Modified coat protein forms the flexible spindle-shaped virion of haloarchaeal virus His1

Evolutionary Adaptation of an RNA Bacteriophage to Repeated Freezing and Thawing Cycles

Bacteriophage fitness is determined by factors influencing both their replication within bacteria and their ability to maintain infectivity between infections. The latter becomes particularly crucial under adverse environmental conditions or when host density is low. In such scenarios, the damage experienced by viral particles could lead to the loss of infectivity, which might be mitigated if the virus undergoes evolutionary optimization through replication. In this study, we conducted an evolution experiment involving bacteriophage Qβ, wherein it underwent 30 serial transfers, each involving a cycle of freezing and thawing followed by replication of the surviving viruses. Our findings show that Qβ was capable of enhancing its resistance to this selective pressure through various adaptive pathways that did not impair the virus replicative capacity. Notably, these adaptations predominantly involved mutations located within genes encoding capsid proteins. The adapted populations exhibited higher resistance levels than individual viruses isolated from them, and the latter surpassed those observed in single mutants generated via site-directed mutagenesis. This suggests potential interactions among mutants and mutations. In conclusion, our study highlights the significant role of extracellular selective pressures in driving the evolution of phages, influencing both the genetic composition of their populations and their phenotypic properties.

Hiding in plain sight: The discovery of complete genomes of 11 hypothetical spindle-shaped viruses that putatively infect mesophilic ammonia-oxidizing archaea

The genome of a putative Nitrosopumilaceae virus with a hypothetical spindle-shaped particle morphology was identified in the Yangshan Harbour metavirome from the East China Sea through protein similarity comparison and structure analysis. This discovery was accompanied by a set of 10 geographically dispersed close relatives found in the environmental virus datasets from typical locations of ammonia-oxidizing archaeon distribution. Its host prediction was supported by iPHoP prediction and protein sequence similarity. The structure of the predicted major capsid protein, together with the overall N-glycosylation site, the transmembrane helices prediction, the hydrophilicity profile, and the docking simulation of the major capsid proteins, indicate that these viruses resemble spindle-shaped viruses. It suggests a similarly assembled structure and, consequently, a possibly spindle-shaped morphology of these newly discovered archaeal viruses.

Mechanical tomography of an archaeal lemon-shaped virus reveals membrane-like fluidity of the capsid and liquid nucleoprotein cargo

Archaeal lemon-shaped viruses have unique helical capsids composed of highly hydrophobic protein strands which can slide past each other resulting in remarkable morphological reorganization. Here, using atomic force microscopy, we explore the biomechanical properties of the lemon-shaped virions of Sulfolobus monocaudavirus 1 (SMV1), a double-stranded DNA virus which infects hyperthermophilic (~80 °C) and acidophilic (pH ~ 2) archaea. Our results reveal that SMV1 virions are extremely soft and withstand repeated extensive deformations, reaching remarkable strains of 80% during multiple cycles of consecutive mechanical assaults, yet showing scarce traces of disruption. SMV1 virions can reversibly collapse wall-to-wall, reducing their volume by ~90%. Beyond revealing the exceptional malleability of the SMV1 protein shell, our data also suggest a fluid-like nucleoprotein cargo which can flow inside the capsid, resisting and accommodating mechanical deformations without further alteration. Our experiments suggest a packing fraction of the virus core to be as low as 11%, with the amount of the accessory proteins almost four times exceeding that of the viral genome. Our findings indicate that SMV1 protein capsid displays biomechanical properties of lipid membranes, which is not found among protein capsids of other viruses. The remarkable malleability and fluidity of the SMV1 virions are likely necessary for the structural transformations during the infection and adaptation to extreme environmental conditions.

Genomic Characterization of a Halovirus Representing a Novel Siphoviral Cluster

Salt mines are a special type of hypersaline environment. Current research mainly focuses on prokaryotes, and the understanding of viruses in salt mines remains limited. Understanding viruses in hypersaline environments is of great significance for revealing the formation and maintenance of microbial communities, energy flow and element cycling, and host ecological functions. A phage infecting Halomonas titanicae was isolated from Yipinglang Salt Mine in China, designated Halomonas titanicae phage vB_HtiS_YPHTV-1 (YPHTV-1). Transmission electron microscopy revealed that YPHTV-1 had an icosahedral head with a diameter of 49.12 ± 0.15 nm (n = 5) and a long noncontractile tail with a length of 141.7 ± 0.58 nm (n = 5), indicating that it was a siphovirus. The one-step growth curve showed that the burst size of YPHTV-1 was 69 plaque forming units (PFUs) cell^-1. The genome of YPHTV-1 was 37,980 bp with a GC content of 36.2%. The phylogenetic analysis of the six conserved proteins indicated that YPHTV-1 formed a cluster with Bacillus phages and was separated from phages infecting Halomonas. The average nucleotide identity (ANI), phylogenetic, and network analyses indicated that the phage YPHTV-1 represented a new genus under Caudoviricetes. In total, 57 open reading frames (ORFs) were predicted in the YPHTV-1 genome, 30 of which could be annotated in the database. Notably, several auxiliary metabolic genes were encoded by YPHTV-1, such as ImmA/IrrE family metalloendopeptidase, mannose-binding lectin (MBL) folding metallohydrolase, M15 family of metal peptidases, MazG-like family protein, O antigen ligase, and acyltransferase. These genes potentially enabled the host bacterium to resist ionizing radiation, ultraviolet light (UV), mitomycin C, β-lactam antibiotic, high osmotic pressure, and nutritional deficiencies. These findings highlight the role of haloviruses in the life cycle of halobacteria.

