Viral evolution revealed by bacteriophage PRD1 and human adenovirus coat protein structures

Insights into membrane interactions and their therapeutic potential

Recent research into membrane interactions has uncovered a diverse range of therapeutic opportunities through the bioengineering of human and non-human macromolecules. Although the majority of this research is focussed on fundamental developments, emerging studies are showcasing promising new technologies to combat conditions such as cancer, Alzheimer's and inflammatory and immune-based disease, utilising the alteration of bacteriophage, adenovirus, bacterial toxins, type 6 secretion systems, annexins, mitochondrial antiviral signalling proteins and bacterial nano-syringes. To advance the field further, each of these opportunities need to be better understood, and the therapeutic models need to be further optimised. Here, we summarise the knowledge and insights into several membrane interactions and detail their current and potential uses therapeutically.

Viruses of the Turriviridae: an emerging model system for studying archaeal virus-host interactions

Viruses have played a central role in the evolution and ecology of cellular life since it first arose. Investigations into viral molecular biology and ecological dynamics have propelled abundant progress in our understanding of living systems, including genetic inheritance, cellular signaling and trafficking, and organismal development. As well, the discovery of viral lineages that infect members of all three domains suggest that these lineages originated at the earliest stages of biological evolution. Research into these viruses is helping to elucidate the conditions under which life arose, and the dynamics that directed its early development. Archaeal viruses have only recently become a subject of intense study, but investigations have already produced intriguing and exciting results. STIV was originally discovered in Yellowstone National Park and has been the focus of concentrated research. Through this research, a viral genetic system was created, a novel lysis mechanism was discovered, and the interaction of the virus with cellular ESCRT machinery was revealed. This review will summarize the discoveries within this group of viruses and will also discuss future work.

Cryo-EM structure of ssDNA bacteriophage ΦCjT23 provides insight into early virus evolution

The origin of viruses remains an open question. While lack of detectable sequence similarity hampers the analysis of distantly related viruses, structural biology investigations of conserved capsid protein structures facilitate the study of distant evolutionary relationships. Here we characterize the lipid-containing ssDNA temperate bacteriophage ΦCjT23, which infects Flavobacterium sp. (Bacteroidetes). We report ΦCjT23-like sequences in the genome of strains belonging to several Flavobacterium species. The virion structure determined by cryogenic electron microscopy reveals similarities to members of the viral kingdom Bamfordvirae that currently consists solely of dsDNA viruses with a major capsid protein composed of two upright β-sandwiches. The minimalistic structure of ΦCjT23 suggests that this phage serves as a model for the last common ancestor between ssDNA and dsDNA viruses in the Bamfordvirae. Both ΦCjT23 and the related phage FLiP infect Flavobacterium species found in several environments, suggesting that these types of viruses have a global distribution and a shared evolutionary origin. Detailed comparisons to related, more complex viruses not only expand our knowledge about this group of viruses but also provide a rare glimpse into early virus evolution.

Genomic evaluation of novel Kenyan virulent phage isolates infecting carbapenemase-producing Klebsiella pneumoniae and safety determination of their lysates in Balb/c mice

This study aimed to evaluate the genomic features of novel Kenyan virulent phage isolates infecting carbapenemase-producing Klebsiella pneumoniae and to determine the safety of their lysates using mice model in a preclinical study. The genomics showed that the Klebsiella phages vB_KpM_CPRSA and vB_KpM_CPRSB belonged to the genus Slopekvirus with a similarity index of less than 92% compared to the most closest relative species. Their genomes did not contain antimicrobial resistance and toxin genes. Then endotoxin levels in the Klebsiella phage lysates were statistically significant (p value ˃ 0.05). The serum activities of aspartate aminotransferase, alanine aminotransferase and urea in the group of balb/c mice injected with bacteriophage lysates through the intravenous route were higher compared to that of the intranasal route. Unexpectedly, there was mild congestion of the central veins of kidneys and liver without damage to renal tubules and hepatocytes and a lack of physical discomfort and pain in the mice. Our study isolated and characterised Klebsiella phages against carbapenem-resistant K. pneumoniae, which are promising therapeutic agents for the treatment of respiratory tract infections using the topical mode of administration as the preferred route of bacteriophage delivery.

Bacteriophage PRD1 as a nanoscaffold for drug loading

Viruses are very attractive biomaterials owing to their capability as nanocarriers of genetic material. Efforts have been made to functionalize self-assembling viral protein capsids on their exterior or interior to selectively take up different payloads. PRD1 is a double-stranded DNA bacteriophage comprising an icosahedral protein outer capsid and an inner lipidic vesicle. Here, we report the three-dimensional structure of PRD1 in complex with the antipsychotic drug chlorpromazine (CPZ) by cryo-electron microscopy. We show that the jellyrolls of the viral major capsid protein P3, protruding outwards from the capsid shell, serve as scaffolds for loading heterocyclic CPZ molecules. Additional X-ray studies and molecular dynamics simulations show the binding modes and organization of CPZ molecules when complexed with P3 only and onto the virion surface. Collectively, we provide a proof of concept for the possible use of the lattice-like organisation and the quasi-symmetric morphology of virus capsomers for loading heterocyclic drugs with defined properties.

Basics for Improved Use of Phages for Therapy

Blood-borne therapeutic phages and phage capsids increasingly reach therapeutic targets as they acquire more persistence, i.e., become more resistant to non-targeted removal from blood. Pathogenic bacteria are targets during classical phage therapy. Metastatic tumors are potential future targets, during use of drug delivery vehicles (DDVs) that are phage derived. Phage therapy has, to date, only sometimes been successful. One cause of failure is low phage persistence. A three-step strategy for increasing persistence is to increase (1) the speed of lytic phage isolation, (2) the diversity of phages isolated, and (3) the effectiveness and speed of screening phages for high persistence. The importance of high persistence-screening is illustrated by our finding here of persistence dramatically higher for coliphage T3 than for its relative, coliphage T7, in murine blood. Coliphage T4 is more persistent, long-term than T3. Pseudomonas chlororaphis phage 201phi2-1 has relatively low persistence. These data are obtained with phages co-inoculated and separately assayed. In addition, highly persistent phage T3 undergoes dispersal to several murine organs and displays tumor tropism in epithelial tissue (xenografted human oral squamous cell carcinoma). Dispersal is an asset for phage therapy, but a liability for phage-based DDVs. We propose increased focus on phage persistence—and dispersal—screening.

