Structure, assembly, and DNA packaging of the bacteriophage T4 head

Methods for Studying Motor-Driven Viral DNA Packaging in Bacteriophages phi29, Lambda, and T4 via Single DNA Molecule Manipulation and Rapid Solution Exchange

Viral DNA packaging is a required step in the assembly of many dsDNA viruses. A molecular motor fueled by ATP hydrolysis packages the viral genome to near crystalline density inside a pre-formed prohead shell in ~5 min at room temperature in vitro. We describe procedures for measuring the packaging of single DNA molecules into single viral proheads with optical tweezers. Additionally, we describe techniques for conducting rapid solution exchange measurements that can be used to probe nucleotide-dependent motor-DNA interactions, stall and restart the motor to probe DNA conformational relaxation, and to probe the dynamics of release of the DNA, akin to ejection, in conditions where the motor fully releases its grip. Three viral packaging systems are described in detail: bacteriophages phi29 (φ29), lambda (λ), and T4. Two different approaches are described: 1. With φ29 and T4, prohead-motor complexes can be pre-assembled in bulk and packaging can be initiated in the optical tweezers by "feeding" a single DNA molecule to one of the complexes; 2. with φ29 and λ, packaging can be initiated in bulk then stalled, and a single prohead-motor-DNA complex can then be captured with optical tweezers and restarted. In both cases, the prohead is ultimately attached to one trapped microsphere and the end of the DNA being packaged is attached to a second trapped microsphere such that packaging of the DNA pulls the two microspheres together and the rate of packaging and force generated by the motor is directly measured in real time. These protocols allow for the effect of many experimental parameters on packaging dynamics to be studied such as temperature, ATP concentration, ionic conditions, structural changes to the DNA substrate, and mutations in the motor proteins. Procedures for capturing microspheres with the optical traps and different measurement modes are also described.

Cryo-EM analysis of Pseudomonas phage Pa193 structural components

The World Health Organization has designated Pseudomonas aeruginosa as a critical pathogen for the development of new antimicrobials. Bacterial viruses, or bacteriophages, have been used in various clinical settings, commonly called phage therapy, to address this growing public health crisis. Here, we describe a high-resolution structural atlas of a therapeutic, contractile-tailed Pseudomonas phage, Pa193. We used bioinformatics, proteomics, and cryogenic electron microscopy single particle analysis to identify, annotate, and build atomic models for 21 distinct structural polypeptide chains forming the icosahedral capsid, neck, contractile tail, and baseplate. We identified a putative scaffolding protein stabilizing the interior of the capsid 5-fold vertex. We also visualized a large portion of Pa193 ~ 500 Å long tail fibers and resolved the interface between the baseplate and tail fibers. The work presented here provides a framework to support a better understanding of phages as biomedicines for phage therapy and inform engineering opportunities.

A large-scale type I CBASS antiphage screen identifies the phage prohead protease as a key determinant of immune activation and evasion

Cyclic oligonucleotide-based signaling system (CBASS) is an antiviral system that protects bacteria from phage infection and is evolutionarily related to human cGAS-STING immunity. cGAS-STING signaling is initiated by the recognition of viral DNA, but the molecular cues activating CBASS are incompletely understood. Using a screen of 975 type I CBASS operon-phage challenges, we show that operons with distinct cGAS/DncV-like nucleotidyltransferases (CD-NTases) and CD-NTase-associated protein (Cap) effectors exhibit marked patterns of phage restriction. We find that some type I CD-NTase enzymes require a C-terminal AGS-C immunoglobulin (Ig)-like fold domain for defense against select phages. Escaper phages evade CBASS via protein-coding mutations in virion assembly proteins, and acquired resistance is largely operon specific. We demonstrate that the phage Bas13 prohead protease interacts with the CD-NTase EcCdnD12 and can induce CBASS-dependent growth arrest in cells. Our results define phage virion assembly as a determinant of type I CBASS immune evasion and support viral protein recognition as a putative mechanism of cGAS-like enzyme activation.

Phage-based delivery systems: engineering, applications, and challenges in nanomedicines

Bacteriophages (phages) represent a unique category of viruses with a remarkable ability to selectively infect host bacteria, characterized by their assembly from proteins and nucleic acids. Leveraging their exceptional biological properties and modifiable characteristics, phages emerge as innovative, safe, and efficient delivery vectors. The potential drawbacks associated with conventional nanocarriers in the realms of drug and gene delivery include a lack of cell-specific targeting, cytotoxicity, and diminished in vivo transfection efficiency. In contrast, engineered phages, when employed as cargo delivery vectors, hold the promise to surmount these limitations and attain enhanced delivery efficacy. This review comprehensively outlines current strategies for the engineering of phages, delineates the principal types of phages utilized as nanocarriers in drug and gene delivery, and explores the application of phage-based delivery systems in disease therapy. Additionally, an incisive analysis is provided, critically examining the challenges confronted by phage-based delivery systems within the domain of nanotechnology. The primary objective of this article is to furnish a theoretical reference that contributes to the reasoned design and development of potent phage-based delivery systems.