Archaeal Host Cell Recognition and Viral Binding of HFTV1 to Its Haloferax Host

Viruses are highly abundant and the main predator of microorganisms. Microorganisms of each domain of life are infected by dedicated viruses. Viruses infecting archaea are genomically and structurally highly diverse. Archaea are undersampled for viruses in comparison with bacteria and eukaryotes. Consequently, the infection mechanisms of archaeal viruses are largely unknown, and most available knowledge stems from viruses infecting a select group of archaea, such as crenarchaea. We employed Haloferax tailed virus 1 (HFTV1) and its host, Haloferax gibbonsii LR2-5, to study viral infection in euryarchaea. We found that HFTV1, which has a siphovirus morphology, is virulent, and interestingly, viral particles adsorb to their host several orders of magnitude faster than most studied haloarchaeal viruses. As the binding site for infection, HFTV1 uses the cell wall component surface (S)-layer protein. Electron microscopy of infected cells revealed that viral particles often made direct contact with their heads to the cell surface, whereby the virion tails were perpendicular to the surface. This seemingly unfavorable orientation for genome delivery might represent a first reversible contact between virus and cell and could enhance viral adsorption rates. In a next irreversible step, the virion tail is orientated toward the cell surface for genome delivery. With these findings, we uncover parallels between entry mechanisms of archaeal viruses and those of bacterial jumbo phages and bacterial gene transfer agents. IMPORTANCE Archaeal viruses are the most enigmatic members of the virosphere. These viruses infect ubiquitous archaea and display an unusually high structural and genetic diversity. Unraveling their mechanisms of infection will shed light on the question if entry and egress mechanisms are highly conserved between viruses infecting a single domain of life or if these mechanisms are dependent on the morphology of the virus and the growth conditions of the host. We studied the entry mechanism of the tailed archaeal virus HFTV1. This showed that despite "typical" siphovirus morphology, the infection mechanism is different from standard laboratory models of tailed phages. We observed that particles bound first with their head to the host cell envelope, and, as such, we discovered parallels between archaeal viruses and nonmodel bacteriophages. This work contributes to a better understanding of entry mechanisms of archaeal viruses and a more complete view of microbial viruses in general.

Viruses in astrobiology

Viruses are the most abundant biological entities on Earth, and yet, they have not received enough consideration in astrobiology. Viruses are also extraordinarily diverse, which is evident in the types of relationships they establish with their host, their strategies to store and replicate their genetic information and the enormous diversity of genes they contain. A viral population, especially if it corresponds to a virus with an RNA genome, can contain an array of sequence variants that greatly exceeds what is present in most cell populations. The fact that viruses always need cellular resources to multiply means that they establish very close interactions with cells. Although in the short term these relationships may appear to be negative for life, it is evident that they can be beneficial in the long term. Viruses are one of the most powerful selective pressures that exist, accelerating the evolution of defense mechanisms in the cellular world. They can also exchange genetic material with the host during the infection process, providing organisms with capacities that favor the colonization of new ecological niches or confer an advantage over competitors, just to cite a few examples. In addition, viruses have a relevant participation in the biogeochemical cycles of our planet, contributing to the recycling of the matter necessary for the maintenance of life. Therefore, although viruses have traditionally been excluded from the tree of life, the structure of this tree is largely the result of the interactions that have been established throughout the intertwined history of the cellular and the viral worlds. We do not know how other possible biospheres outside our planet could be, but it is clear that viruses play an essential role in the terrestrial one. Therefore, they must be taken into account both to improve our understanding of life that we know, and to understand other possible lives that might exist in the cosmos.

The Viral Susceptibility of the Haloferax Species

Viruses can infect members of all three domains of life. However, little is known about viruses infecting archaea and the mechanisms that determine their host interactions are poorly understood. Investigations of molecular mechanisms of viral infection rely on genetically accessible virus–host model systems. Euryarchaea belonging to the genus Haloferax are interesting models, as a reliable genetic system and versatile microscopy methods are available. However, only one virus infecting the Haloferax species is currently available. In this study, we tested ~100 haloarchaeal virus isolates for their infectivity on 14 Haloferax strains. From this, we identified 10 virus isolates in total capable of infecting Haloferax strains, which represented myovirus or siphovirus morphotypes. Surprisingly, the only susceptible strain of all 14 tested was Haloferax gibbonsii LR2-5, which serves as an auspicious host for all of these 10 viruses. By applying comparative genomics, we shed light on factors determining the host range of haloarchaeal viruses on Haloferax. We anticipate our study to be a starting point in the study of haloarchaeal virus–host interactions.

Spindle-shaped archaeal viruses evolved from rod-shaped ancestors to package a larger genome

Spindle- or lemon-shaped viruses infect archaea in diverse environments. Due to the highly pleomorphic nature of these virions, which can be found with cylindrical tails emanating from the spindle-shaped body, structural studies of these capsids have been challenging. We have determined the atomic structure of the capsid of Sulfolobus monocaudavirus 1, a virus that infects hosts living in nearly boiling acid. A highly hydrophobic protein, likely integrated into the host membrane before the virions assemble, forms 7 strands that slide past each other in both the tails and the spindle body. We observe the discrete steps that occur as the tail tubes expand, and these are due to highly conserved quasiequivalent interactions with neighboring subunits maintained despite significant diameter changes. Our results show how helical assemblies can vary their diameters, becoming nearly spherical to package a larger genome and suggest how all spindle-shaped viruses have evolved from archaeal rod-like viruses.