The Trimeric Major Capsid Protein of Mavirus is stabilized by its Interlocked N-termini Enabling Core Flexibility for Capsid Assembly

Icosahedral viral capsids assemble with high fidelity from a large number of identical buildings blocks. The mechanisms that enable individual capsid proteins to form stable oligomeric units (capsomers) while affording structural adaptability required for further assembly into capsids are mostly unknown. Understanding these mechanisms requires knowledge of the capsomers' dynamics, especially for viruses where no additional helper proteins are needed during capsid assembly like for the Mavirus virophage that despite its complexity (triangulation number T = 27) can assemble from its major capsid protein (MCP) alone. This protein forms the basic building block of the capsid namely a trimer (MCP₃) of double-jelly roll protomers with highly intertwined N-terminal arms of each protomer wrapping around the other two at the base of the capsomer, secured by a clasp that is formed by part of the C-terminus. Probing the dynamics of the capsomer with HDX mass spectrometry we observed differences in conformational flexibility between functional elements of the MCP trimer. While the N-terminal arm and clasp regions show above average deuterium incorporation, the two jelly-roll units in each protomer also differ in their structural plasticity, which might be needed for efficient assembly. Assessing the role of the N-terminal arm in maintaining capsomer stability showed that its detachment is required for capsomer dissociation, constituting a barrier towards capsomer monomerisation. Surprisingly, capsomer dissociation was irreversible since it was followed by a global structural rearrangement of the protomers as indicated by computational studies showing a rearrangement of the N-terminus blocking part of the capsomer forming interface.

Indirect Selection against Antibiotic Resistance via Specialized Plasmid-Dependent Bacteriophages

Antibiotic resistance genes of important Gram-negative bacterial pathogens are residing in mobile genetic elements such as conjugative plasmids. These elements rapidly disperse between cells when antibiotics are present and hence our continuous use of antimicrobials selects for elements that often harbor multiple resistance genes. Plasmid-dependent (or male-specific or, in some cases, pilus-dependent) bacteriophages are bacterial viruses that infect specifically bacteria that carry certain plasmids. The introduction of these specialized phages into a plasmid-abundant bacterial community has many beneficial effects from an anthropocentric viewpoint: the majority of the plasmids are lost while the remaining plasmids acquire mutations that make them untransferable between pathogens. Recently, bacteriophage-based therapies have become a more acceptable choice to treat multi-resistant bacterial infections. Accordingly, there is a possibility to utilize these specialized phages, which are not dependent on any particular pathogenic species or strain but rather on the resistance-providing elements, in order to improve or enlengthen the lifespan of conventional antibiotic approaches. Here, we take a snapshot of the current knowledge of plasmid-dependent bacteriophages.

Biological characteristics and genome analysis of a novel phage vB_KpnP_IME279 infecting Klebsiella pneumoniae

Klebsiella pneumoniae (family Enterobacteriaceae) is a gram-negative bacterium that has strong pathogenicity to humans and can cause sepsis, pneumonia, and urinary tract infection. In recent years, the unreasonable use of antibacterial drugs has led to an increase in drug-resistant strains of K. pneumoniae, a serious threat to public health. Bacteriophages, viruses that infect bacteria, are ubiquitous in the natural environment. They are considered to be the most promising substitute for antibiotics because of their high specificity, high efficiency, high safety, low cost, and short development cycle. In this study, a novel phage designated vB_KpnP_IME279 was successfully isolated from hospital sewage using a multidrug-resistant strain of K. pneumoniae as an indicator. A one-step growth curve showed that vB_KpnP_IME279 has a burst size of 140 plaque-forming units/cell and a latent period of 20 min at its optimal multiplicity of infection (MOI = 0.1). Phage vB_KpnP_IME279 survives in a wide pH range between 3 and 11 and is stable at temperatures ranging from 40 to 60 °C. Ten of the 20 strains of K. pneumoniae including the host bacteria were lysed by the phage vB_KpnP_IME279, and the multilocus sequence typing and wzi typing of the 10 strains were ST11, ST37, ST375, wzi209, wzi52, and wzi72, respectively. The genome of vB_KpnP_IME279 is 42,518 bp long with a G + C content of 59.3%. Electron microscopic observation showed that the phage belongs to the family Podoviridae. BLASTN alignment showed that the genome of the phage has low similarity with currently known phages. The evolutionary relationship between phage vB_KpnP_IME279 and other Podoviridae was analyzed using a phylogenetic tree based on sequences of phage major capsid protein and indicates that the phage vB_KpnP_IME279 belongs to the Podoviridae subfamily. These data enhance understanding of K. pneumoniae phages and will help in development of treatments for multidrug-resistant bacteria using phages.

Superimposition of Viral Protein Structures: A Means to Decipher the Phylogenies of Viruses

Superimposition of protein structures is key in unravelling structural homology across proteins whose sequence similarity is lost. Structural comparison provides insights into protein function and evolution. Here, we review some of the original findings and thoughts that have led to the current established structure-based phylogeny of viruses: starting from the original observation that the major capsid proteins of plant and animal viruses possess similar folds, to the idea that each virus has an innate “self”. This latter idea fueled the conceptualization of the PRD1-adenovirus lineage whose members possess a major capsid protein (innate “self”) with a double jelly roll fold. Based on this approach, long-range viral evolutionary relationships can be detected allowing the virosphere to be classified in four structure-based lineages. However, this process is not without its challenges or limitations. As an example of these hurdles, we finally touch on the difficulty of establishing structural “self” traits for enveloped viruses showcasing the coronaviruses but also the power of structure-based analysis in the understanding of emerging viruses

Current capsid assembly models of icosahedral nucleocytoviricota viruses

Nucleocytoviricota viruses (NCVs) belong to a newly established phylum originally grouped as Nucleocytoplasmic large DNA viruses. NCVs are unique because of their large and complicated genomes that contain cellular genes with homologs from all kingdoms of life, raising intensive debates on their evolutional origins. Many NCVs pack their genomes inside massive icosahedral capsids assembled from thousands of proteins. Studying the assembly mechanism of such capsids has been challenging until breakthroughs from structural studies. Subsequently, several models of the capsid assembly were proposed, which provided some interesting insights on this elaborate process. In this review, we discuss three of the most recent assembly models as well as supporting experimental observations. Furthermore, we propose a new model that combines research developments from multiple sources. Investigation of the assembly process of these vast NCV capsids will facilitate future deciphering of the molecular mechanisms driving the formation of similar supramolecular complexes.

ICTV Virus Taxonomy Profile: Finnlakeviridae

Finnlakeviridae is a family of icosahedral, internal membrane-containing bacterial viruses with circular, single-stranded DNA genomes. The family includes the genus, Finnlakevirus, with the species, Flavobacterium virus FLiP. Flavobacterium phage FLiP was isolated with its Gram-negative host bacterium from a boreal freshwater habitat in Central Finland in 2010. It is the first described single-stranded DNA virus with an internal membrane and shares minimal sequence similarity with other known viruses. The virion organization (pseudo T=21 dextro) and major capsid protein fold (double-β-barrel) resemble those of Pseudoalteromonas phage PM2 (family Corticoviridae), which has a double-stranded DNA genome. A similar major capsid protein fold is also found in other double-stranded DNA viruses in the kingdom Bamfordvirae. This is a summary of the International Committee on Taxonomy of Viruses (ICTV) report on the family Finnlakeviridae, which is available at ictv.global/report/finnlakeviridae.