Mutagenesis and functional analysis of the varicella-zoster virus portal protein

Herpesviruses replicate by cleaving concatemeric dsDNA into single genomic units that are packaged through an oligomeric portal present in preformed procapsids. In contrast to what is known about phage portal proteins, details concerning herpesvirus portal structure and function are not as well understood. A panel of 65 Varicella-Zoster virus (VZV) recombinant portal proteins with five amino acid in-frame insertions were generated by random transposon mutagenesis of the VZV portal gene, ORF54. Subsequently, 65 VZVLUC recombinant viruses (TNs) were generated via recombineering. Insertions were mapped to predicted portal domains (clip, wing, stem, wall, crown, and β-hairpin tunnel-loop) and recombinant viruses were characterized for plaque morphology, replication kinetics, pORF54 expression, and classified based on replication in non-complementing (ARPE19) or complementing (ARPE54C50) cell lines. The N- and C-termini were tolerant to insertion mutagenesis, as were certain clip sub-domains. The majority of mutants mapping to the wing, wall, β-hairpin tunnel loop, and stem domains were lethal. Elimination of the predicted ORF54 start codon revealed that the first 40 amino acids of the N-terminus were not required for viral replication. Stop codon insertions in the C-terminus showed that the last 100 amino acids were not required for viral replication. Lastly, a putative protease cleavage site was identified in the C-terminus of pORF54. Cleavage was likely orchestrated by a viral protease; however, processing was not required for DNA encapsidation and viral replication. The panel of recombinants should prove valuable in future studies to dissect mammalian portal structure and function.IMPORTANCEThough nucleoside analogs and a live-attenuated vaccine are currently available to treat some human herpesvirus family members, alternate methods of combating herpesvirus infection could include blocking viral replication at the DNA encapsidation stage. The approval of Letermovir provided proof of concept regarding the use of encapsidation inhibitors to treat herpesvirus infections in the clinic. We propose that small-molecule compounds could be employed to interrupt portal oligomerization, assembly into the capsid vertex, or affect portal function/dynamics. Targeting portal at any of these steps would result in disruption of viral DNA packaging and a decrease or absence of mature infectious herpesvirus particles. The oligomeric portals of herpesviruses are structurally conserved, and therefore, it may be possible to find a single compound capable of targeting portals from one or more of the herpesvirus subfamilies. Drug candidates from such a series would be effective against viruses resistant to the currently available antivirals.

Cryo-EM analysis of Pseudomonas phage Pa193 structural components

The World Health Organization has designated Pseudomonas aeruginosa as a critical pathogen for the development of new antimicrobials. Bacterial viruses, or bacteriophages, have been used in various clinical settings, commonly called phage therapy, to address this growing public health crisis. Here, we describe a high-resolution structural atlas of a therapeutic, contractile-tailed Pseudomonas phage, Pa193. We used bioinformatics, proteomics, and cryogenic electron microscopy single particle analysis to identify, annotate, and build atomic models for 21 distinct structural polypeptide chains forming the icosahedral capsid, neck, contractile tail, and baseplate. We identified a putative scaffolding protein stabilizing the interior of the capsid 5-fold vertex. We also visualized a large portion of Pa193 ~ 500 Å long tail fibers and resolved the interface between the baseplate and tail fibers. The work presented here provides a framework to support a better understanding of phages as biomedicines for phage therapy and inform engineering opportunities.

Mechanism of Viral DNA Packaging in Phage T4 Using Single-Molecule Fluorescence Approaches

In all tailed phages, the packaging of the double-stranded genome into the head by a terminase motor complex is an essential step in virion formation. Despite extensive research, there are still major gaps in the understanding of this highly dynamic process and the mechanisms responsible for DNA translocation. Over the last fifteen years, single-molecule fluorescence technologies have been applied to study viral nucleic acid packaging using the robust and flexible T4 in vitro packaging system in conjunction with genetic, biochemical, and structural analyses. In this review, we discuss the novel findings from these studies, including that the T4 genome was determined to be packaged as an elongated loop via the colocalization of dye-labeled DNA termini above the portal structure. Packaging efficiency of the TerL motor was shown to be inherently linked to substrate structure, with packaging stalling at DNA branches. The latter led to the design of multiple experiments whose results all support a proposed torsional compression translocation model to explain substrate packaging. Evidence of substrate compression was derived from FRET and/or smFRET measurements of stalled versus resolvase released dye-labeled Y-DNAs and other dye-labeled substrates relative to motor components. Additionally, active in vivo T4 TerS fluorescent fusion proteins facilitated the application of advanced super-resolution optical microscopy toward the visualization of the initiation of packaging. The formation of twin TerS ring complexes, each expected to be ~15 nm in diameter, supports a double protein ring-DNA synapsis model for the control of packaging initiation, a model that may help explain the variety of ring structures reported among pac site phages. The examination of the dynamics of the T4 packaging motor at the single-molecule level in these studies demonstrates the value of state-of-the-art fluorescent tools for future studies of complex viral replication mechanisms.

Neck and capsid architecture of the robust Agrobacterium phage Milano

Large gaps exist in our understanding of how bacteriophages, the most abundant biological entities on Earth, assemble and function. The structure of the "neck" region, where the DNA-filled capsid is connected to the host-recognizing tail remains poorly understood. We describe cryo-EM structures of the neck, the neck-capsid and neck-tail junctions, and capsid of the Agrobacterium phage Milano. The Milano neck 1 protein connects the 12-fold symmetrical neck to a 5-fold vertex of the icosahedral capsid. Comparison of Milano neck 1 homologs leads to four proposed classes, likely evolved from the simplest one in siphophages to more complex ones in myo- and podophages. Milano neck is surrounded by the atypical collar, which covalently crosslinks the tail sheath to neck 1. The Milano capsid is decorated with three types of proteins, a minor capsid protein (mCP) and two linking proteins crosslinking the mCP to the major capsid protein. The extensive network of disulfide bonds within and between neck, collar, capsid and tail provides an exceptional structural stability to Milano.

High-resolution cryo-EM structure of the Pseudomonas bacteriophage E217

E217 is a Pseudomonas phage used in an experimental cocktail to eradicate cystic fibrosis-associated Pseudomonas aeruginosa. Here, we describe the structure of the whole E217 virion before and after DNA ejection at 3.1 Å and 4.5 Å resolution, respectively, determined using cryogenic electron microscopy (cryo-EM). We identify and build de novo structures for 19 unique E217 gene products, resolve the tail genome-ejection machine in both extended and contracted states, and decipher the complete architecture of the baseplate formed by 66 polypeptide chains. We also determine that E217 recognizes the host O-antigen as a receptor, and we resolve the N-terminal portion of the O-antigen-binding tail fiber. We propose that E217 design principles presented in this paper are conserved across PB1-like Myoviridae phages of the Pbunavirus genus that encode a ~1.4 MDa baseplate, dramatically smaller than the coliphage T4.