Archaeal Viruses: Production of Virus Particles and Vesicle-like Viruses and Purification Using Asymmetrical Flow Field-Flow Fractionation

Asymmetrical flow field-flow fractionation (AF4) is a separation method based on hydrodynamic size of the sample components. It can separate a broad size range of components (~10³ to 10⁹ Da; particle diameter from ~1 nm to ~1 μm), but is especially well suited for high molecular weight samples such as virus-sized particles and extracellular vesicles. Separation takes place in an open channel where the flows control sample elution. Separation does not involve stationary phase, allowing gentle separation and good recoveries. The method is compatible with a wide variety of buffers. Coupling to various analytical detectors enables rapid assays on the molecular weight and size and their distribution, degradation, and aggregation of the sample components giving information on the sample quality. In addition to being an advanced analytical method, AF4 can be used in a semipreparative mode for purification. Here, we summarize archaeal virus production methods and virus purification by AF4 and provide examples on the steps that need optimization for obtaining good separation with the focus on halophilic archaeal viruses. Importantly, AF4 method is suitable for a variety of viruses and extracellular vesicles regardless of their host organism.

The Novel Halovirus Hardycor1, and the Presence of Active (Induced) Proviruses in Four Haloarchaea

The virus Hardycor1 was isolated in 1998 and infects the haloarchaeon Halorubrum coriense. DNA from a frozen stock (HC1) was sequenced and the viral genome found to be 45,142 bp of dsDNA, probably having redundant, circularly permuted termini. The genome showed little similarity (BLASTn) to known viruses. Only twenty-two of the 53 (41%) predicted proteins were significantly similar to sequences in the NCBI nr protein database (E-value ≤ 10^-15). Six caudovirus-like proteins were encoded, including large subunit terminase (TerL), major capsid protein (Mcp) and tape measure protein (Tmp). Hardycor1 was predicted to be a siphovirus (VIRFAM). No close relationship to other viruses was found using phylogenetic tree reconstructions based on TerL and Mcp. Unexpectedly, the sequenced virus stock HC1 also revealed two induced proviruses of the host: a siphovirus (Humcor1) and a pleolipovirus (Humcor2). A re-examination of other similarly sequenced, archival virus stocks revealed induced proviruses of Haloferax volcanii, Haloferax gibbonsii and Haloarcula hispanica, three of which were pleolipoviruses. One provirus (Halfvol2) of Hfx. volcanii showed little similarity (BLASTn) to known viruses and probably represents a novel virus group. The attP sequences of many pleolipoproviruses were found to be embedded in a newly detected coding sequence, split in the provirus state, that spans between genes for integrase and a downstream CxxC-motif protein. This gene might play an important role in regulation of the temperate state.

Growth Phase Dependent Cell Shape of Haloarcula

Several haloarchaea are reported to be pleomorphic, while others exhibit remarkable shapes, such as squares. Recently, Haloferax volcanii was found to alter its morphology during growth. Cells are motile rods in early exponential phase, and immotile plates in stationary phase. It is unknown if this growth phase dependent cell shape alteration is a specific feature of Hfx. volcanii, or conserved amongst haloarchaea. Here, we studied the cell shape and motility of two haloarchaea species Haloarcula hispanica and Haloarcula californiae. With a combination of light and electron microscopy, we observed that both strains undergo a growth phase dependent morphological development, albeit in a slightly different fashion as Hfx. volcanii. For both Haloarcula strains, the cell size is changing throughout growth. Cell shape seems to be related with motility, as highly motile cells on semi-solid agar plates are predominantly rod-shaped. We conclude that the growth phase dependent cell morphology alteration might be a common feature amongst haloarchaea, and that cell shape is generally linked with a motile life style. The conservation of this phenomenon underscores the importance of studies of the molecular mechanisms regulating cell shape in archaea.

Virus and Potential Host Microbes from Viral-Enriched Metagenomic Characterization in the High-Altitude Wetland, Salar de Huasco, Chile

Salar de Huasco is a wetland in the Andes mountains, located 3800 m above sea level at the Chilean Altiplano. Here we present a study aimed at characterizing the viral fraction and the microbial communities through metagenomic analysis. Two ponds (H0 and H3) were examined in November 2015. Water samples were processed using tangential flow filtration to obtain metagenomes from which the DNA fraction of the sample was amplified and sequenced (HiSeq system, Illumina). The ponds were characterized by freshwater and the viral-like particles to picoplankton ratio was 12.1 and 2.3 for H0 and H3, respectively. A great number of unassigned viral sequences were found in H0 (55.8%) and H3 (32.8%), followed by the family Fuselloviridae 20.8% (H0) and other less relatively abundant groups such as Microviridae (H0, 11.7% and H3, 3.3%) and Inoviridae (H3, 2.7%). The dominant viral sequences in both metagenomes belong to the order Caudovirales, with Siphoviridae being the most important family, especially in H3 (32.7%). The most important bacteria phyla were Proteobacteria, Bacteroidetes and Firmicutes in both sites, followed by Cyanobacteria (H0). Genes encoding lysogenic and lytic enzymes (i.e., recombinases and integrases) were found in H0 and H3, indicating a potential for active viral replication at the time of sampling; this was supported by the presence of viral metabolic auxiliary genes at both sites (e.g., cysteine hydrolase). In total, our study indicates a great novelty of viral groups, differences in taxonomic diversity and replication pathways between sites, which contribute to a better understanding of how viruses balance the cycling of energy and matter in this extreme environment.

Taxonomy of prokaryotic viruses: 2018-2019 update from the ICTV Bacterial and Archaeal Viruses Subcommittee

This article is a summary of the activities of the ICTV's Bacterial and Archaeal Viruses Subcommittee for the years 2018 and 2019. Highlights include the creation of a new order, 10 families, 22 subfamilies, 424 genera and 964 species. Some of our concerns about the ICTV's ability to adjust to and incorporate new DNA- and protein-based taxonomic tools are discussed.

