X-ray crystal structure of the polymerase domain of the bacteriophage N4 virion RNA polymerase

Asymmetric structure of podophage N4 from the Schitoviridae family reveals a type of tube-sheath short-tail architecture

The tails of the majority of reported podophages are typically composed of an adaptor, a nozzle, and a needle, and flanked by six or twelve fibers. However, the Schitoviridae family, as represented by podophage N4, exhibits a different tail architecture that remains poorly understood. In this study, we employed cryoelectron microscopy (cryo-EM) to determine the atomic structures of mature and empty podophage N4 particles. The N4 tail, which is connected to the head by a portal and flanked by 12 fibers, comprises an adaptor, a 12-fold extended tail tube encircled by a 6-fold tail sheath, and a plug. The extended tail sheath is composed of two proteins, gp65 and gp64. Furthermore, we identified two distinct tail conformations in the mature podophage N4. Our structures provide insights into the mechanisms of ejection and early transcription of podophage N4, as well as for N4-like phages and CrAssphages.

Viral Genome Delivery Across Bacterial Cell Surfaces

In 1952, Hershey and Chase used bacteriophage T2 genome delivery inside Escherichia coli to demonstrate that DNA, not protein, is the genetic material. Over 70 years later, our understanding of bacteriophage structure has grown dramatically, mainly thanks to the cryogenic electron microscopy revolution. In stark contrast, phage genome delivery in prokaryotes remains poorly understood, mainly due to the inherent challenge of studying such a transient and complex process. Here, we review the current literature on viral genome delivery across bacterial cell surfaces. We focus on icosahedral bacterial viruses that we arbitrarily sort into three groups based on the presence and size of a tail apparatus. We inventory the building blocks implicated in genome delivery and critically analyze putative mechanisms of genome ejection. Bacteriophage genome delivery into bacteria is a topic of growing interest, given the renaissance of phage therapy in Western medicine as a therapeutic alternative to face the antibiotic resistance crisis.

Integrative structural analysis of Pseudomonas phage DEV reveals a genome ejection motor

DEV is an obligatory lytic Pseudomonas phage of the N4-like genus, recently reclassified as Schitoviridae. The DEV genome encodes 91 ORFs, including a 3398 amino acid virion-associated RNA polymerase (vRNAP). Here, we describe the complete architecture of DEV, determined using a combination of cryo-electron microscopy localized reconstruction, biochemical methods, and genetic knockouts. We built de novo structures of all capsid factors and tail components involved in host attachment. We demonstrate that DEV long tail fibers are essential for infection of Pseudomonas aeruginosa but dispensable for infecting mutants with a truncated lipopolysaccharide devoid of the O-antigen. We determine that DEV vRNAP is part of a three-gene operon conserved in 191 Schitoviridae genomes. We propose these three proteins are ejected into the host to form a genome ejection motor spanning the cell envelope. We posit that the design principles of the DEV ejection apparatus are conserved in all Schitoviridae.

Tailed Lytic Bacteriophages of Soft Rot Pectobacteriaceae

The study of the ecological and evolutionary traits of Soft Rot Pectobacteriaceae (SRP) comprising genera Pectobacterium and Dickeya often involves bacterial viruses (bacteriophages). Bacteriophages are considered to be a prospective tool for the ecologically safe and highly specific protection of plants and harvests from bacterial diseases. Information concerning bacteriophages has been growing rapidly in recent years, and this has included new genomics-based principles of taxonomic distribution. In this review, we summarise the data on phages infecting Pectobacterium and Dickeya that are available in publications and genomic databases. The analysis highlights not only major genomic properties that assign phages to taxonomic families and genera, but also the features that make them potentially suitable for phage control applications. Specifically, there is a discussion of the molecular mechanisms of receptor recognition by the phages and problems concerning the evolution of phage-resistant mutants.