Structure and Function of Hoc-A Novel Environment Sensing Device Encoded by T4 and Other Bacteriophages

Bacteriophage T4 is decorated with 155 180 Å-long fibers of the highly antigenic outer capsid protein (Hoc). In this study, we describe a near-atomic structural model of Hoc by combining cryo-electron microscopy and AlphaFold structure predictions. It consists of a conserved C-terminal capsid-binding domain attached to a string of three variable immunoglobulin (Ig)-like domains, an architecture well-preserved in hundreds of Hoc molecules found in phage genomes. Each T4-Hoc fiber attaches randomly to the center of gp23* hexameric capsomers in one of the six possible orientations, though at the vertex-proximal hexamers that deviate from 6-fold symmetry, Hoc binds in two preferred orientations related by 180° rotation. Remarkably, each Hoc fiber binds to all six subunits of the capsomer, though the interactions are greatest with three of the subunits, resulting in the off-centered attachment of the C-domain. Biochemical analyses suggest that the acidic Hoc fiber (pI, ~4-5) allows for the clustering of virions in acidic pH and dispersion in neutral/alkaline pH. Hoc appears to have evolved as a sensing device that allows the phage to navigate its movements through reversible clustering-dispersion transitions so that it reaches its destination, the host bacterium, and persists in various ecological niches such as the human/mammalian gut.

Revolving hexameric ATPases as asymmetric motors to translocate double-stranded DNA genome along one strand

DsDNA translocation through nanoscale pores is generally accomplished by ATPase biomotors. The discovery of the revolving dsDNA translocation mechanism, as opposed to rotation, in bacteriophage phi29 elucidated how ATPase motors move dsDNA. Revolution-driven, hexameric dsDNA motors have been reported in herpesvirus, bacterial FtsK, Streptomyces TraB, and T7 phage. This review explores the common relationship between their structure and mechanisms. Commonalities include moving along the 5'→3' strand, inchworm sequential action leading to an asymmetrical structure, channel chirality, channel size, and 3-step channel gating for controlling motion direction. The revolving mechanism and contact with one of the dsDNA strands addresses the historic controversy of dsDNA packaging using nicked, gapped, hybrid, or chemically modified DNA. These controversies surrounding dsDNA packaging activity using modified materials can be answered by whether the modification was introduced into the 3'→5' or 5'→3' strand. Perspectives concerning solutions to the controversy of motor structure and stoichiometry are also discussed.

Convergent mutations in phage virion assembly proteins enable evasion of Type I CBASS immunity

CBASS is an anti-phage defense system that protects bacteria from phage infection and is evolutionarily related to human cGAS-STING immunity. cGAS-STING signaling is initiated by viral DNA but the stage of phage replication which activates bacterial CBASS remains unclear. Here we define the specificity of Type I CBASS immunity using a comprehensive analysis of 975 operon-phage pairings and show that Type I CBASS operons composed of distinct CD-NTases, and Cap effectors exhibit striking patterns of defense against dsDNA phages across five diverse viral families. We demonstrate that escaper phages evade CBASS immunity by acquiring mutations in structural genes encoding the prohead protease, capsid, and tail fiber proteins. Acquired CBASS resistance is highly operon-specific and typically does not affect overall fitness. However, we observe that some resistance mutations drastically alter phage infection kinetics. Our results define late-stage virus assembly as a critical determinant of CBASS immune activation and evasion by phages.

Duck plague virus pUL15 performs a nonspecial cleavage activity through its C terminal nuclease domain in vitro

Duck plague virus (DPV), also known as anatid herpesvirus, is a double-stranded DNA virus and a member of α herpesvirus. DPV pUL15 is a homolog of herpes simplex virus 1 (HSV-1) pUL15, a terminase large subunit, and plays a key role in the cleavage and packaging of the viral concatemeric genome. However, the sequence similarity between DPV pUL15 and its homologs is low, and it is not sure if DPV pUL15 has the potential to cleave the concatemeric genome as same as its homologs. Here, we expressed the C terminal domain of DPV pUL15 to explore the nuclease function of DPV pUL15. The main results showed that (Ⅰ) DPV pUL15 C-terminal domain possesses nonspecific nuclease activity and lacks the DNA binding ability. (Ⅱ) DPV pUL15 nuclease activity needs to coordinate with divalent metal ions and tends to be more active at high temperatures. (Ⅲ) Even though the structure of DPV pUL15 nuclease domain is relatively conserved, the mutations of conserved amino acids on the nuclease domain do not significantly inhibit the nuclease activity.

Bacteriophage T4 Head: Structure, Assembly, and Genome Packaging

Bacteriophage (phage) T4 has served as an extraordinary model to elucidate biological structures and mechanisms. Recent discoveries on the T4 head (capsid) structure, portal vertex, and genome packaging add a significant body of new literature to phage biology. Head structures in unexpanded and expanded conformations show dramatic domain movements, structural remodeling, and a ~70% increase in inner volume while creating high-affinity binding sites for the outer decoration proteins Soc and Hoc. Small changes in intercapsomer interactions modulate angles between capsomer planes, leading to profound alterations in head length. The in situ cryo-EM structure of the symmetry-mismatched portal vertex shows the remarkable structural morphing of local regions of the portal protein, allowing similar interactions with the capsid protein in different structural environments. Conformational changes in these interactions trigger the structural remodeling of capsid protein subunits surrounding the portal vertex, which propagate as a wave of expansion throughout the capsid. A second symmetry mismatch is created when a pentameric packaging motor assembles at the outer "clip" domains of the dodecameric portal vertex. The single-molecule dynamics of the packaging machine suggests a continuous burst mechanism in which the motor subunits adjusted to the shape of the DNA fire ATP hydrolysis, generating speeds as high as 2000 bp/s.

Assembly and Capsid Expansion Mechanism of Bacteriophage P22 Revealed by High-Resolution Cryo-EM Structures

The formation of many double-stranded DNA viruses, such as herpesviruses and bacteriophages, begins with the scaffolding-protein-mediated assembly of the procapsid. Subsequently, the procapsid undergoes extensive structural rearrangement and expansion to become the mature capsid. Bacteriophage P22 is an established model system used to study virus maturation. Here, we report the cryo-electron microscopy structures of procapsid, empty procapsid, empty mature capsid, and mature capsid of phage P22 at resolutions of 2.6 Å, 3.9 Å, 2.8 Å, and 3.0 Å, respectively. The structure of the procapsid allowed us to build an accurate model of the coat protein gp5 and the C-terminal region of the scaffolding protein gp8. In addition, interactions among the gp5 subunits responsible for procapsid assembly and stabilization were identified. Two C-terminal α-helices of gp8 were observed to interact with the coat protein in the procapsid. The amino acid interactions between gp5 and gp8 in the procapsid were consistent with the results of previous biochemical studies involving mutant proteins. Our structures reveal hydrogen bonds and salt bridges between the gp5 subunits in the procapsid and the conformational changes of the gp5 domains involved in the closure of the local sixfold opening and a thinner capsid shell during capsid maturation.