Spindle-shaped viruses infect marine ammonia-oxidizing thaumarchaea

Ammonia-oxidizing archaea (AOA) from the phylum Thaumarchaeota are ubiquitous in marine ecosystems and play a prominent role in carbon and nitrogen cycling. Previous studies have suggested that, like all microbes, thaumarchaea are infected by viruses and that viral predation has a profound impact on thaumarchaeal functioning and mortality, thereby regulating global biogeochemical cycles. However, not a single virus capable of infecting thaumarchaea has been reported thus far. Here we describe the isolation and characterization of three Nitrosopumilus spindle-shaped viruses (NSVs) that infect AOA and are distinct from other known marine viruses. Although NSVs have a narrow host range, they efficiently infect autochthonous Nitrosopumilus strains and display high rates of adsorption to their host cells. The NSVs have linear double-stranded DNA genomes of ∼28 kb that do not display appreciable sequence similarity to genomes of other known archaeal or bacterial viruses and could be considered as representatives of a new virus family, the "Thaspiviridae." Upon infection, NSV replication leads to inhibition of AOA growth, accompanied by severe reduction in the rate of ammonia oxidation and nitrite reduction. Nevertheless, unlike in the case of lytic bacteriophages, NSV propagation is not associated with detectable degradation of the host chromosome or a decrease in cell counts. The broad distribution of NSVs in AOA-dominated marine environments suggests that NSV predation might regulate the diversity and dynamics of AOA communities. Collectively, our results shed light on the diversity, evolution, and potential impact of the virosphere associated with ecologically important mesophilic archaea.

Survey of high-resolution archaeal virus structures

Archaeal viruses exhibit diverse morphologies whose structures are just beginning to be explored at high-resolution. In this review, we update recent findings on archaeal structural proteins and virion architectures and place them in the biological context in which these viruses replicate. We conclude that many of the unusual structural features and dynamics of archaeal viruses aid their replication and survival in the chemically harsh environments, in which they replicate. Furthermore, we should expect to find more novel features from examining the high-resolution structures of additional archaeal viruses.

Broad Environmental Tolerance for a Salicola Host-Phage Pair Isolated from the Cargill Solar Saltworks, Newark, CA, USA

Phages greatly influence the ecology and evolution of their bacterial hosts; however, compared to hosts, a relatively low number of phages, especially halophilic phages, have been studied. This study describes a comparative investigation of physicochemical tolerance between a strain of the halophilic bacterium, Salicola, isolated from the Cargill Saltworks (Newark, CA, USA) and its associated phage. The host grew in media between pH 6-8.5, had a salinity growth optimum of 20% total salts (ranging from 10%-30%) and an upper temperature growth limit of 48 °C. The host utilized 61 of 190 substrates tested using BIOLOG Phenotype MicroArrays. The CGφ29 phage, one of only four reported Salicola phages, is a DNA virus of the Siphoviridae family. Overall, the phage tolerated a broader range of environmental conditions than its host (salinity 0-30% total salts; pH 3-9; upper thermal limit 80 °C) and is the most thermotolerant halophilic phage ever reported. This study is the most comprehensive investigation to date of a Salicola host-phage pair and provides novel insights into extreme environmental tolerances among bacteriophages.

The PL6-Family Plasmids of Haloquadratum Are Virus-Related

Plasmids PL6A and PL6B are both carried by the C23^T strain of the square archaeon Haloquadratum walsbyi, and are closely related (76% nucleotide identity), circular, about 6 kb in size, and display the same gene synteny. They are unrelated to other known plasmids and all of the predicted proteins are cryptic in function. Here we describe two additional PL6-related plasmids, pBAJ9-6 and pLT53-7, each carried by distinct isolates of Haloquadratum walsbyi that were recovered from hypersaline waters in Australia. A third PL6-like plasmid, pLTMV-6, was assembled from metavirome data from Lake Tyrell, a salt-lake in Victoria, Australia. Comparison of all five plasmids revealed a distinct plasmid family with strong conservation of gene content and synteny, an average size of 6.2 kb (range 5.8-7.0 kb) and pairwise similarities between 61-79%. One protein (F3) was closely similar to a protein carried by betapleolipoviruses while another (R6) was similar to a predicted AAA-ATPase of His 1 halovirus (His1V_gp16). Plasmid pLT53-7 carried a gene for a FkbM family methyltransferase that was not present in any of the other plasmids. Comparative analysis of all PL6-like plasmids provided better resolution of conserved sequences and coding regions, confirmed the strong link to haloviruses, and showed that their sequences are highly conserved among examples from Haloquadratum isolates and metagenomic data that collectively cover geographically distant locations, indicating that these genetic elements are widespread.

Structural studies of Acidianus tailed spindle virus reveal a structural paradigm used in the assembly of spindle-shaped viruses

The spindle-shaped virion morphology is common among archaeal viruses, where it is a defining characteristic of many viral families. However, structural heterogeneity intrinsic to spindle-shaped viruses has seriously hindered efforts to elucidate the molecular architecture of these lemon-shaped capsids. We have utilized a combination of cryo-electron microscopy and X-ray crystallography to study Acidianus tailed spindle virus (ATSV). These studies reveal the architectural principles that underlie assembly of a spindle-shaped virus. Cryo-electron tomography shows a smooth transition from the spindle-shaped capsid into the tubular-shaped tail and allows low-resolution structural modeling of individual virions. Remarkably, higher-dose 2D micrographs reveal a helical surface lattice in the spindle-shaped capsid. Consistent with this, crystallographic studies of the major capsid protein reveal a decorated four-helix bundle that packs within the crystal to form a four-start helical assembly with structural similarity to the tube-shaped tail structure of ATSV and other tailed, spindle-shaped viruses. Combined, this suggests that the spindle-shaped morphology of the ATSV capsid is formed by a multistart helical assembly with a smoothly varying radius and allows construction of a pseudoatomic model for the lemon-shaped capsid that extends into a tubular tail. The potential advantages that this novel architecture conveys to the life cycle of spindle-shaped viruses, including a role in DNA ejection, are discussed.