Isolation and characterization of Vibrio parahaemolyticus bacteriophage vB_VpaS_PG07

A novel bacteriophage vB_VpaS_PG07 (hereafter designated PG07) that infects Vibrio parahaemolyticus was isolated. The bacteriophage was examined by transmission electron microscopy, and the result showed that PG07 belonged to family Siphoviridae, with an isometric polyhedral head (80 nm in diameter) and a long tail (175 nm in length). The one-step growth curve showed that the latent period and burst size were 10 min and 60 PFUs/infected cell, respectively. PG07 had double-stranded DNA genome of 112, 106 bp with 43.65 % G+C content. A total of 158 putative open reading frames (ORFs) were identified in the genome of PG07, including functional genes associated with integration, nucleotide metabolism and replication, structure and packaging and bacterial lysis. Sixteen tRNA genes were discovered, and no genes associated with pathogenicity and virulence were identified. The genome of PG07 showed very low similarity to phage genomes deposited in public databases (77.65 % nucleotide identity and 9 % query coverage). The newly sequenced PG07 could be considered as a novel T5-like virus. PG07 significantly reduced the mortality of shrimps challenged with V. parahaemolyticus, a bacterium causing acute hepatopancreatic necrosis disease (AHPND). The findings highlight the potential of PG07 as an effective antibacterial agent for phage prophylaxis and phage therapy in aquaculture.

Adenoviruses - Infection, pathogenesis and therapy

Both well-known and emerging viruses increasingly affect humans and cause disease, sometimes with devastating impact on society. The viruses present in the biosphere are the top predators in the life chain, virtually without enemies, except perhaps the immune system, and harsh environmental physicochemical conditions restricting their dissemination. We know a lot about viruses, but do we know enough? This series of reviews is dedicated to adenoviruses (AdVs), a family of nonenveloped DNA viruses occurring in vertebrates, including humans. AdVs have been the focus of intense research for more than 67 years. Besides causing disease, they have immensely contributed to the advance of life sciences and medicine over the past decades. Recently, AdVs have been widely used as vehicles in gene therapy and vaccination. They continue to provide fundamental insights into virus-host interactions in cells, tissues and organisms, as well as systems and metabolic networks. This special issue of FEBS Letters presents a unique collection of 23 state-of-the-art review articles by leading adenovirologists. In this prelude, I present the chapters, which provide a solid basis for further exploring the rich heritage in adenovirus molecular cell biology, structural biology, genetics, immunology, gene therapy and epidemiology. I conclude with an essential discussion of six blind spots in adenovirology.

The Stabilizing Excipients in Dry State Therapeutic Phage Formulations

Phage therapy has gained prominence due to the increasing pathogenicity of "super bugs" and the rise of their multidrug resistance to conventional antibiotics. Dry state formulation of therapeutic phage is attractive to improve their "druggability" by increasing their shelf life, improving their ease of handling, and ultimately retaining their long-term potency. The use and selection of excipients are critical to stabilize phage in solid formulations and protect their viability from stresses encountered during the solidification process and long-term storage prior to use. Here, this review focuses on the current classes of excipients used to manufacture dry state phage formulations and their ability to stabilize and protect phage throughout the process, as discussed in the literature. We provide perspective of outstanding challenges involved in the formulation of dry state phage. We suggest strategies to improve excipient identification and selection, optimize the potential excipient combinations to improve phage viability during formulation, and evaluate new methodologies that can provide greater insight into phage-excipient interactions to improve design criteria to improve formulation of dry state phage therapeutics. Addressing these challenges opens up new opportunities to re-design and re-imagine phage formulations for improved efficacy as a pharmaceutical product.

Unraveling virus relationships by structure-based phylogenetic classification

Delineation of the intricacies of protein function from macromolecular structure constitutes a continual obstacle in the study of cell and pathogen biology. Structure-based phylogenetic analysis has emerged as a powerful tool for addressing this challenge, allowing the detection and quantification of conserved architectural properties between proteins, including those with low or no detectable sequence homology. With a focus on viral protein structure, we highlight how a number of investigations have utilized this powerful method to infer common functionality and ancestry.

Phage diversity, genomics and phylogeny

Recent advances in viral metagenomics have enabled the rapid discovery of an unprecedented catalogue of phages in numerous environments, from the human gut to the deep ocean. Although these advances have expanded our understanding of phage genomic diversity, they also revealed that we have only scratched the surface in the discovery of novel viruses. Yet, despite the remarkable diversity of phages at the nucleotide sequence level, the structural proteins that form viral particles show strong similarities and conservation. Phages are uniquely interconnected from an evolutionary perspective and undergo multiple events of genetic exchange in response to the selective pressure of their hosts, which drives their diversity. In this Review, we explore phage diversity at the structural, genomic and community levels as well as the complex evolutionary relationships between phages, moulded by the mosaicity of their genomes.

Lessons learned from adenovirus (1970-2019)

Animal viruses are well recognized for their ability to uncover fundamental cell and molecular processes, and adenovirus certainly provides a prime example. This review illustrates the lessons learned from studying adenovirus over the past five decades. We take a look back at the key studies of adenovirus structure and biophysical properties, which revealed the mechanisms of adenovirus association with antibody, cell receptor, and immune molecules that regulate infection. In addition, we discuss the critical contribution of studies of adenovirus gene expression to elucidation of fundamental reactions in pre-mRNA processing and its regulation. Other pioneering studies furnished the first examples of protein-primed initiation of DNA synthesis and viral small RNAs. As a nonenveloped virus, adenoviruses have furnished insights into the modes of virus attachment, entry, and penetration of host cells, and we discuss the diversity of cell receptors that support these processes, as well as membrane penetration. As a result of these extensive studies, adenovirus vectors were among the first to be developed for therapeutic applications. We highlight some of the early (unsuccessful) trials and the lessons learned from them.

Genomic foundations of evolution and ocular pathogenesis in human adenovirus species D

Human adenovirus commonly causes infections of respiratory, gastrointestinal, genitourinary, and ocular surface mucosae. Although most adenovirus eye infections are mild and self-limited, specific viruses within human adenovirus species D are associated with epidemic keratoconjunctivitis (EKC), a severe and highly contagious ocular surface infection, which can lead to chronic and/or recurrent, visually disabling keratitis. In this review, we discuss the links between adenovirus ontogeny, genomics, immune responses, and corneal pathogenesis, for those viruses that cause EKC.

Virus-Host Coevolution with a Focus on Animal and Human DNA Viruses

Viruses have been infecting their host cells since the dawn of life, and this extremely long-term coevolution gave rise to some surprising consequences for the entire tree of life. It is hypothesised that viruses might have contributed to the formation of the first cellular life form, or that even the eukaryotic cell nucleus originates from an infection by a coated virus. The continuous struggle between viruses and their hosts to maintain at least a constant fitness level led to the development of an unceasing arms race, where weapons are often shuttled between the participants. In this literature review we try to give a short insight into some general consequences or traits of virus–host coevolution, and after this we zoom in to the viral clades of adenoviruses, herpesviruses, nucleo-cytoplasmic large DNA viruses, polyomaviruses and, finally, circoviruses.