Structures of a large prolate virus capsid in unexpanded and expanded states generate insights into the icosahedral virus assembly

Many icosahedral viruses assemble proteinaceous precursors called proheads or procapsids. Proheads are metastable structures that undergo a profound structural transition known as expansion that transforms an immature unexpanded head into a mature genome-packaging head. Bacteriophage T4 is a model virus, well studied genetically and biochemically, but its structure determination has been challenging because of its large size and unusually prolate-shaped, ∼1,200-Å-long and ∼860-Å-wide capsid. Here, we report the cryogenic electron microscopy (cryo-EM) structures of T4 capsid in both of its major conformational states: unexpanded at a resolution of 5.1 Å and expanded at a resolution of 3.4 Å. These are among the largest structures deposited in Protein Data Bank to date and provide insights into virus assembly, head length determination, and shell expansion. First, the structures illustrate major domain movements and ∼70% additional gain in inner capsid volume, an essential transformation to contain the entire viral genome. Second, intricate intracapsomer interactions involving a unique insertion domain dramatically change, allowing the capsid subunits to rotate and twist while the capsomers remain fastened at quasi-threefold axes. Third, high-affinity binding sites emerge for a capsid decoration protein that clamps adjacent capsomers, imparting extraordinary structural stability. Fourth, subtle conformational changes at capsomers' periphery modulate intercapsomer angles between capsomer planes that control capsid length. Finally, conformational changes were observed at the symmetry-mismatched portal vertex, which might be involved in triggering head expansion. These analyses illustrate how small changes in local capsid subunit interactions lead to profound shifts in viral capsid morphology, stability, and volume.

Bacteriophage T4 as a nanovehicle for delivery of genes and therapeutics into human cells

The ability to deliver therapeutic genes and biomolecules into a human cell and restore a defective function has been the holy grail of medicine. Adeno-associated viruses and lentiviruses have been extensively used as delivery vehicles, but their capacity is limited to one (or two) gene(s). Bacteriophages are emerging as novel vehicles for gene therapy. The large 120 × 86-nm T4 capsid allows engineering of both its surface and its interior to incorporate combinations of DNAs, RNAs, proteins, and their complexes. In vitro assembly using purified components allows customization for various applications and for individualized therapies. Its large capacity, cell-targeting capability, safety, and inexpensive manufacturing could open unprecedented new possibilities for gene, cancer, and stem cell therapies. However, efficient entry into primary human cells and intracellular trafficking are significant barriers that must be overcome by gene engineering and evolution in order to translate phage-delivery technology from bench to bedside.

Isolation and Characterization of a Novel Cyanophage Encoding Multiple Auxiliary Metabolic Genes

As significant drivers of cyanobacteria mortality, cyanophages have been known to regulate the population dynamics, metabolic activities, and community structure of this most important marine autotrophic picoplankton and, therefore, influence the global primary production and biogeochemical cycle in aquatic ecosystems. In the present study, a lytic Synechococcus phage, namely S-SZBM1, was isolated and identified. Cyanophage S-SZBM1 has a double-stranded DNA genome of 177,834 bp with a G+C content of 43.31% and contains a total of 218 predicted ORFs and six tRNA genes. Phylogenetic analysis and nucleotide-based intergenomic similarity suggested that cyanophage S-SZBM1 belongs to a new genus under the family Kyanoviridae. A variety of auxiliary metabolic genes (AMGs) that have been proved or speculated to relate to photosynthesis, carbon metabolism, nucleotide synthesis and metabolism, cell protection, and other cell metabolism were identified in cyanophage S-SZBM1 genome and may affect host processes during infection. In addition, 24 of 32 predicted structural proteins were identified by a high-throughput proteome analysis which were potentially involved in the assembly processes of virion. The genomic and proteomic analysis features of cyanophage S-SZBM1 offer a valuable insight into the interactions between cyanophages and their hosts during infection.

Hysteresis in Hepatitis B Virus (HBV) Requires Assembly of Near-Perfect Capsids

The hepatitis B virus (HBV) must release its contents to initiate infection, making capsid disassembly critical to the viral life cycle. Capsid assembly proceeds through a cascade of weak interactions between copies of capsid protein (Cp) to yield uniform particles. However, there is a hysteresis to capsid dissociation that allows capsids to persist under conditions where they could not assemble. In this study, we have sought to define the basis of hysteresis by examining urea-induced dissociation of in vitro-assembled HBV capsids. In general, capsid samples show a mixture of two pools, differentiated by stability. Labile capsid dissociation corresponds to an ∼5 μM pseudocritical concentration of assembly (pcc), the same as that observed in assembly reactions. Dissociation of the stable pool corresponds to a subfemtomolar pcc, indicative of hysteresis. The fraction of stable capsids in an assembly reaction increases with the integrity of the Cp preparation and when association is performed at a higher ionic strength, which modifies the Cp conformation. Labile complexes are more prevalent when assembly conditions yield many kinetically trapped (incomplete and overgrown) products. Cp isolated from stable capsids reassembles into a mixture of stable and labile capsids. These results suggest that hysteresis arises from an ideal capsid lattice, even when some of the substituents in that lattice have defects. Consistent with structural studies that show a subtle difference between Cp dimers and Cp in capsid, we propose that hysteresis arises when HBV capsids undergo a lattice-dependent structural transition.

