immunoelectron microscopy analyses revealed that the 10-kDa P98 protein encoded by the SIRV2 virus is the sole component of the VAPs. The VAPs were produced in Sulfolobus acidocaldarius and Escherichia coli by heterologous expression of the SIRV2-P98 gene. The results confirm that P98 is the only constituent of the VAPs and demonstrate that no other viral protein is involved in the assembly of pyramids. P98 was able to produce stable structures under conditions ranging from moderate to extremely high temperatures (80°C) and from neutral to extremely acidic pH (pH 2), demonstrating another remarkable property of this exceptional viral protein.hyperthermophile | lysis | virion release | sevenfold symmetry T he vast majority of known archaeal viruses carry doublestranded DNA genomes and differ morphologically from double-stranded DNA viruses of the two other domains of life, Bacteria and Eukarya; an exception are archaeal head-and-tail viruses, which are related to the bacterial Caudivirales (1). The genomes of the majority of archaeal viruses are unique as well. The functions of more than 90% of putative genes cannot be identified due to the lack of homologs in the extant databases (1) and limited knowledge of the biology of archaeal viruses. Archaeal viral cycles can have unusual features, as demonstrated by the recent discovery of a unique virion release mechanism exploited by the Sulfolobus islandicus rod-shaped virus 2 (SIRV2) (2) and the Sulfolobus turreted icosahedral virus (STIV) (3).Among archaeal viruses, SIRV2 and STIV are studied best with respect to host cell interactions. Both are lytic viruses, and SIRV2 causes massive degradation of the host chromosome. Virion assembly takes place in the cytoplasm and coincides with the appearance of numerous prominent virus-associated pyramids (VAPs) on the host cell surface, which point outward and rupture the S-layer. Shortly after their formation, VAPs open to the outside and create large apertures through which virions escape from the cell (2, 3).The discovery of this unique virion release system has raised questions regarding the nature of the VAPs and the mechanism of their formation. Here we report the...

A decisive step in a virus infection cycle is the recognition of a specific receptor present on the host cell surface, subsequently leading to the delivery of the viral genome into the cell interior. Until now, the early stages of infection have not been thoroughly investigated for any virus infecting hyperthermophilic archaea. Here, we present the first study focusing on the primary interactions between the archaeal rod-shaped virus Sulfolobus islandicus rod-shaped virus 2 (SIRV2) (family Rudiviridae) and its hyperthermoacidophilic host, S. islandicus. We show that SIRV2 adsorption is very rapid, with the majority of virions being irreversibly bound to the host cell within 1 min. We utilized transmission electron microscopy and whole-cell electron cryotomography to demonstrate that SIRV2 virions specifically recognize the tips of pilus-like filaments, which are highly abundant on the host cell surface. Following the initial binding, the viral particles are found attached to the sides of the filaments, suggesting a movement along these appendages toward the cell surface. Finally, we also show that SIRV2 establishes superinfection exclusion, a phenomenon not previously described for archaeal viruses.