Halophilic viruses with varying biochemical and biophysical properties are amenable to purification with asymmetrical flow field-flow fractionation

Viruses come in various shapes and sizes, and a number of viruses originate from extremities, e.g. high salinity or elevated temperature. One challenge for studying extreme viruses is to find efficient purification conditions where viruses maintain their infectivity. Asymmetrical flow field-flow fractionation (AF4) is a gentle native chromatography-like technique for size-based separation. It does not have solid stationary phase and the mobile phase composition is readily adjustable according to the sample needs. Due to the high separation power of specimens up to 50 µm, AF4 is suitable for virus purification. Here, we applied AF4 for extremophilic viruses representing four morphotypes: lemon-shaped, tailed and tailless icosahedral, as well as pleomorphic enveloped. AF4 was applied to input samples of different purity: crude supernatants of infected cultures, polyethylene glycol-precipitated viruses and viruses purified by ultracentrifugation. All four virus morphotypes were successfully purified by AF4. AF4 purification of culture supernatants or polyethylene glycol-precipitated viruses yielded high recoveries, and the purities were comparable to those obtained by the multistep ultracentrifugation purification methods. In addition, we also demonstrate that AF4 is a rapid monitoring tool for virus production in slowly growing host cells living in extreme conditions.

Virus-host interplay in high salt environments

Interaction of viruses and cells has tremendous impact on cellular and viral evolution, nutrient cycling and decay of organic matter. Thus, viruses can indirectly affect complex processes such as climate change and microbial pathogenicity. During recent decades, studies on extreme environments have introduced us to archaeal viruses and viruses infecting extremophilic bacteria or eukaryotes. Hypersaline environments are known to contain strikingly high numbers of viruses (∼10(9) particles per ml). Halophilic archaea, bacteria and eukaryotes inhabiting hypersaline environments have only a few cellular predators, indicating that the role of viruses is highly important in these ecosystems. Viruses thriving in high salt are called haloviruses and to date more than 100 such viruses have been described. Virulent, temperate, and persistent halovirus life cycles have been observed among the known isolates including the recently described SNJ1-SNJ2 temperate virus pair which is the first example of an interplay between two haloviruses in one host cell. In addition to direct virus and cell isolations, metagenomics have provided a wealth of information about virus-host dynamics in hypersaline environments suggesting that halovirus populations and halophilic microorganisms are dynamic over time and spatially distributed around the highly saline environments on the Earth.

Life Cycle Characterization of Sulfolobus Monocaudavirus 1, an Extremophilic Spindle-Shaped Virus with Extracellular Tail Development

We provide here, for the first time, insights into the initial infection stages of a large spindle-shaped archaeal virus and explore the following life cycle events. Our observations suggest that Sulfolobus monocaudavirus 1 (SMV1) exhibits a high adsorption rate and that virions adsorb to the host cells via three distinct attachment modes: nosecone association, body association, and body/tail association. In the body/tail association mode, the entire virion, including the tail(s), aligns to the host cell surface and the main body is greatly flattened, suggesting a possible fusion entry mechanism. Upon infection, the intracellular replication cycle lasts about 8 h, at which point the virions are released as spindle-shaped tailless particles. Replication of the virus retarded host growth but did not cause lysis of the host cells. Once released from the host and at temperatures resembling that of its natural habitat, SMV1 starts developing one or two tails. This exceptional property of undergoing a major morphological development outside, and independently of, the host cell has been reported only once before for the related Acidianus two-tailed virus. Here, we show that SMV1 can develop tails of more than 900 nm in length, more than quadrupling the total virion length.

IMPORTANCE

Very little is known about the initial life cycle stages of viruses infecting hosts of the third domain of life, Archaea This work describes the first example of an archaeal virus employing three distinct association modes. The virus under study, Sulfolobus monocaudavirus 1, is a representative of the large spindle-shaped viruses that are frequently found in acidic hot springs. The results described here will add valuable knowledge about Archaea, the least studied domain in the virology field.

Monitoring Physiological Changes in Haloarchaeal Cell during Virus Release

The slow rate of adsorption and non-synchronous release of some archaeal viruses have hindered more thorough analyses of the mechanisms of archaeal virus release. To address this deficit, we utilized four viruses that infect Haloarcula hispanica that represent the four virion morphotypes currently known for halophilic euryarchaeal viruses: (1) icosahedral internal membrane-containing SH1; (2) icosahedral tailed HHTV-1; (3) spindle-shaped His1; and (4) pleomorphic His2. To discern the events occurring as the progeny viruses exit, we monitored culture turbidity, as well as viable cell and progeny virus counts of infected and uninfected cultures. In addition to these traditional metrics, we measured three parameters associated with membrane integrity: the binding of the lipophilic anion phenyldicarbaundecaborane, oxygen consumption, and both intra- and extra-cellular ATP levels.

Temperature and pH dependence of DNA ejection from archaeal lemon-shaped virus His1

The archaeal virus His1 isolated from a hypersaline environment infects an extremely halophilic archaeon Haloarcula hispanica. His1 features a lemon-shaped capsid, which is so far found only in archaeal viruses. This unique capsid can withstand high salt concentrations, and can transform into a helical tube, which in turn is resistant to extremely harsh conditions. Hypersaline environments exhibit a wide range of temperatures and pH conditions, which present an extra challenge to their inhabitants. We investigated the influence of pH and temperature on DNA ejection from His1 virus using single-molecule fluorescence experiments. The observed number of ejecting viruses is constant in pH 5 to 9, while the ejection process is suppressed at pH below 5. Similarly, the number of ejections within 15-42 °C shows only a minor increase around 25-37 °C. The maximum velocity of single ejected DNA increases with temperature, in qualitative agreement with the continuum model of dsDNA ejection.