The Beauty of Bacteriophage T4 Research: Lindsay W. Black and the T4 Head Assembly

Viruses are biochemically complex structures and mainly consist of folded proteins that contain nucleic acids. Bacteriophage T4 is one of most prominent examples, having a tail structure that contracts during the infection process. Intracellular phage multiplication leads to separate self-directed assembly reactions of proheads, tails and tail fibers. The proheads are packaged with concatemeric DNA produced by tandem replication reactions of the parental DNA molecule. Once DNA packaging is completed, the head is joined with the tail and six long fibers are attached. The mature particles are then released from the cell via lysis, another tightly regulated process. These processes have been studied in molecular detail leading to a fascinating view of the protein-folding dynamics that direct the structural interplay of assembled complexes. Lindsay W. Black dedicated his career to identifying and defining the molecular events required to form the T4 virion. He leaves us with rich insights into the astonishingly precise molecular clockwork that co-ordinates all of the players in T4 assembly, both viral and cellular. Here, we summarize Lindsay’s key research contributions that are certain to stimulate our future science for many years to come.

Effects of the Newly Isolated T4-like Phage on Transmission of Plasmid-Borne Antibiotic Resistance Genes via Generalized Transduction

Bacteriophages are the most abundant biological entities on earth and may play an important role in the transmission of antibiotic resistance genes (ARG) from host bacteria. Although the specialized transduction mediated by the temperate phage targeting a specific insertion site is widely explored, the carrying characteristics of “transducing particles” for different ARG subtypes in the process of generalized transduction remains largely unclear. Here, we isolated a new T4-like lytic phage targeting transconjugant Escherichia coli C600 that contained plasmid pHNAH67 (KX246266) and encoded 11 different ARG subtypes. We found that phage AH67C600_Q9 can misload plasmid-borne ARGs and package host DNA randomly. Moreover, for any specific ARG subtype, the carrying frequency was negatively correlated with the multiplicity of infection (MOI). Further, whole genome sequencing (WGS) identified that only 0.338% (4/1183) of the contigs of an entire purified phage population contained ARG sequences; these were floR, sul2, aph(4)-Ia, and fosA. The low coverage indicated that long-read sequencing methods are needed to explore the mechanism of ARG transmission during generalized transduction.

The remarkable viral portal vertex: structure and a plausible model for mechanism

Many icosahedral viruses including tailed bacteriophages and herpes viruses have a unique portal vertex where a dodecameric protein ring is associated with a fivefold capsid shell. While the peripheral regions of the portal ring are involved in capsid assembly, its central channel is used to transport DNA into and out of capsid during genome packaging and infection. Though the atomic structure of this highly conserved, turbine-shaped, portal is known for nearly two decades, its molecular mechanism remains a mystery. Recent high-resolution in situ structures reveal various conformational states of the portal and the asymmetric interactions between the 12-fold portal and the fivefold capsid. These lead to a valve-like mechanism for this symmetry-mismatched portal vertex that regulates DNA flow through the channel, a critical function for high fidelity assembly of an infectious virion.

Bacteriophage T4 Escapes CRISPR Attack by Minihomology Recombination and Repair

Bacteria and bacteriophages (phages) have evolved potent defense and counterdefense mechanisms that allowed their survival and greatest abundance on Earth. CRISPR (clustered regularly interspaced short palindromic repeat)-Cas (CRISPR-associated) is a bacterial defense system that inactivates the invading phage genome by introducing double-strand breaks at targeted sequences. While the mechanisms of CRISPR defense have been extensively investigated, the counterdefense mechanisms employed by phages are poorly understood. Here, we report a novel counterdefense mechanism by which phage T4 restores the genomes broken by CRISPR cleavages. Catalyzed by the phage-encoded recombinase UvsX, this mechanism pairs very short stretches of sequence identity (minihomology sites), as few as 3 or 4 nucleotides in the flanking regions of the cleaved site, allowing replication, repair, and stitching of genomic fragments. Consequently, a series of deletions are created at the targeted site, making the progeny genomes completely resistant to CRISPR attack. Our results demonstrate that this is a general mechanism operating against both type II (Cas9) and type V (Cas12a) CRISPR-Cas systems. These studies uncovered a new type of counterdefense mechanism evolved by T4 phage where subtle functional tuning of preexisting DNA metabolism leads to profound impact on phage survival. IMPORTANCE Bacteriophages (phages) are viruses that infect bacteria and use them as replication factories to assemble progeny phages. Bacteria have evolved powerful defense mechanisms to destroy the invading phages by severing their genomes soon after entry into cells. We discovered a counterdefense mechanism evolved by phage T4 to stitch back the broken genomes and restore viral infection. In this process, a small amount of genetic material is deleted or another mutation is introduced, making the phage resistant to future bacterial attack. The mutant virus might also gain survival advantages against other restriction conditions or DNA damaging events. Thus, bacterial attack not only triggers counterdefenses but also provides opportunities to generate more fit phages. Such defense and counterdefense mechanisms over the millennia led to the extraordinary diversity and the greatest abundance of bacteriophages on Earth. Understanding these mechanisms will open new avenues for engineering recombinant phages for biomedical applications.

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.

The T4 TerL Prohead Packaging Motor Does Not Drive DNA Translocation by a Proposed Dehydration Mechanism

A "DNA crunching" linear motor mechanism that employs a grip-and-release transient spring like compression of B- to A-form DNA has been found in our previous studies. Our FRET measurements in vitro show a decrease in distance from TerL to portal during packaging; furthermore, there is a decrease in distance between closely positioned dye pairs in the Y-stem of translocating Y-DNA that conforms to B- and A- structure. In normal translocation into the prohead the TerL motor expels all B-form tightly binding YOYO-1 dye that cannot bind A-form. The TerL motor cannot package A-form dsRNA. Our work reported here shows that addition of helper B form DNA:DNA (D:D) 20mers allows increased packaging of heteroduplex A-form DNA:RNA 20mers (D:R), evidence for a B- to A-form spring motor pushing duplex nucleic acid. A-form DNA:RNA 25mers, 30mers, and 35mers alone are efficiently packaged into proheads by the TerL motor showing that a proposed hypothetical dehydration motor mechanism operating on duplex substrates does not provide the packaging motor force. Taken together with our previous studies showing TerL motor protein motion toward the portal during DNA packaging, our present studies of short D:D and D:R duplex nucleic acid substrates strongly supports our previous evidence that the protein motor pushes rather than pulls or dehydrates duplex substrates to provide the translocation into prohead packaging force.