Jumbo Phages: A Comparative Genomic Overview of Core Functions and Adaptions for Biological Conflicts

Jumbo phages have attracted much attention by virtue of their extraordinary genome size and unusual aspects of biology. By performing a comparative genomics analysis of 224 jumbo phages, we suggest an objective inclusion criterion based on genome size distributions and present a synthetic overview of their manifold adaptations across major biological systems. By means of clustering and principal component analysis of the phyletic patterns of conserved genes, all known jumbo phages can be classified into three higher-order groups, which include both myoviral and siphoviral morphologies indicating multiple independent origins from smaller predecessors. Our study uncovers several under-appreciated or unreported aspects of the DNA replication, recombination, transcription and virion maturation systems. Leveraging sensitive sequence analysis methods, we identify novel protein-modifying enzymes that might help hijack the host-machinery. Focusing on host–virus conflicts, we detect strategies used to counter different wings of the bacterial immune system, such as cyclic nucleotide- and NAD⁺-dependent effector-activation, and prevention of superinfection during pseudolysogeny. We reconstruct the RNA-repair systems of jumbo phages that counter the consequences of RNA-targeting host effectors. These findings also suggest that several jumbo phage proteins provide a snapshot of the systems found in ancient replicons preceding the last universal ancestor of cellular life.

Structure and function of virion RNA polymerase of a crAss-like phage

CrAss-like phages are a recently described expansive group of viruses that includes the most abundant virus in the human gut1-3. The genomes of all crAss-like phages encode a large virion-packaged protein2,4 that contains a DFDxD sequence motif, which forms the catalytic site in cellular multisubunit RNA polymerases (RNAPs)5. Here, using Cellulophaga baltica crAss-like phage phi14:2 as a model system, we show that this protein is a DNA-dependent RNAP that is translocated into the host cell along with the phage DNA and transcribes early phage genes. We determined the crystal structure of this 2,180-residue enzyme in a self-inhibited state, which probably occurs before virion packaging. This conformation is attained with the help of a cleft-blocking domain that interacts with the active site and occupies the cavity in which the RNA-DNA hybrid binds. Structurally, phi14:2 RNAP is most similar to eukaryotic RNAPs that are involved in RNA interference6,7, although most of the phi14:2 RNAP structure (nearly 1,600 residues) maps to a new region of the protein fold space. Considering this structural similarity, we propose that eukaryal RNA interference polymerases have their origins in phage, which parallels the emergence of the mitochondrial transcription apparatus8.

Evaluation of the genomic diversity of viruses infecting bacteria, archaea and eukaryotes using a common bioinformatic platform: steps towards a unified taxonomy

Genome Relationship Applied to Virus Taxonomy (GRAViTy) is a genetics-based tool that computes sequence relatedness between viruses. Composite generalized Jaccard (CGJ) distances combine measures of homology between encoded viral genes and similarities in genome organizational features (gene orders and orientations). This scoring framework effectively recapitulates the current, largely morphology and phenotypic-based, family-level classification of eukaryotic viruses. Eukaryotic virus families typically formed monophyletic groups with consistent CGJ distance cut-off dividing between and within family divergence ranges. In the current study, a parallel analysis of prokaryotic virus families revealed quite different sequence relationships, particularly those of tailed phage families (Siphoviridae, Myoviridae and Podoviridae), where members of the same family were generally far more divergent and often not detectably homologous to each other. Analysis of the 20 currently classified prokaryotic virus families indeed split them into 70 separate clusters of tailed phages genetically equivalent to family-level assignments of eukaryotic viruses. It further divided several bacterial (Sphaerolipoviridae, Tectiviridae) and archaeal (Lipothrixviridae) families. We also found that the subfamily-level groupings of tailed phages were generally more consistent with the family assignments of eukaryotic viruses, and this supports ongoing reclassifications, including Spounavirinae and Vi1virus taxa as new virus families. The current study applied a common benchmark with which to compare taxonomies of eukaryotic and prokaryotic viruses. The findings support the planned shift away from traditional morphology-based classifications of prokaryotic viruses towards a genome-based taxonomy. They demonstrate the feasibility of a unified taxonomy of viruses into which the vast body of metagenomic viral sequences may be consistently assigned.

Minimalism and functionality: Structural lessons from the heterodimeric N4 bacteriophage RNA polymerase II