Electron cryo-microscopy of bacteriophage PR772 reveals the elusive vertex complex and the capsid architecture

Bacteriophage PR772, a member of the Tectiviridae family, has a 70 nm diameter icosahedral protein capsid that encapsulates a lipid membrane, dsDNA, and various internal proteins. An icosahedrally averaged CryoEM reconstruction of the wild-type virion and a localized reconstruction of the vertex region reveal the composition and the structure of the vertex complex along with new protein conformations that play a vital role in maintaining the capsid architecture of the virion. The overall resolution of the virion is 2.75 Å, while the resolution of the protein capsid is 2.3 Å. The conventional penta-symmetron formed by the capsomeres is replaced by a large vertex complex in the pseudo T = 25 capsid. All the vertices contain the host-recognition protein, P5; two of these vertices show the presence of the receptor-binding protein, P2. The 3D structure of the vertex complex shows interactions with the viral membrane, indicating a possible mechanism for viral infection.

Complete genome analysis of a novel temperate bacteriophage induced from Corynebacterium striatum

A temperate bacteriophage, IME1320_01, was induced by mitomycin C treatment from Corynebacterium striatum. This phage possesses a double-stranded DNA genome of 40,086 bp with a G+C content of 58%. A total of 53 putative open reading frames (ORFs) were identified in its genome. BLASTn analysis revealed that IME1320_01 had the highest sequence similarity to Corynebacterium striatum strain 216, with a genome homology coverage of 44% and highest sequence identity of 95%. The termini of the phage genome was non-redundant, with a 13-nt 3'-protruding cohesive end. To the best of our knowledge, phage IME1320_01 is the first inducible phage to be identified in Corynebacterium striatum.

Lytic gene expression in the temperate bacteriophage GIL01 is activated by a phage-encoded LexA homologue

The GIL01 bacteriophage is a temperate phage that infects the insect pathogen Bacillus thuringiensis. During the lytic cycle, phage gene transcription is initiated from three promoters: P1 and P2, which control the expression of the early phage genes involved in genome replication and P3, which controls the expression of the late genes responsible for virion maturation and host lysis. Unlike most temperate phages, GIL01 lysogeny is not maintained by a dedicated phage repressor but rather by the host's regulator of the SOS response, LexA. Previously we showed that the lytic cycle was induced by DNA damage and that LexA, in conjunction with phage-encoded protein gp7, repressed P1. Here we examine the lytic/lysogenic switch in more detail and show that P3 is also repressed by a LexA-gp7 complex, binding to tandem LexA boxes within the promoter. We also demonstrate that expression from P3 is considerably delayed after DNA damage, requiring the phage-encoded DNA binding protein, gp6. Surprisingly, gp6 is homologous to LexA itself and, thus, is a rare example of a LexA homologue directly activating transcription. We propose that the interplay between these two LexA family members, with opposing functions, ensures the timely expression of GIL01 phage late genes.

Editorial: Physical Virology and the Nature of Virus Infections

Virus particles, 'virions', range in size from nano-scale to micro-scale. They have many different shapes and are composed of proteins, sugars, nucleic acids, lipids, water and solutes. Virions are autonomous entities and affect all forms of life in a parasitic relationship. They infect prokaryotic and eukaryotic cells. The physical properties of virions are tuned to the way they interact with cells. When virions interact with cells, they gain huge complexity and give rise to an infected cell, also known as 'virus'. Virion-cell interactions entail the processes of entry, replication and assembly, as well as egress from the infected cell. Collectively, these steps can result in progeny virions, which is a productive infection, or in silencing of the virus, an abortive or latent infection. This book explores facets of the physical nature of virions and viruses and the impact of mechanical properties on infection processes at the cellular and subcellular levels.

Characterization and genome analysis of a novel bacteriophage vB_SpuP_Spp16 that infects Salmonella enterica serovar pullorum

A novel virulent bacteriophage vB_SpuP_Spp16 (hereafter designated Spp16) that infects Salmonella enterica serovar pullorum was isolated. Transmission electron microscopy showed that Spp16 possessed an isometric polyhedral head (60 nm in diameter) and a short tail (10 nm in length) belonging to the family Podoviridae. Its complete genome was determined to be 41,832 bp, with a 39.46% GC content by next-generation sequencing. The genome contains 53 proposed open reading frames that are involved in DNA replication and modification, transcriptional regulation, phage structural and packaging proteins and bacterial lysis. No transfer RNA genes were identified. The termini of genome were determined using our previously proposed termini identification method, which suggests that this phage has redundant termini with 421 bp direct terminal repeats. BLASTn analysis revealed the highest sequence similarity with Yersinia phage phi80-18, with a genome coverage of 33% and highest sequence identity of 69%. The phylogenetic analysis indicated that Spp16 forms a distinct branch of the subfamily Autographivirinae. Comparative genomics analysis showed that the phage Spp16 should be regarded as a new subcluster within the GAP227-like cluster in the Autographivirinae subfamily. The phage Spp16 has an obligate lytic life cycle demonstrated by experimental data and genomic analysis. These results suggest that Spp16 may be a proper candidate to control diseases caused by Salmonella enterica serovar pullorum.

Structural basis for assembly of vertical single β-barrel viruses

The vertical double β-barrel major capsid protein (MCP) fold, fingerprint of the PRD1-adeno viral lineage, is widespread in many viruses infecting organisms across the three domains of life. The discovery of PRD1-like viruses with two MCPs challenged the known assembly principles. Here, we present the cryo-electron microscopy (cryo-EM) structures of the archaeal, halophilic, internal membrane-containing Haloarcula californiae icosahedral virus 1 (HCIV-1) and Haloarcula hispanica icosahedral virus 2 (HHIV-2) at 3.7 and 3.8 Å resolution, respectively. Our structures reveal proteins located beneath the morphologically distinct two- and three-tower capsomers and homopentameric membrane proteins at the vertices that orchestrate the positioning of pre-formed vertical single β-barrel MCP heterodimers. The cryo-EM based structures together with the proteomics data provide insights into the assembly mechanism of this type of viruses and into those with membrane-less double β-barrel MCPs.

Here, the authors present the cryo-EM structures of two archaeal, halophilic, internal membrane-containing icosahedral viruses at 3.7 and 3.8 Å resolution, providing insights into the assembly process of these and related PRD1-adeno lineage viruses.

Half a Century of Research on Membrane-Containing Bacteriophages: Bringing New Concepts to Modern Virology

Half a century of research on membrane-containing phages has had a major impact on virology, providing new insights into virus diversity, evolution and ecological importance. The recent revolutionary technical advances in imaging, sequencing and lipid analysis have significantly boosted the depth and volume of knowledge on these viruses. This has resulted in new concepts of virus assembly, understanding of virion stability and dynamics, and the description of novel processes for viral genome packaging and membrane-driven genome delivery to the host. The detailed analyses of such processes have given novel insights into DNA transport across the protein-rich lipid bilayer and the transformation of spherical membrane structures into tubular nanotubes, resulting in the description of unexpectedly dynamic functions of the membrane structures. Membrane-containing phages have provided a framework for understanding virus evolution. The original observation on membrane-containing bacteriophage PRD1 and human pathogenic adenovirus has been fundamental in delineating the concept of "viral lineages", postulating that the fold of the major capsid protein can be used as an evolutionary fingerprint to trace long-distance evolutionary relationships that are unrecognizable from the primary sequences. This has brought the early evolutionary paths of certain eukaryotic, bacterial, and archaeal viruses together, and potentially enables the reorganization of the nearly immeasurable virus population (~1 × 1031) on Earth into a reasonably low number of groups representing different architectural principles. In addition, the research on membrane-containing phages can support the development of novel tools and strategies for human therapy and crop protection.