Gene Transfer Agents in Symbiotic Microbes

Prokaryotes commonly undergo genome reduction, particularly in the case of symbiotic bacteria. Genome reductions tend toward the energetically favorable removal of unnecessary, redundant, or nonfunctional genes. However, without mechanisms to compensate for these losses, deleterious mutation and genetic drift might otherwise overwhelm a population. Among the mechanisms employed to counter gene loss and share evolutionary success within a population, gene transfer agents (GTAs) are increasingly becoming recognized as important contributors. Although viral in origin, GTA particles package fragments of their "host" genome for distribution within a population of cells, often in a synchronized manner, rather than selfishly packaging genes necessary for their spread. Microbes as diverse as archaea and alpha-proteobacteria have been known to produce GTA particles, which are capable of transferring selective advantages such as virulence factors and antibiotic resistance. In this review, we discuss the various types of GTAs identified thus far, focusing on a defined set of symbiotic alpha-proteobacteria known to carry them. Drawing attention to the predicted presence of these genes, we discuss their potential within the selective marine and terrestrial environments occupied by mutualistic, parasitic, and endosymbiotic microbes.

Portal Protein: The Orchestrator of Capsid Assembly for the dsDNA Tailed Bacteriophages and Herpesviruses

Tailed, double-stranded DNA bacteriophages provide a well-characterized model system for the study of viral assembly, especially for herpesviruses and adenoviruses. A wealth of genetic, structural, and biochemical work has allowed for the development of assembly models and an understanding of the DNA packaging process. The portal complex is an essential player in all aspects of bacteriophage and herpesvirus assembly. Despite having low sequence similarity, portal structures across bacteriophages share the portal fold and maintain a conserved function. Due to their dynamic role, portal proteins are surprisingly plastic, and their conformations change for each stage of assembly. Because the maturation process is dependent on the portal protein, researchers have been working to validate this protein as a potential antiviral drug target. Here we review recent work on the role of portal complexes in capsid assembly, including DNA packaging, as well as portal ring assembly and incorporation and analysis of portal structures.

Resistance in marine cyanobacteria differs against specialist and generalist cyanophages

Long-term coexistence between unicellular cyanobacteria and their lytic viruses (cyanophages) in the oceans is thought to be due to the presence of sensitive cells in which cyanophages reproduce, ultimately killing the cell, while other cyanobacteria survive due to resistance to infection. Here, we investigated resistance in marine cyanobacteria from the genera Synechococcus and Prochlorococcus and compared modes of resistance against specialist and generalist cyanophages belonging to the T7-like and T4-like cyanophage families. Resistance was extracellular in most interactions against specialist cyanophages irrespective of the phage family, preventing entry into the cell. In contrast, resistance was intracellular in practically all interactions against generalist T4-like cyanophages. The stage of intracellular arrest was interaction-specific, halting at various stages of the infection cycle. Incomplete infection cycles proceeded to various degrees of phage genome transcription and translation as well as phage genome replication in numerous interactions. In a particularly intriguing case, intracellular capsid assembly was observed, but the phage genome was not packaged. The cyanobacteria survived the encounter despite late-stage infection and partial genome degradation. We hypothesize that this is tolerated due to genome polyploidy, which we found for certain strains of both Synechococcus and Prochlorococcus Our findings unveil a heavy cost of promiscuous entry of generalist phages into nonhost cells that is rarely paid by specialist phages and suggests the presence of unknown mechanisms of intracellular resistance in the marine unicellular cyanobacteria. Furthermore, these findings indicate that the range for virus-mediated horizontal gene transfer extends beyond hosts to nonhost cyanobacterial cells.

Bacteriophage T4 nanoparticles for vaccine delivery against infectious diseases

Subunit vaccines containing one or more target antigens from pathogenic organisms represent safer alternatives to whole pathogen vaccines. However, the antigens by themselves are not sufficiently immunogenic and require additives known as adjuvants to enhance immunogenicity and protective efficacy. Assembly of the antigens into virus-like nanoparticles (VLPs) is a better approach as it allows presentation of the epitopes in a more native context. The repetitive, symmetrical, and high density display of antigens on the VLPs mimic pathogen-associated molecular patterns seen on bacteria and viruses. The antigens, thus, might be better presented to stimulate host's innate as well as adaptive immune systems thereby eliciting both humoral and cellular immune responses. Bacteriophages such as phage T4 provide excellent platforms to generate the nanoparticle vaccines. The T4 capsid containing two non-essential outer proteins Soc and Hoc allow high density array of antigen epitopes in the form of peptides, domains, full-length proteins, or even multi-subunit complexes. Co-delivery of DNAs, targeting molecules, and/or molecular adjuvants provides additional advantages. Recent studies demonstrate that the phage T4 VLPs are highly immunogenic, do not need an adjuvant, and provide complete protection against bacterial and viral pathogens. Thus, phage T4 could potentially be developed as a "universal" VLP platform to design future multivalent vaccines against complex and emerging pathogens.

DNA Conformational Changes Play a Force-Generating Role during Bacteriophage Genome Packaging

Motors that move DNA, or that move along DNA, play essential roles in DNA replication, transcription, recombination, and chromosome segregation. The mechanisms by which these DNA translocases operate remain largely unknown. Some double-stranded DNA (dsDNA) viruses use an ATP-dependent motor to drive DNA into preformed capsids. These include several human pathogens as well as dsDNA bacteriophages-viruses that infect bacteria. We previously proposed that DNA is not a passive substrate of bacteriophage packaging motors but is instead an active component of the machinery. We carried out computational studies on dsDNA in the channels of viral portal proteins, and they reveal DNA conformational changes consistent with that hypothesis. dsDNA becomes longer ("stretched") in regions of high negative electrostatic potential and shorter ("scrunched") in regions of high positive potential. These results suggest a mechanism that electrostatically couples the energy released by ATP hydrolysis to DNA translocation: The chemical cycle of ATP binding, hydrolysis, and product release drives a cycle of protein conformational changes. This produces changes in the electrostatic potential in the channel through the portal, and these drive cyclic changes in the length of dsDNA as the phosphate groups respond to the protein's electrostatic potential. The DNA motions are captured by a coordinated protein-DNA grip-and-release cycle to produce DNA translocation. In short, the ATPase, portal, and dsDNA work synergistically to promote genome packaging.