Genomes of phages, mitochondria, and chloroplasts are transcribed by a diverse group of transcriptional machineries with structurally related single-subunit RNA polymerases (RNAPs). Our understanding of transcription mechanisms of these enzymes is predominantly based on biochemical and structural studies of three most-studied members, transcription factor-independent phage T7 RNAP, transcription factor-dependent phage N4 virion-encapsidated RNAP, and transcription factor-dependent mitochondrial RNAPs (mtRNAP). Although these RNAPs employ completely different mechanisms for promoter recognition and transcription termination, these enzymes are relatively large and formed by single polypeptides. Historically being a model enzyme for studying the mechanisms of transcription by T7-like RNAPs, however, T7 RNAP represents only a small group of RNAPs in this family. The vast majority of T7-like RNAPs are transcription factor-dependent, and several of them are heterodimeric enzymes. Here, we report X-ray crystal structures of transcription complexes of the smallest and heterodimeric form of T7-like RNAP, bacteriophage N4 RNAPII, providing insights into the structural organization of a minimum RNAP in this family. We analyze structural and functional aspects of heterodimeric architecture of N4 RNAPII concerning the mechanisms of transcription initiation and transition to processive RNA elongation. Interestingly, N4 RNAPII maintains the same conformation in promoter-bound and elongation transcription complexes, revealing a novel transcription mechanism for single-subunit RNAPs. This work establishes a structural basis for studying mechanistic aspects of transcription by factor-dependent minimum RNAP.

Polyvalent Proteins, a Pervasive Theme in the Intergenomic Biological Conflicts of Bacteriophages and Conjugative Elements

Intense biological conflicts between prokaryotic genomes and their genomic parasites have resulted in an arms race in terms of the molecular “weaponry” deployed on both sides. Using a recursive computational approach, we uncovered a remarkable class of multidomain proteins with 2 to 15 domains in the same polypeptide deployed by viruses and plasmids in such conflicts. Domain architectures and genomic contexts indicate that they are part of a widespread conflict strategy involving proteins injected into the host cell along with parasite DNA during the earliest phase of infection. Their unique feature is the combination of domains with highly disparate biochemical activities in the same polypeptide; accordingly, we term them polyvalent proteins. Of the 131 domains in polyvalent proteins, a large fraction are enzymatic domains predicted to modify proteins, target nucleic acids, alter nucleotide signaling/metabolism, and attack peptidoglycan or cytoskeletal components. They further contain nucleic acid-binding domains, virion structural domains, and 40 novel uncharacterized domains. Analysis of their architectural network reveals both pervasive common themes and specialized strategies for conjugative elements and plasmids or (pro)phages. The themes include likely processing of multidomain polypeptides by zincin-like metallopeptidases and mechanisms to counter restriction or CRISPR/Cas systems and jump-start transcription or replication. DNA-binding domains acquired by eukaryotes from such systems have been reused in XPC/RAD4-dependent DNA repair and mitochondrial genome replication in kinetoplastids. Characterization of the novel domains discovered here, such as RNases and peptidases, are likely to aid in the development of new reagents and elucidation of the spread of antibiotic resistance.

IMPORTANCE This is the first report of the widespread presence of large proteins, termed polyvalent proteins, predicted to be transmitted by genomic parasites such as conjugative elements, plasmids, and phages during the initial phase of infection along with their DNA. They are typified by the presence of multiple domains with disparate activities combined in the same protein. While some of these domains are predicted to assist the invasive element in replication, transcription, or protection of their DNA, several are likely to target various host defense systems or modify the host to favor the parasite's life cycle. Notably, DNA-binding domains from these systems have been transferred to eukaryotes, where they have been incorporated into DNA repair and mitochondrial genome replication systems.

Novel N4 Bacteriophages Prevail in the Cold Biosphere

Coliphage N4 is a lytic bacteriophage discovered nearly half a century ago, and it was considered to be a "genetic orphan" until very recently, when several additional N4-like phages were discovered to infect nonenteric bacterial hosts. Interest in this genus of phages is stimulated by their unique genetic features and propagation strategies. To better understand the ecology of N4-like phages, we investigated the diversity and geographic patterns of N4-like phages by examining 56 Chesapeake Bay viral communities, using a PCR-clone library approach targeting a diagnostic N4-like DNA polymerase gene. Many new lineages of N4-like phages were found in the bay, and their genotypes shift from the lower to the upper bay. Interestingly, signature sequences of N4-like phages were recovered only from winter month samples, when water temperatures were below 4°C. An analysis of existing metagenomic libraries from various aquatic environments supports the hypothesis that N4-like phages are most prolific in colder waters. In particular, a high number of N4-like phages were detected in Organic Lake, Antarctica, a cold and hypersaline system. The prevalence of N4-like phages in the cold biosphere suggests these viruses possess yet-to-be-determined mechanisms that facilitate lytic infections under cold conditions.