The so far farthest reaches of the double jelly roll capsid protein fold

BACKGROUND

During the last two decades, structural biology analyses have shown that viruses infecting hosts far apart in evolution share similar architectural features, prompting a new virus classification based on structural lineages. Until recently, only a few prokaryotic viruses had been described for one of the lineages, whose main characteristic is a capsid protein with a perpendicular double jelly roll.

MAIN BODY

Metagenomics analyses are showing that the variety of prokaryotic viruses encoding double jelly roll capsid proteins is much larger than previously thought. The newly discovered viruses have novel genome organisations with interesting implications for virus structure, function and evolution. There are also indications of their having a significant ecological impact.

CONCLUSION

Viruses with double jelly roll capsid proteins that infect prokaryotic hosts form a large part of the virosphere that had so far gone unnoticed. Their discovery by metagenomics is only a first step towards many more exciting findings. Work needs to be invested in isolating these viruses and their hosts, characterizing the structure and function of the proteins their genomes encode, and eventually access the wealth of biological information they may hold.

Virus Proteins and Nucleoproteins: An Overview

Genetic economy is a key feature in all aspects of viral replication. Some viruses are able to function as independently evolving entities with as few as two genes, while satellite viruses have been described that encode a single gene product. To accommodate the need for genetic economy, the viral infectious entity - the virion, generally assembles in a highly-symmetrical manner. Viral structural proteins are multifunctional, accomplishing several tasks beyond their primary role of forming protective shells. These include mediating attachment and entry, avoiding and regulating host responses to infection and sometimes mediating gene expression and genome replication. Here we introduce some of the basic principles of virus assembly with examples to show how recurring motifs are seen in spherical viruses that infect diverse host species, how some viruses use helical assemblies to encapsidate their genomes and how viral envelope glycoproteins accomplish membrane fusion.

Bacterial RecA Protein Promotes Adenoviral Recombination during In Vitro Infection

Adenoviruses are common human mucosal pathogens of the gastrointestinal, respiratory, and genitourinary tracts and ocular surface. Here, we report finding Chi-like sequences in adenovirus recombination hot spots. Adenovirus coinfection in the presence of bacterial RecA protein facilitated homologous recombination between viruses. Genetic recombination led to evolution of an important external feature on the adenoviral capsid, namely, the penton base protein hypervariable loop 2, which contains the arginine-glycine-aspartic acid motif critical to viral internalization. We speculate that free Rec proteins present in gastrointestinal secretions upon bacterial cell death facilitate the evolution of human adenoviruses through homologous recombination, an example of viral commensalism and the complexity of virus-host interactions, including regional microbiota.

Adenovirus infections in humans are common and sometimes lethal. Adenovirus-derived vectors are also commonly chosen for gene therapy in human clinical trials. We have shown in previous work that homologous recombination between adenoviral genomes of human adenovirus species D (HAdV-D), the largest and fastest growing HAdV species, is responsible for the rapid evolution of this species. Because adenovirus infection initiates in mucosal epithelia, particularly at the gastrointestinal, respiratory, genitourinary, and ocular surfaces, we sought to determine a possible role for mucosal microbiota in adenovirus genome diversity. By analysis of known recombination hot spots across 38 human adenovirus genomes in species D (HAdV-D), we identified nucleotide sequence motifs similar to bacterial Chi sequences, which facilitate homologous recombination in the presence of bacterial Rec enzymes. These motifs, referred to here as Chi_AD, were identified immediately 5′ to the sequence encoding penton base hypervariable loop 2, which expresses the arginine-glycine-aspartate moiety critical to adenoviral cellular entry. Coinfection with two HAdV-Ds in the presence of an Escherichia coli lysate increased recombination; this was blocked in a RecA mutant strain, E. coli DH5α, or upon RecA depletion. Recombination increased in the presence of E. coli lysate despite a general reduction in viral replication. RecA colocalized with viral DNA in HAdV-D-infected cell nuclei and was shown to bind specifically to Chi_AD sequences. These results indicate that adenoviruses may repurpose bacterial recombination machinery, a sharing of evolutionary mechanisms across a diverse microbiota, and unique example of viral commensalism.

IMPORTANCE Adenoviruses are common human mucosal pathogens of the gastrointestinal, respiratory, and genitourinary tracts and ocular surface. Here, we report finding Chi-like sequences in adenovirus recombination hot spots. Adenovirus coinfection in the presence of bacterial RecA protein facilitated homologous recombination between viruses. Genetic recombination led to evolution of an important external feature on the adenoviral capsid, namely, the penton base protein hypervariable loop 2, which contains the arginine-glycine-aspartic acid motif critical to viral internalization. We speculate that free Rec proteins present in gastrointestinal secretions upon bacterial cell death facilitate the evolution of human adenoviruses through homologous recombination, an example of viral commensalism and the complexity of virus-host interactions, including regional microbiota.

Protein cage assembly across multiple length scales

Within the materials science community, proteins with cage-like architectures are being developed as versatile nanoscale platforms for use in protein nanotechnology. Much effort has been focused on the functionalization of protein cages with biological and non-biological moieties to bring about new properties of not only individual protein cages, but collective bulk-scale assemblies of protein cages. In this review, we report on the current understanding of protein cage assembly, both of the cages themselves from individual subunits, and the assembly of the individual protein cages into higher order structures. We start by discussing the key properties of natural protein cages (for example: size, shape and structure) followed by a review of some of the mechanisms of protein cage assembly and the factors that influence it. We then explore the current approaches for functionalizing protein cages, on the interior or exterior surfaces of the capsids. Lastly, we explore the emerging area of higher order assemblies created from individual protein cages and their potential for new and exciting collective properties.

Vast diversity of prokaryotic virus genomes encoding double jelly-roll major capsid proteins uncovered by genomic and metagenomic sequence analysis

BACKGROUND

Analysis of metagenomic sequences has become the principal approach for the study of the diversity of viruses. Many recent, extensive metagenomic studies on several classes of viruses have dramatically expanded the visible part of the virosphere, showing that previously undetected viruses, or those that have been considered rare, actually are important components of the global virome.