Bacteriophage T4 capsid as a nanocarrier for Peptide-N-Glycosidase F immobilization through self-assembly

Enzyme immobilization is widely applied in biocatalysis to improve stability and facilitate recovery and reuse of enzymes. However, high cost of supporting materials and laborious immobilization procedures has limited its industrial application and commercialization. In this study, we report a novel self-assembly immobilization system using bacteriophage T4 capsid as a nanocarrier. The system utilizes the binding sites of the small outer capsid protein, Soc, on the T4 capsid. Enzymes as Soc fusions constructed with regular molecular cloning technology expressed at the appropriate time during phage assembly and self-assembled onto the capsids. The proof of principle experiment was carried out by immobilizing β-galactosidase, and the system was successfully applied to the immobilization of an important glycomics enzyme, Peptide-N-Glycosidase F. Production of Peptide-N-Glycosidase F and simultaneous immobilization was finished within seven hours. Characterizations of the immobilized Peptide-N-Glycosidase F indicated high retention of activity and well reserved deglycosylation capacity. The immobilized Peptide-N-Glycosidase F was easily recycled by centrifugation and exhibited good stability that sustained five repeated uses. This novel system uses the self-amplified T4 capsid as the nanoparticle-type of supporting material, and operates with a self-assembly procedure, making it a simple and low-cost enzyme immobilization technology with promising application potentials.

Terminase Large Subunit Provides a New Drug Target for Herpesvirus Treatment

Herpesvirus infection is an orderly, regulated process. Among these viruses, the encapsidation of viral DNA is a noteworthy link; the entire process requires a powered motor that binds to viral DNA and carries it into the preformed capsid. Studies have shown that this power motor is a complex composed of a large subunit, a small subunit, and a third subunit, which are collectively known as terminase. The terminase large subunit is highly conserved in herpesvirus. It mainly includes two domains: the C-terminal nuclease domain, which cuts the viral concatemeric DNA into a monomeric genome, and the N-terminal ATPase domain, which hydrolyzes ATP to provide energy for the genome cutting and transfer activities. Because this process is not present in eukaryotic cells, it provides a reliable theoretical basis for the development of safe and effective anti-herpesvirus drugs. This article reviews the genetic characteristics, protein structure, and function of the herpesvirus terminase large subunit, as well as the antiviral drugs that target the terminase large subunit. We hope to provide a theoretical basis for the prevention and treatment of herpesvirus.

Packaging of Dinoroseobacter shibae DNA into Gene Transfer Agent Particles Is Not Random

Gene transfer agents (GTAs) are phage-like particles which contain a fragment of genomic DNA of the bacterial or archaeal producer and deliver this to a recipient cell. GTA gene clusters are present in the genomes of almost all marine Rhodobacteraceae (Roseobacters) and might be important contributors to horizontal gene transfer in the world’s oceans. For all organisms studied so far, no obvious evidence of sequence specificity or other nonrandom process responsible for packaging genomic DNA into GTAs has been found. Here, we show that knock-out of an autoinducer synthase gene of Dinoroseobacter shibae resulted in overproduction and release of functional GTA particles (DsGTA). Next-generation sequencing of the 4.2-kb DNA fragments isolated from DsGTAs revealed that packaging was not random. DNA from low-GC conjugative plasmids but not from high-GC chromids was excluded from packaging. Seven chromosomal regions were strongly overrepresented in DNA isolated from DsGTA. These packaging peaks lacked identifiable conserved sequence motifs that might represent recognition sites for the GTA terminase complex. Low-GC regions of the chromosome, including the origin and terminus of replication, were underrepresented in DNA isolated from DsGTAs. DNA methylation reduced packaging frequency while the level of gene expression had no influence. Chromosomal regions found to be over- and underrepresented in DsGTA-DNA were regularly spaced. We propose that a “headful” type of packaging is initiated at the sites of coverage peaks and, after linearization of the chromosomal DNA, proceeds in both directions from the initiation site. GC-content, DNA-modifications, and chromatin structure might influence at which sides GTA packaging can be initiated.

Human herpesvirus portal proteins: Structure, function, and antiviral prospects

Herpesviruses (Herpesvirales) and tailed bacteriophages (Caudovirales) package their dsDNA genomes through an evolutionarily conserved mechanism. Much is known about the biochemistry and structural biology of phage portal proteins and the DNA encapsidation (viral genome cleavage and packaging) process. Although not at the same level of detail, studies on HSV-1, CMV, VZV, and HHV-8 have revealed important information on the function and structure of herpesvirus portal proteins. During dsDNA phage and herpesviral genome replication, concatamers of viral dsDNA are cleaved into single length units by a virus-encoded terminase and packaged into preformed procapsids through a channel located at a single capsid vertex (portal). Oligomeric portals are formed by the interaction of identical portal protein monomers. Comparing portal protein primary aa sequences between phage and herpesviruses reveals little to no sequence similarity. In contrast, the secondary and tertiary structures of known portals are remarkable. In all cases, function is highly conserved in that portals are essential for DNA packaging and also play a role in releasing viral genomic DNA during infection. Preclinical studies have described small molecules that target the HSV-1 and VZV portals and prevent viral replication by inhibiting encapsidation. This review summarizes what is known concerning the structure and function of herpesvirus portal proteins primarily based on their conserved bacteriophage counterparts and the potential to develop novel portal-specific DNA encapsidation inhibitors.

Genomic characterization and phylogenetic analysis of the novel Pseudomonas phage PPSC2

We isolated a Pseudomonas phage infecting Pseudomonas fluorescens SA1 separated from a soil sample collected in Sichuan Province, China. This phage, which we named PPSC2, has a genome that is composed of a 97,330-bp-long linear double-stranded DNA with 47.51% G+C content and 168 putative protein-coding genes. We identified 20 tRNA genes in the genome of PPSC2, and the tRNA GC content ranged from 44.2% to 58.4%. Phylogenetic and BLASTn analysis revealed that the Pseudomonas phage PPSC2 should be considered a new member of the family Myoviridae.