Metaviromics of Namib Desert Salt Pans: A Novel Lineage of Haloarchaeal Salterproviruses and a Rich Source of ssDNA Viruses

Viral communities of two different salt pans located in the Namib Desert, Hosabes and Eisfeld, were investigated using a combination of multiple displacement amplification of metaviromic DNA and deep sequencing, and provided comprehensive sequence data on both ssDNA and dsDNA viral community structures. Read and contig annotations through online pipelines showed that the salt pans harbored largely unknown viral communities. Through network analysis, we were able to assign a large portion of the unknown reads to a diverse group of ssDNA viruses. Contigs belonging to the subfamily Gokushovirinae were common in both environmental datasets. Analysis of haloarchaeal virus contigs revealed the presence of three contigs distantly related with His1, indicating a possible new lineage of salterproviruses in the Hosabes playa. Based on viral richness and read mapping analyses, the salt pan metaviromes were novel and most closely related to each other while showing a low degree of overlap with other environmental viromes.

Analysis of metagenomic data reveals common features of halophilic viral communities across continents

Microbial communities from hypersaline ponds, dominated by halophilic archaea, are considered specific of such extreme conditions. The associated viral communities have accordingly been shown to display specific features, such as similar morphologies among different sites. However, little is known about the genetic diversity of these halophilic viral communities across the Earth. Here, we studied viral communities in hypersaline ponds sampled on the coast of Senegal (8-36% of salinity) using metagenomics approach, and compared them with hypersaline viromes from Australia and Spain. The specificity of hyperhalophilic viruses could first be demonstrated at a community scale, salinity being a strong discriminating factor between communities. For the major viral group detected in all samples (Caudovirales), only a limited number of halophilic Caudovirales clades were highlighted. These clades gather viruses from different continents and display consistent genetic composition, indicating that they represent related lineages with a worldwide distribution. Non-tailed hyperhalophilic viruses display a greater rate of gene transfer and recombination, with uncharacterized genes conserved across different kind of viruses and plasmids. Thus, hypersaline viral communities around the world appear to form a genetically consistent community that are likely to harbour new genes coding for enzymes specifically adapted to these environments.

Pleolipoviridae, a newly proposed family comprising archaeal pleomorphic viruses with single-stranded or double-stranded DNA genomes

Viruses infecting archaea show a variety of virion morphotypes, and they are currently classified into more than ten viral families or corresponding groups. A pleomorphic virus morphotype is very common among haloarchaeal viruses, and to date, several such viruses have been isolated. Here, we propose the classification of eight such viruses and formation of a new family, Pleolipoviridae (from the Greek pleo for more or many and lipos for lipid), containing three genera, Alpha-, Beta-, and Gammapleolipovirus. The proposal is currently under review by the International Committee on Taxonomy of Viruses (ICTV). The members of the proposed family Pleolipoviridae infect halophilic archaea and are nonlytic. They share structural and genomic features and differ from any other classified virus. The virion of pleolipoviruses is composed of a pleomorphic membrane vesicle enclosing the genome. All pleolipoviruses have two major structural protein species, internal membrane and spike proteins. Although the genomes of the pleolipoviruses are single- or double-stranded, linear or circular DNA molecules, they share the same genome organization and gene synteny and show significant similarity at the amino acid level. The canonical features common to all members of the proposed family Pleolipoviridae show that they are closely related and thus form a new viral family.

Identification and characterization of SNJ2, the first temperate pleolipovirus integrating into the genome of the SNJ1-lysogenic archaeal strain

Proviral regions have been identified in the genomes of many haloarchaea, but only a few archaeal halophilic temperate viruses have been studied. Here, we report a new virus, SNJ2, originating from archaeal strain Natrinema sp. J7-1. We demonstrate that this temperate virus coexists with SNJ1 virus and is dependent on SNJ1 for efficient production. Here, we show that SNJ1 is an icosahedral membrane-containing virus, whereas SNJ2 is a pleomorphic one. Instead of producing progeny virions and forming plaques, SNJ2 integrates into the host tRNA(Met) gene. The virion contains a discontinuous, circular, double-stranded DNA genome of 16 992 bp, in which both nicks and single-stranded regions are present preceded by a 'GCCCA' motif. Among 25 putative SNJ2 open reading frames (ORFs), five of them form a cluster of conserved ORFs homologous to archaeal pleolipoviruses isolated from hypersaline environments. Two structural protein encoding genes in the conserved cluster were verified in SNJ2. Furthermore, SNJ2-like proviruses containing the conserved gene cluster were identified in the chromosomes of archaea belonging to 10 different genera. Comparison of SNJ2 and these proviruses suggests that they employ a similar integration strategy into a tRNA gene.

Sulfolobus Spindle-Shaped Virus 1 Contains Glycosylated Capsid Proteins, a Cellular Chromatin Protein, and Host-Derived Lipids

Geothermal and hypersaline environments are rich in virus-like particles, among which spindle-shaped morphotypes dominate. Currently, viruses with spindle- or lemon-shaped virions are exclusive to Archaea and belong to two distinct viral families. The larger of the two families, the Fuselloviridae, comprises tail-less, spindle-shaped viruses, which infect hosts from phylogenetically distant archaeal lineages. Sulfolobus spindle-shaped virus 1 (SSV1) is the best known member of the family and was one of the first hyperthermophilic archaeal viruses to be isolated. SSV1 is an attractive model for understanding virus-host interactions in Archaea; however, the constituents and architecture of SSV1 particles remain only partially characterized. Here, we have conducted an extensive biochemical characterization of highly purified SSV1 virions and identified four virus-encoded structural proteins, VP1 to VP4, as well as one DNA-binding protein of cellular origin. The virion proteins VP1, VP3, and VP4 undergo posttranslational modification by glycosylation, seemingly at multiple sites. VP1 is also proteolytically processed. In addition to the viral DNA-binding protein VP2, we show that viral particles contain the Sulfolobus solfataricus chromatin protein Sso7d. Finally, we provide evidence indicating that SSV1 virions contain glycerol dibiphytanyl glycerol tetraether (GDGT) lipids, resolving a long-standing debate on the presence of lipids within SSV1 virions. A comparison of the contents of lipids isolated from the virus and its host cell suggests that GDGTs are acquired by the virus in a selective manner from the host cytoplasmic membrane, likely during progeny egress.