RESULTS

We investigated the provenance of viruses related to tail-less bacteriophages of the family Tectiviridae by searching genomic and metagenomics sequence databases for distant homologs of the tectivirus-like Double Jelly-Roll major capsid proteins (DJR MCP). These searches resulted in the identification of numerous genomes of virus-like elements that are similar in size to tectiviruses (10-15 kilobases) and have diverse gene compositions. By comparison of the gene repertoires, the DJR MCP-encoding genomes were classified into 6 distinct groups that can be predicted to differ in reproduction strategies and host ranges. Only the DJR MCP gene that is present by design is shared by all these genomes, and most also encode a predicted DNA-packaging ATPase; the rest of the genes are present only in subgroups of this unexpectedly diverse collection of DJR MCP-encoding genomes. Only a minority encode a DNA polymerase which is a hallmark of the family Tectiviridae and the putative family "Autolykiviridae". Notably, one of the identified putative DJR MCP viruses encodes a homolog of Cas1 endonuclease, the integrase involved in CRISPR-Cas adaptation and integration of transposon-like elements called casposons. This is the first detected occurrence of Cas1 in a virus. Many of the identified elements are individual contigs flanked by inverted or direct repeats and appear to represent complete, extrachromosomal viral genomes, whereas others are flanked by bacterial genes and thus can be considered as proviruses. These contigs come from metagenomes of widely different environments, some dominated by archaea and others by bacteria, suggesting that collectively, the DJR MCP-encoding elements have a broad host range among prokaryotes.

CONCLUSIONS

The findings reported here greatly expand the known host range of (putative) viruses of bacteria and archaea that encode a DJR MCP. They also demonstrate the extreme diversity of genome architectures in these viruses that encode no universal proteins other than the capsid protein that was used as the marker for their identification. From a supposedly minor group of bacterial and archaeal viruses, these viruses are emerging as a substantial component of the prokaryotic virome.

The genomic underpinnings of eukaryotic virus taxonomy: creating a sequence-based framework for family-level virus classification

BACKGROUND

The International Committee on Taxonomy of Viruses (ICTV) classifies viruses into families, genera and species and provides a regulated system for their nomenclature that is universally used in virus descriptions. Virus taxonomic assignments have traditionally been based upon virus phenotypic properties such as host range, virion morphology and replication mechanisms, particularly at family level. However, gene sequence comparisons provide a clearer guide to their evolutionary relationships and provide the only information that may guide the incorporation of viruses detected in environmental (metagenomic) studies that lack any phenotypic data.

RESULTS

The current study sought to determine whether the existing virus taxonomy could be reproduced by examination of genetic relationships through the extraction of protein-coding gene signatures and genome organisational features. We found large-scale consistency between genetic relationships and taxonomic assignments for viruses of all genome configurations and genome sizes. The analysis pipeline that we have called 'Genome Relationships Applied to Virus Taxonomy' (GRAViTy) was highly effective at reproducing the current assignments of viruses at family level as well as inter-family groupings into orders. Its ability to correctly differentiate assigned viruses from unassigned viruses, and classify them into the correct taxonomic group, was evaluated by threefold cross-validation technique. This predicted family membership of eukaryotic viruses with close to 100% accuracy and specificity potentially enabling the algorithm to predict assignments for the vast corpus of metagenomic sequences consistently with ICTV taxonomy rules. In an evaluation run of GRAViTy, over one half (460/921) of (near)-complete genome sequences from several large published metagenomic eukaryotic virus datasets were assigned to 127 novel family-level groupings. If corroborated by other analysis methods, these would potentially more than double the number of eukaryotic virus families in the ICTV taxonomy.

CONCLUSIONS

A rapid and objective means to explore metagenomic viral diversity and make informed recommendations for their assignments at each taxonomic layer is essential. GRAViTy provides one means to make rule-based assignments at family and order levels in a manner that preserves the integrity and underlying organisational principles of the current ICTV taxonomy framework. Such methods are increasingly required as the vast virosphere is explored.

Characterization of a Novel Tectivirus Phage Toil and Its Potential as an Agent for Biolipid Extraction

The oleaginous bacterium Rhodococcus opacus PD630 is metabolically diverse and can be cultivated on various renewable resources to serve as a sustainable triacylglycerol (TAG) feedstock for biodiesel production. Current methods for TAG extraction are costly, but infection of cultures by lytic bacteriophages (phages) may be a viable approach for achieving release of intracellular lipid from oleaginous bacteria such as R. opacus. This study reports the novel tectiviral phage Toil capable of releasing intracellular contents including a fluorescent protein marker and TAGs into the supernatant after phage infection of R. opacus PD631, a domesticated derivative of strain PD630. Phage Toil is placed in the Tectiviridae by its morphology, the presence of a lipid membrane, its genome architecture and the presence of terminal covalently-linked proteins. Toil is the first tectivirus capable of infecting a member of the Actinobacteria. Microscopy shows that infected cells do not undergo sudden lysis but instead maintain their original shape for several hours, with the cellular morphology gradually deteriorating. Approximately 30% of intracellular TAGs could be recovered from the culture supernatants of Toil-infected PD631 cells. Phage Toil has potential to be used as an agent in extraction of TAGs from oleaginous bacterium R. opacus.

IMPORTANCE

This study reported the first tectivirus (Phage Toil) capable of infecting a member of the Actinobacteria. In this study, we showed that Phage Toil can infect oleaginous bacterium Rhodococcus opacus to release intracellular contents such as a fluorescent protein marker and TAG lipid granules, which can serve as a starting material for biodiesel production. This study demonstrates a new method to extract TAGs by using this phage. Additionally, Phage Toil can be a new model phage to advance knowledge regarding phage infection mechanisms in Rhodococcus and other mycolic acid-containing bacteria such as Mycobacterium.

The number of genes encoding repeat domain-containing proteins positively correlates with genome size in amoebal giant viruses

Curiously, in viruses, the virion volume appears to be predominantly driven by genome length rather than the number of proteins it encodes or geometric constraints. With their large genome and giant particle size, amoebal viruses (AVs) are ideally suited to study the relationship between genome and virion size and explore the role of genome plasticity in their evolutionary success. Different genomic regions of AVs exhibit distinct genealogies. Although the vertically transferred core genes and their functions are universally conserved across the nucleocytoplasmic large DNA virus (NCLDV) families and are essential for their replication, the horizontally acquired genes are variable across families and are lineage-specific. When compared with other giant virus families, we observed a near–linear increase in the number of genes encoding repeat domain-containing proteins (RDCPs) with the increase in the genome size of AVs. From what is known about the functions of RDCPs in bacteria and eukaryotes and their prevalence in the AV genomes, we envisage important roles for RDCPs in the life cycle of AVs, their genome expansion, and plasticity. This observation also supports the evolution of AVs from a smaller viral ancestor by the acquisition of diverse gene families from the environment including RDCPs that might have helped in host adaption.

Capsids and Genomes of Jumbo-Sized Bacteriophages Reveal the Evolutionary Reach of the HK97 Fold

Large icosahedral viruses that infect bacteria represent an extreme of the coevolution of capsids and the genomes they accommodate. One subset of these large viruses is the jumbophages, tailed phages with double-stranded DNA genomes of at least 200,000 bp. We explored the mechanism leading to increased capsid and genome sizes by characterizing structures of several jumbophage capsids and the DNA packaged within them. Capsid structures determined for six jumbophages were consistent with the canonical phage HK97 fold, and three had capsid geometries with novel triangulation numbers (T=25, T=28, and T=52). Packaged DNA (chromosome) sizes were larger than the genome sizes, indicating that all jumbophages use a head-full DNA packaging mechanism. For two phages (PAU and G), the sizes appeared very much larger than their genome length. We used two-dimensional DNA gel electrophoresis to show that these two DNAs migrated abnormally due to base modifications and to allow us to calculate their actual chromosome sizes. Our results support a ratchet model of capsid and genome coevolution whereby mutations lead to increased capsid volume and allow the acquisition of additional genes. Once the added genes and larger capsid are established, mutations that restore the smaller size are disfavored.