Structural and biochemical investigation of bacteriophage N4-encoded RNA polymerases

Bacteriophage N4 regulates the temporal expression of its genome through the activity of three distinct RNA polymerases (RNAP). Expression of the early genes is carried out by a phage-encoded, virion-encapsidated RNAP (vRNAP) that is injected into the host at the onset of infection and transcribes the early genes. These encode the components of new transcriptional machinery (N4 RNAPII and cofactors) responsible for the synthesis of middle RNAs. Both N4 RNAPs belong to the T7-like "single-subunit" family of polymerases. Herein, we describe their mechanisms of promoter recognition, regulation, and roles in the phage life cycle.

Automated structural comparisons clarify the phylogeny of the right-hand-shaped polymerases

Polymerases are essential for life, being responsible for replication, transcription, and the repair of nucleic acid molecules. Those that share a right-hand-shaped fold and catalytic site structurally similar to the DNA polymerase I of Escherichia coli may catalyze RNA- or DNA-dependent RNA polymerization, reverse transcription, or DNA replication in eukarya, archaea, bacteria, and their viruses. We have applied novel computational methods for structure-based clustering and phylogenetic analyses of this functionally diverse polymerase superfamily, which currently comprises six families. We identified a structural core common to all right-handed polymerases, composed of 57 amino acid residues, harboring two positionally and chemically conserved residues, the catalytic aspartates. The structural conservation within each of the six families is considerable, for example, the structural core shared by family Y DNA polymerases covers over 90% of the polymerase domain of the Sulfolobus solfataricus Dpo4. Our phylogenetic analyses propose an early separation of RNA-dependent polymerases that use primers from those that are primer-independent. Furthermore, the exchange of polymerase genes between viruses and their hosts is evident. Because of this horizontal gene transfer, the phylogeny of polymerases does not always reflect the evolutionary history of the corresponding organisms.

Clustering of Alpers disease mutations and catalytic defects in biochemical variants reveal new features of molecular mechanism of the human mitochondrial replicase, Pol γ

Mutations in Pol γ represent a major cause of human mitochondrial diseases, especially those affecting the nervous system in adults and in children. Recessive mutations in Pol γ represent nearly half of those reported to date, and they are nearly uniformly distributed along the length of the POLG1 gene (Human DNA Polymerase gamma Mutation Database); the majority of them are linked to the most severe form of POLG syndrome, Alpers–Huttenlocher syndrome. In this report, we assess the structure–function relationships for recessive disease mutations by reviewing existing biochemical data on site-directed mutagenesis of the human, Drosophila and yeast Pol γs, and their homologs from the family A DNA polymerase group. We do so in the context of a molecular model of Pol γ in complex with primer–template DNA, which we have developed based upon the recently solved crystal structure of the apoenzyme form. We present evidence that recessive mutations cluster within five distinct functional modules in the catalytic core of Pol γ. Our results suggest that cluster prediction can be used as a diagnosis-supporting tool to evaluate the pathogenic role of new Pol γ variants.

X-ray crystal structures elucidate the nucleotidyl transfer reaction of transcript initiation using two nucleotides

We have determined the X-ray crystal structures of the pre- and postcatalytic forms of the initiation complex of bacteriophage N4 RNA polymerase that provide the complete set of atomic images depicting the process of transcript initiation by a single-subunit RNA polymerase. As observed during T7 RNA polymerase transcript elongation, substrate loading for the initiation process also drives a conformational change of the O-helix, but only the correct base pairing between the +2 substrate and DNA base is able to complete the O-helix conformational transition. Substrate binding also facilitates catalytic metal binding that leads to alignment of the reactive groups of substrates for the nucleotidyl transfer reaction. Although all nucleic acid polymerases use two divalent metals for catalysis, they differ in the requirements and the timing of binding of each metal. In the case of bacteriophage RNA polymerase, we propose that catalytic metal binding is the last step before the nucleotidyl transfer reaction.

A cluster of pathogenic mutations in the 3'-5' exonuclease domain of DNA polymerase gamma defines a novel module coupling DNA synthesis and degradation

Mutations in DNA polymerase gamma (pol g), the unique replicase inside mitochondria, cause a broad and complex spectrum of diseases in human. We have used Mip1, the yeast pol g, as a model enzyme to characterize six pathogenic pol g mutations. Four mutations clustered in a highly conserved 3'-5' exonuclease module are localized in the DNA-binding channel in close vicinity to the polymerase domain. They result in an increased frequency of point mutations and high instability of the mitochondrial DNA (mtDNA) in yeast cells, and unexpectedly for mutator mutations in the exonuclease domain, they favour exonucleolysis versus polymerization. This trait is associated with highly decreased DNA-binding affinity and poorly processive DNA synthesis. Our data show for the first time that a 3'-5' exonuclease module of pol g plays a crucial role in the coordination of the polymerase and exonuclease functions and they strongly suggest that in patients the disease is not caused by defective proofreading but results from poor mtDNA replication generated by a severe imbalance between DNA synthesis and degradation.