IMPORTANCE

Although spindle-shaped viruses represent one of the most prominent viral groups in Archaea, structural data on their virion constituents and architecture still are scarce. The comprehensive biochemical characterization of the hyperthermophilic virus SSV1 presented here brings novel and significant insights into the organization and architecture of spindle-shaped virions. The obtained data permit the comparison between spindle-shaped viruses residing in widely different ecological niches, improving our understanding of the adaptation of viruses with unusual morphotypes to extreme environmental conditions.

Haloarchaeal virus morphotypes

Hypersaline waters and salt crystals are known to contain high numbers of haloarchaeal cells and their viruses. Both culture-dependent and culture-independent studies indicate that these viruses represent a world-wide distributed reservoir of orphan genes and possibly novel virion morphotypes. To date, 90 viruses have been described for halophilic archaeal hosts, all belonging to the Halobacteriaceae family. This number is higher than that described for the members of any other archaeal family, but still very low compared to the viruses of bacteria and eukaryotes. The known haloarchaeal viruses represent icosahedral tailed, icosahedral internal membrane-containing, pleomorphic, and spindle-shaped virion morphotypes. This morphotype distribution is low, especially when compared to the astronomical number (>10(31)) of viruses on Earth. This strongly suggests that only certain protein folds are capable of making a functional virion. Viruses infecting cells belonging to any of the three domains of life are known to share similar major capsid protein folds which can be used to classify viruses into structure-based lineages. The latest observation supporting this proposal comes from the studies of icosahedral tailed haloarchaeal viruses which are the most abundant virus isolates from hypersaline environments. These viruses were shown to have the same major capsid protein fold (HK97-fold) with tailed bacteriophages belonging to the order Caudovirales and with eukaryotic herpes viruses. This proposes that these viruses have a common origin dating back to ancient times. Here we summarize the current knowledge of haloarchaeal viruses from the perspective of virus morphotypes.

Archaeal viruses multiply: temporal screening in a solar saltern

Hypersaline environments around the world are dominated by archaea and their viruses. To date, very little is known about these viruses and their interaction with the host strains when compared to bacterial and eukaryotic viruses. We performed the first culture-dependent temporal screening of haloarchaeal viruses and their hosts in the saltern of Samut Sakhon, Thailand, during two subsequent years (2009, 2010). Altogether we obtained 36 haloarchaeal virus isolates and 36 archaeal strains, significantly increasing the number of known archaeal virus isolates. Interestingly, the morphological distribution of our temporal isolates (head-tailed, pleomorphic, and icosahedral membrane-containing viruses) was similar to the outcome of our previous spatial survey supporting the observations of a global resemblance of halophilic microorganisms and their viruses. Myoviruses represented the most abundant virus morphotype with strikingly broad host ranges. The other viral morphotypes (siphoviruses, as well as pleomorphic and icosahedral internal membrane-containing viruses) were more host-specific. We also identified a group of Halorubrum strains highly susceptible to numerous different viruses (up to 26). This high virus sensitivity, the abundance of broad host range viruses, and the maintenance of infectivity over a period of one year suggest constant interplay of halophilic microorganisms and their viruses within an extreme environment.

Lemon-shaped halo archaeal virus His1 with uniform tail but variable capsid structure

Lemon-shaped viruses are common in nature but so far have been observed to infect only archaea. Due to their unusual shape, the structures of these viruses are challenging to study and therefore poorly characterized. Here, we have studied haloarchaeal virus His1 using cryo-electron tomography as well as biochemical dissociation. The virions have different sizes, but prove to be extremely stable under various biochemical treatments. Subtomogram averaging of the computationally extracted virions resolved a tail-like structure with a central tail hub density and six tail spikes. Inside the tail there are two cavities and a plug density that separates the tail hub from the interior genome. His1 most likely uses the tail spikes to anchor to host cells and the tail hub to eject the genome, analogous to classic tailed bacteriophages. Upon biochemical treatment that releases the genome, the lemon-shaped virion transforms into an empty tube. Such a dramatic transformation demonstrates that the capsid proteins are capable of undergoing substantial quaternary structural changes, which may occur at different stages of the virus life cycle.

Comparison of lipid-containing bacterial and archaeal viruses

Lipid-containing bacteriophages were discovered late and considered to be rare. After further phage isolations and the establishment of the domain Archaea, several new prokaryotic viruses with lipids were observed. Consequently, the presence of lipids in prokaryotic viruses is reasonably common. The wealth of information about how prokaryotic viruses use their lipids comes from a few well-studied model viruses (PM2, PRD1, and ϕ6). These bacteriophages derive their lipid membranes selectively from the host during the virion assembly process which, in the case of PM2 and PRD1, culminates in the formation of protein capsid with an inner membrane, and for ϕ6 an outer envelope. Several inner membrane-containing viruses have been described for archaea, and their lipid acquisition models are reminiscent to those of PM2 and PRD1. Unselective acquisition of lipids has been observed for bacterial mycoplasmaviruses and archaeal pleolipoviruses, which resemble each other by size, morphology, and life style. In addition to these shared morphotypes of bacterial and archaeal viruses, archaea are infected by viruses with unique morphotypes, such as lemon-shaped, helical, and globular ones. It appears that structurally related viruses may or may not have a lipid component in the virion, suggesting that the significance of viral lipids might be to provide viruses extended means to interact with the host cell.