A large family of viruses share the same fold of the capsid protein as bacteriophage HK97, a virus that infects bacteria. Members of this family use different numbers of the capsid protein to build capsids of different sizes. Here, we examined the structures of extremely large capsids and measured their DNA content relative to the sequenced genome lengths, aiming to understand the process that increases size. We concluded that mutational changes leading to larger capsids become locked in by subsequent changes to the genome organization.

Virus found in a boreal lake links ssDNA and dsDNA viruses

Viruses have impacted the biosphere in numerous ways since the dawn of life. However, the evolution, genetic, structural, and taxonomic diversity of viruses remain poorly understood, in part because sparse sampling of the virosphere has concentrated mostly on exploring the abundance and diversity of dsDNA viruses. Furthermore, viral genomes are highly diverse, and using only the current sequence-based methods for classifying viruses and studying their phylogeny is complicated. Here we describe a virus, FLiP (Flavobacterium-infecting, lipid-containing phage), with a circular ssDNA genome and an internal lipid membrane enclosed in the icosahedral capsid. The 9,174-nt-long genome showed limited sequence similarity to other known viruses. The genetic data imply that this virus might use replication mechanisms similar to those found in other ssDNA replicons. However, the structure of the viral major capsid protein, elucidated at near-atomic resolution using cryo-electron microscopy, is strikingly similar to that observed in dsDNA viruses of the PRD1-adenovirus lineage, characterized by a major capsid protein bearing two β-barrels. The strong similarity between FLiP and another member of the structural lineage, bacteriophage PM2, extends to the capsid organization (pseudo T = 21 dextro) despite the difference in the genetic material packaged and the lack of significant sequence similarity.

Nucleic and Amino Acid Sequences Support Structure-Based Viral Classification

Viral capsids ensure viral genome integrity by protecting the enclosed nucleic acids. Interactions between the genome and capsid and between individual capsid proteins (i.e., capsid architecture) are intimate and are expected to be characterized by strong evolutionary conservation. For this reason, a capsid structure-based viral classification has been proposed as a way to bring order to the viral universe. The seeming lack of sufficient sequence similarity to reproduce this classification has made it difficult to reject structural convergence as the basis for the classification. We reinvestigate whether the structure-based classification for viral coat proteins making icosahedral virus capsids is in fact supported by previously undetected sequence similarity. Since codon choices can influence nascent protein folding cotranslationally, we searched for both amino acid and nucleotide sequence similarity. To demonstrate the sensitivity of the approach, we identify a candidate gene for the pandoravirus capsid protein. We show that the structure-based classification is strongly supported by amino acid and also nucleotide sequence similarities, suggesting that the similarities are due to common descent. The correspondence between structure-based and sequence-based analyses of the same proteins shown here allow them to be used in future analyses of the relationship between linear sequence information and macromolecular function, as well as between linear sequence and protein folds.

IMPORTANCE Viral capsids protect nucleic acid genomes, which in turn encode capsid proteins. This tight coupling of protein shell and nucleic acids, together with strong functional constraints on capsid protein folding and architecture, leads to the hypothesis that capsid protein-coding nucleotide sequences may retain signatures of ancient viral evolution. We have been able to show that this is indeed the case, using the major capsid proteins of viruses forming icosahedral capsids. Importantly, we detected similarity at the nucleotide level between capsid protein-coding regions from viruses infecting cells belonging to all three domains of life, reproducing a previously established structure-based classification of icosahedral viral capsids.

HCIV-1 and Other Tailless Icosahedral Internal Membrane-Containing Viruses of the Family Sphaerolipoviridae

Members of the virus family Sphaerolipoviridae include both archaeal viruses and bacteriophages that possess a tailless icosahedral capsid with an internal membrane. The genera Alpha- and Betasphaerolipovirus comprise viruses that infect halophilic euryarchaea, whereas viruses of thermophilic Thermus bacteria belong to the genus Gammasphaerolipovirus. Both sequence-based and structural clustering of the major capsid proteins and ATPases of sphaerolipoviruses yield three distinct clades corresponding to these three genera. Conserved virion architectural principles observed in sphaerolipoviruses suggest that these viruses belong to the PRD1-adenovirus structural lineage. Here we focus on archaeal alphasphaerolipoviruses and their related putative proviruses. The highest sequence similarities among alphasphaerolipoviruses are observed in the core structural elements of their virions: the two major capsid proteins, the major membrane protein, and a putative packaging ATPase. A recently described tailless icosahedral haloarchaeal virus, Haloarcula californiae icosahedral virus 1 (HCIV-1), has a double-stranded DNA genome and an internal membrane lining the capsid. HCIV-1 shares significant similarities with the other tailless icosahedral internal membrane-containing haloarchaeal viruses of the family Sphaerolipoviridae. The proposal to include a new virus species, Haloarcula virus HCIV1, into the genus Alphasphaerolipovirus was submitted to the International Committee on Taxonomy of Viruses (ICTV) in 2016.

Crystal Structure of Major Envelope Protein VP24 from White Spot Syndrome Virus

White spot syndrome virus (WSSV) is one of the major and most serious pathogen in the shrimp industry. As one of the most abundant envelope protein, VP24 acts as a core protein interacting with other structure proteins and plays an important role in virus assembly and infection. Here, we have presented the crystal structure of VP24 from WSSV. In the structure, VP24 consists of a nine-stranded β–barrel fold with mostly antiparallel β-strands, and the loops extending out the β–barrel at both N-terminus and C-terminus, which is distinct to those of the other two major envelope proteins VP28 and VP26. Structural comparison of VP24 with VP26 and VP28 reveals opposite electrostatic surface potential properties of them. These structural differences could provide insight into their differential functional mechanisms and roles for virus assembly and infection. Moreover, the structure reveals a trimeric assembly, suggesting a likely natural conformation of VP24 in viral envelope. Therefore, in addition to confirming the evolutionary relationship among the three abundant envelope proteins of WSSV, our structural studies also facilitate a better understanding of the molecular mechanism underlying special roles of VP24 in WSSV assembly and infection.