A promoter recognition mechanism common to yeast mitochondrial and phage t7 RNA polymerases

Yeast mitochondrial (YMt) and phage T7 RNA polymerases (RNAPs) are two divergent representatives of a large family of single subunit RNAPs that are also found in the mitochondria and chloroplasts of higher eukaryotes, mammalian nuclei, and many other bacteriophage. YMt and phage T7 promoters differ greatly in sequence and length, and the YMt RNAP uses an accessory factor for initiation, whereas T7 RNAP does not. We obtain evidence here that, despite these apparent differences, both the YMt and T7 RNAPs utilize a similar promoter recognition loop to bind their respective promoters. Mutations in this element in YMt RNAP specifically disrupt mitochondrial promoter utilization, and experiments with site-specifically tethered chemical nucleases indicate that this element binds the mitochondrial promoter almost identically to how the promoter recognition loop from the phage RNAP binds its promoter. Sequence comparisons reveal that the other members of the single subunit RNAP family display loops of variable sequence and size at a position corresponding to the YMt and T7 RNAP promoter recognition loops. We speculate that these elements may be involved in promoter recognition in most or all of these enzymes and that this element's structure allows it to accommodate significant sequence and length variation to provide a mechanism for rapid evolution of new promoter specificities in this RNAP family.

Structural basis for DNA-hairpin promoter recognition by the bacteriophage N4 virion RNA polymerase

Coliphage N4 virion-encapsidated RNA polymerase (vRNAP) is a member of the phage T7-like single-subunit RNA polymerase (RNAP) family. Its central domain (mini-vRNAP) contains all RNAP functions of the full-length vRNAP, which recognizes a 5 to 7 base pair stem and 3 nucleotide loop hairpin DNA promoter. Here, we report the X-ray crystal structures of mini-vRNAP bound to promoters. Mini-vRNAP uses four structural motifs to recognize DNA sequences at the hairpin loop and stem and to unwind DNA. Despite their low sequence similarity, three out of four motifs are shared with T7 RNAP that recognizes a double-stranded DNA promoter. The binary complex structure and results of engineered disulfide linkage experiments reveal that the plug and motif B loop, which block the access of template DNA to the active site in the apo-form mini-vRNAP, undergo a large-scale conformational change upon promoter binding, explaining the restricted promoter specificity that is critical for N4 phage early transcription.

Identification of bacteriophage N4 virion RNA polymerase-nucleic acid interactions in transcription complexes

Bacteriophage N4 mini-virion RNA polymerase (mini-vRNAP), the 1106-amino acid transcriptionally active domain of vRNAP, recognizes single-stranded DNA template-containing promoters composed of conserved sequences and a 3-base loop-5-base pair stem hairpin structure. The major promoter recognition determinants are a purine located at the center of the hairpin loop (-11G) and a base at the hairpin stem (-8G). Mini-vRNAP is an evolutionarily highly diverged member of the T7 family of RNAPs. A two-plasmid system was developed to measure the in vivo activity of mutant mini-vRNAP enzymes. Five mini-vRNAP derivatives, each containing a pair of cysteine residues separated by approximately 100 amino acids and single cysteine-containing enzymes, were generated. These reagents were used to determine the smallest catalytically active polypeptide and to map promoter, substrate, and RNA-DNA hybrid contact sites to single amino acid residues in the enzyme by using end-labeled 5-iododeoxyuridine- and azidophenacyl-substituted oligonucleotides, cross-linkable derivatives of the initiating nucleotide, and RNA products with 5-iodouridine incorporated at specific positions. Localization of functionally important amino acid residues in the recently determined crystal structures of apomini-vRNAP and the mini-vRNAP-promoter complex and comparison with the crystal structures of the T7 RNAP initiation and elongation complexes allowed us to predict major rearrangements in mini-vRNAP in the transition from transcription initiation to elongation similar to those observed in T7 RNAP, a task otherwise precluded by the lack of sequence homology between N4 mini-vRNAP and T7 RNAP.