Viruses of haloarchaea

In hypersaline environments, haloarchaea (halophilic members of the Archaea) are the dominant organisms, and the viruses that infect them, haloarchaeoviruses are at least ten times more abundant. Since their discovery in 1974, described haloarchaeoviruses include head-tailed, pleomorphic, spherical and spindle-shaped morphologies, representing Myoviridae, Siphoviridae, Podoviridae, Pleolipoviridae, Sphaerolipoviridae and Fuselloviridae families. This review overviews current knowledge of haloarchaeoviruses, providing information about classification, morphotypes, macromolecules, life cycles, genetic manipulation and gene regulation, and host-virus responses. In so doing, the review incorporates knowledge from laboratory studies of isolated viruses, field-based studies of environmental samples, and both genomic and metagenomic analyses of haloarchaeoviruses. What emerges is that some haloarchaeoviruses possess unique morphological and life cycle properties, while others share features with other viruses (e.g., bacteriophages). Their interactions with hosts influence community structure and evolution of populations that exist in hypersaline environments as diverse as seawater evaporation ponds, to hot desert or Antarctic lakes. The discoveries of their wide-ranging and important roles in the ecology and evolution of hypersaline communities serves as a strong motivator for future investigations of both laboratory-model and environmental systems.

Adenosine triphosphatases of thermophilic archaeal double-stranded DNA viruses

Adenosine triphosphatases (ATPases) of double-stranded (ds) DNA archaeal viruses are structurally related to the AAA+ hexameric helicases and translocases. These ATPases have been implicated in viral life cycle functions such as DNA entry into the host, and viral genome packaging into preformed procapsids. We summarize bioinformatical analyses of a wide range of archaeal ATPases, and review the biochemical and structural properties of those archaeal ATPases that have measurable ATPase activity. We discuss their potential roles in genome delivery into the host, virus assembly and genome packaging in comparison to hexameric helicases and packaging motors from bacteriophages.

Archaeal Viruses: Diversity, Replication, and Structure

The Archaea-and their viruses-remain the most enigmatic of life's three domains. Once thought to inhabit only extreme environments, archaea are now known to inhabit diverse environments. Even though the first archaeal virus was described over 40 years ago, only 117 archaeal viruses have been discovered to date. Despite this small number, these viruses have painted a portrait of enormous morphological and genetic diversity. For example, research centered around the various steps of the archaeal virus life cycle has led to the discovery of unique mechanisms employed by archaeal viruses during replication, maturation, and virion release. In many instances, archaeal virus proteins display very low levels of sequence homology to other proteins listed in the public database, and therefore, structural characterization of these proteins has played an integral role in functional assignment. These structural studies have not only provided insights into structure-function relationships but have also identified links between viruses across all three domains of life.

Archaeal viruses and bacteriophages: comparisons and contrasts

Isolated archaeal viruses comprise only a few percent of all known prokaryotic viruses. Thus, the study of viruses infecting archaea is still in its early stages. Here we summarize the most recent discoveries of archaeal viruses utilizing a virion-centered view. We describe the known archaeal virion morphotypes and compare them to the bacterial counterparts, if such exist. Viruses infecting archaea are morphologically diverse and present some unique morphotypes. Although limited in isolate number, archaeal viruses reveal new insights into the viral world, such as deep evolutionary relationships between viruses that infect hosts from all three domains of life.

A Glimpse of the genomic diversity of haloarchaeal tailed viruses

Tailed viruses are the most common isolates infecting prokaryotic hosts residing in hypersaline environments. Archaeal tailed viruses represent only a small portion of all characterized tailed viruses of prokaryotes. But even this small dataset revealed that archaeal tailed viruses have many similarities to their counterparts infecting bacteria, the bacteriophages. Shared functional homologs and similar genome organizations suggested that all microbial tailed viruses have common virion architectural and assembly principles. Recent structural studies have provided evidence justifying this thereby grouping archaeal and bacterial tailed viruses into a single lineage. Currently there are 17 haloarchaeal tailed viruses with entirely sequenced genomes. Nine viruses have at least one close relative among the 17 viruses and, according to the similarities, can be divided into three groups. Two other viruses share some homologs and therefore are distantly related, whereas the rest of the viruses are rather divergent (or singletons). Comparative genomics analysis of these viruses offers a glimpse into the genetic diversity and structure of haloarchaeal tailed virus communities.

DNA ejection from an archaeal virus--a single-molecule approach

The translocation of genetic material from the viral capsid to the cell is an essential part of the viral infection process. Whether the energetics of this process is driven by the energy stored within the confined nucleic acid or cellular processes pull the genome into the cell has been the subject of discussion. However, in vitro studies of genome ejection have been limited to a few head-tailed bacteriophages with a double-stranded DNA genome. Here we describe a DNA release system that operates in an archaeal virus. This virus infects an archaeon Haloarcula hispanica that was isolated from a hypersaline environment. The DNA-ejection velocity of His1, determined by single-molecule experiments, is comparable to that of bacterial viruses. We found that the ejection process is modulated by the external osmotic pressure (polyethylene glycol (PEG)) and by increased ion (Mg(2+) and Na(+)) concentration. The observed ejection was unidirectional, randomly paused, and incomplete, which suggests that cellular processes are required to complete the DNA transfer.

Unification of the globally distributed spindle-shaped viruses of the Archaea

Viruses with spindle-shaped virions are abundant in diverse environments. Over the years, such viruses have been isolated from a wide range of archaeal hosts. Evolutionary relationships between them remained enigmatic, however. Here, using structural proteins as markers, we define familial ties among these "dark horses" of the virosphere and segregate all spindle-shaped viruses into two distinct evolutionary lineages, corresponding to Bicaudaviridae and Fuselloviridae. Our results illuminate the utility of structure-based virus classification and bring additional order to the virosphere.

First insights into the entry process of hyperthermophilic archaeal viruses

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.