Archaeal Haloarcula californiae Icosahedral Virus 1 Highlights Conserved Elements in Icosahedral Membrane-Containing DNA Viruses from Extreme Environments

Despite their high genomic diversity, all known viruses are structurally constrained to a limited number of virion morphotypes. One morphotype of viruses infecting bacteria, archaea, and eukaryotes is the tailless icosahedral morphotype with an internal membrane. Although it is considered an abundant morphotype in extreme environments, only seven such archaeal viruses are known. Here, we introduce Haloarcula californiae icosahedral virus 1 (HCIV-1), a halophilic euryarchaeal virus originating from salt crystals. HCIV-1 also retains its infectivity under low-salinity conditions, showing that it is able to adapt to environmental changes. The release of progeny virions resulting from cell lysis was evidenced by reduced cellular oxygen consumption, leakage of intracellular ATP, and binding of an indicator ion to ruptured cell membranes. The virion contains at least 12 different protein species, lipids selectively acquired from the host cell membrane, and a 31,314-bp-long linear double-stranded DNA (dsDNA). The overall genome organization and sequence show high similarity to the genomes of archaeal viruses in the Sphaerolipoviridae family. Phylogenetic analysis based on the major conserved components needed for virion assembly—the major capsid proteins and the packaging ATPase—placed HCIV-1 along with the alphasphaerolipoviruses in a distinct, well-supported clade. On the basis of its virion morphology and sequence similarities, most notably, those of its core virion components, we propose that HCIV-1 is a member of the PRD1-adenovirus structure-based lineage together with other sphaerolipoviruses. This addition to the lineage reinforces the notion of the ancient evolutionary links observed between the viruses and further highlights the limits of the choices found in nature for formation of a virion.

Under conditions of extreme salinity, the majority of the organisms present are archaea, which encounter substantial selective pressure, being constantly attacked by viruses. Regardless of the enormous viral sequence diversity, all known viruses can be clustered into a few structure-based viral lineages based on their core virion components. Our description of a new halophilic virus-host system adds significant insights into the largely unstudied field of archaeal viruses and, in general, of life under extreme conditions. Comprehensive molecular characterization of HCIV-1 shows that this icosahedral internal membrane-containing virus exhibits conserved elements responsible for virion organization. This places the virus neatly in the PRD1-adenovirus structure-based lineage. HCIV-1 further highlights the limited diversity of virus morphotypes despite the astronomical number of viruses in the biosphere. The observed high conservation in the core virion elements should be considered in addressing such fundamental issues as the origin and evolution of viruses and their interplay with their hosts.

Characterizing Bacteriophage PR772 as a Potential Surrogate for Adenovirus in Water Disinfection: A Comparative Analysis of Inactivation Kinetics and Replication Cycle Inhibition by Free Chlorine

Elucidating mechanisms by which pathogenic waterborne viruses become inactivated by drinking water disinfectants would facilitate the development of sensors to detect infectious viruses and novel disinfection strategies to provide safe water. Using bacteriophages as surrogates for human pathogenic viruses could assist in elucidating these mechanisms; however, an appropriate viral surrogate for human adenovirus (HAdV), a medium-sized virus with a double-stranded DNA genome, needs to be identified. Here, we characterized the inactivation kinetics of bacteriophage PR772, a member of the Tectiviridae family with many similarities in structure and replication to HAdV. The inactivation of PR772 and HAdV by free chlorine had similar kinetics that could be represented with a model previously developed for HAdV type 2 (HAdV-2). We developed and tested a quantitative assay to analyze several steps in the PR772 replication cycle to determine if both viruses being inactivated at similar rates resulted from similar replication cycle events being inhibited. Like HAdV-2, we observed that PR772 inactivated by free chlorine still attached to host cells, and viral DNA synthesis and early and late gene transcription were inhibited. Consequently, free chlorine exposure inhibited a replication cycle event that was post-binding but took place prior to early gene synthesis for both PR772 and HAdV-2.

Infectio: a Generic Framework for Computational Simulation of Virus Transmission between Cells

Infectio presents a generalized platform to analyze virus infection spread between cells. It allows the simulation of plaque phenotypes from image-based assays. Viral plaques are the result of virus spreading from primary infected cells to neighboring cells. This is a complex process and involves neighborhood effects at cell-cell contact sites or fluid dynamics in the extracellular medium. Infectio differentiates between two major modes of virus transmission between cells, allowing in silico testing of hypotheses about spreading mechanisms of any virus which can be grown in cell cultures, based on experimentally measured parameters, such as infection intensity or cell killing. The results of these tests can be compared with experimental data and allow interpretations with regard to biophysical mechanisms. Infectio also facilitates characterizations of the mode of action of therapeutic agents, such as oncolytic viruses or other infectious or cytotoxic agents.

Viruses spread between cells, tissues, and organisms by cell-free and cell-cell mechanisms, depending on the cell type, the nature of the virus, or the phase of the infection cycle. The mode of viral transmission has a large impact on disease development, the outcome of antiviral therapies or the efficacy of gene therapy protocols. The transmission mode of viruses can be addressed in tissue culture systems using live-cell imaging. Yet even in relatively simple cell cultures, the mechanisms of viral transmission are difficult to distinguish. Here we present a cross-platform software framework called “Infectio,” which is capable of simulating transmission phenotypes in tissue culture of virtually any virus. Infectio can estimate interdependent biological parameters, for example for vaccinia virus infection, and differentiate between cell-cell and cell-free virus spreading. Infectio assists in elucidating virus transmission mechanisms, a feature useful for designing strategies of perturbing or enhancing viral transmission. The complexity of the Infectio software is low compared to that of other software commonly used to quantitate features of cell biological images, which yields stable and relatively error-free output from Infectio. The software is open source (GPLv3 license), and operates on the major platforms (Windows, Mac, and Linux). The complete source code can be downloaded from http://infectio.github.io/index.html.

IMPORTANCE Infectio presents a generalized platform to analyze virus infection spread between cells. It allows the simulation of plaque phenotypes from image-based assays. Viral plaques are the result of virus spreading from primary infected cells to neighboring cells. This is a complex process and involves neighborhood effects at cell-cell contact sites or fluid dynamics in the extracellular medium. Infectio differentiates between two major modes of virus transmission between cells, allowing in silico testing of hypotheses about spreading mechanisms of any virus which can be grown in cell cultures, based on experimentally measured parameters, such as infection intensity or cell killing. The results of these tests can be compared with experimental data and allow interpretations with regard to biophysical mechanisms. Infectio also facilitates characterizations of the mode of action of therapeutic agents, such as oncolytic viruses or other infectious or cytotoxic agents.

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.

Insight into the Assembly of Viruses with Vertical Single β-barrel Major Capsid Proteins

Archaeal viruses constitute the least explored niche within the virosphere. Structure-based approaches have revealed close relationships between viruses infecting organisms from different domains of life. Here, using biochemical and cryo-electron microscopy techniques, we solved the structure of euryarchaeal, halophilic, internal membrane-containing Haloarcula hispanica icosahedral virus 2 (HHIV-2). We show that the density of the two major capsid proteins (MCPs) recapitulates vertical single β-barrel proteins and that disulfide bridges stabilize the capsid. Below, ordered density is visible close to the membrane and at the five-fold vertices underneath the host-interacting vertex complex underpinning membrane-protein interactions. The HHIV-2 structure exemplifies the division of conserved architectural elements of a virion, such as the capsid, from those that evolve rapidly due to selective environmental pressure such as host-recognizing structures. We propose that in viruses with two vertical single β-barrel MCPs the vesicle is indispensable, and membrane-protein interactions serve as protein-railings for guiding the assembly.

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.