Unexpected evolutionary benefit to phages imparted by bacterial CRISPR-Cas9

Bacteria and bacteriophages arm themselves with various defensive and counterdefensive mechanisms to protect their own genome and degrade the other's. CRISPR (clustered regularly interspaced short palindromic repeat)-Cas (CRISPR-associated) is an adaptive bacterial defense mechanism that recognizes short stretches of invading phage genome and destroys it by nuclease attack. Unexpectedly, we discovered that the CRISPR-Cas system might also accelerate phage evolution. When Escherichia coli bacteria containing CRISPR-Cas9 were infected with phage T4, its cytosine hydroxymethylated and glucosylated genome was cleaved poorly by Cas9 nuclease, but the continuing CRISPR-Cas9 pressure led to rapid evolution of mutants that accumulated even by the time a single plaque was formed. The mutation frequencies are, remarkably, approximately six orders of magnitude higher than the spontaneous mutation frequency in the absence of CRISPR pressure. Our findings lead to the hypothesis that the CRISPR-Cas might be a double-edged sword, providing survival advantages to both bacteria and phages, leading to their coevolution and abundance on Earth.

Single-Molecule Measurements of Motor-Driven Viral DNA Packaging in Bacteriophages Phi29, Lambda, and T4 with Optical Tweezers

Viral DNA packaging is a required step in the assembly of many dsDNA viruses. A molecular motor fueled by ATP hydrolysis packages the viral genome to near crystalline density inside a preformed prohead shell in ~5 min at room temperature. We describe procedures for measuring the packaging of single DNA molecules into single viral proheads with optical tweezers. Three viral packaging systems are described in detail: bacteriophages phi29 (φ29), lambda (λ), and T4. Two different approaches are described: (1) With φ29 and T4, prohead-motor complexes can be preassembled in bulk and packaging can be initiated in the optical tweezers by "feeding" a single DNA molecule to one of the complexes; (2) With φ29 and λ, packaging can be initiated in bulk then stalled, and a single prohead-motor-DNA complex can then be captured with optical tweezers and restarted. In both cases, the prohead is ultimately attached to one trapped microsphere and the end of the DNA being packaged is attached to a second trapped microsphere such that packaging of the DNA pulls the two microspheres together and the rate of packaging and force generated by the motor is directly measured in real time. These protocols allow for the effect of many experimental parameters on packaging dynamics to be studied such as temperature, ATP concentration, ionic conditions, structural changes to the DNA substrate, and mutations in the motor proteins. Procedures for capturing microspheres with the optical traps and different measurement modes are also described.

Altering the speed of a DNA packaging motor from bacteriophage T4

The speed at which a molecular motor operates is critically important for the survival of a virus or an organism but very little is known about the underlying mechanisms. Tailed bacteriophage T4 employs one of the fastest and most powerful packaging motors, a pentamer of gp17 that translocates DNA at a rate of up to ∼2000-bp/s. We hypothesize, guided by structural and genetic analyses, that a unique hydrophobic environment in the catalytic space of gp17-adenosine triphosphatase (ATPase) determines the rate at which the 'lytic water' molecule is activated and OH- nucleophile is generated, in turn determining the speed of the motor. We tested this hypothesis by identifying two hydrophobic amino acids, M195 and F259, in the catalytic space of gp17-ATPase that are in a position to modulate motor speed. Combinatorial mutagenesis demonstrated that hydrophobic substitutions were tolerated but polar or charged substitutions resulted in null or cold-sensitive/small-plaque phenotypes. Quantitative biochemical and single-molecule analyses showed that the mutant motors exhibited 1.8- to 2.5-fold lower rate of ATP hydrolysis, 2.5- to 4.5-fold lower DNA packaging velocity, and required an activator protein, gp16 for rapid firing of ATPases. These studies uncover a speed control mechanism that might allow selection of motors with optimal performance for organisms' survival.

To Be or Not To Be T4: Evidence of a Complex Evolutionary Pathway of Head Structure and Assembly in Giant Salmonella Virus SPN3US

Giant Salmonella phage SPN3US has a 240-kb dsDNA genome and a large complex virion composed of many proteins for which the functions of most are undefined. We recently determined that SPN3US shares a core set of genes with related giant phages and sequenced and characterized 18 amber mutants to facilitate its use as a genetic model system. Notably, SPN3US and related giant phages contain a bolus of ejection proteins within their heads, including a multi-subunit virion RNA polymerase (vRNAP), that enter the host cell with the DNA during infection. In this study, we characterized the SPN3US virion using mass spectrometry to gain insight into its head composition and the features that its head shares with those of related giant phages and with T4 phage. SPN3US has only homologs to the T4 proteins critical for prohead shell formation, the portal and major capsid proteins, as well as to the major enzymes essential for head maturation, the prohead protease and large terminase subunit. Eight of ~50 SPN3US head proteins were found to undergo proteolytic processing at a cleavage motif by the prohead protease gp245. Gp245 undergoes auto-cleavage of its C-terminus, suggesting this is a conserved activation and/or maturation feature of related phage proteases. Analyses of essential head gene mutants showed that the five subunits of the vRNAP must be assembled for any subunit to be incorporated into the prohead, although the assembled vRNAP must then undergo subsequent major conformational rearrangements in the DNA packed capsid to allow ejection through the ~30 Å diameter tail tube for transcription from the injected DNA. In addition, ejection protein candidate gp243 was found to play a critical role in head assembly. Our analyses of the vRNAP and gp243 mutants highlighted an unexpected dichotomy in giant phage head maturation: while all analyzed giant phages have a homologous protease that processes major capsid and portal proteins, processing of ejection proteins is not always a stable/defining feature. Our identification in SPN3US, and related phages, of a diverged paralog to the prohead protease further hints toward a complicated evolutionary pathway for giant phage head structure and assembly.

Engineering of Bacteriophage T4 Genome Using CRISPR-Cas9

Bacteriophages likely constitute the largest biomass on Earth. However, very few phage genomes have been well-characterized, the tailed phage T4 genome being one of them. Even in T4, much of the genome remained uncharacterized. The classical genetic strategies are tedious, compounded by genome modifications such as cytosine hydroxylmethylation and glucosylation which makes T4 DNA resistant to most restriction endonucleases. Here, using the type-II CRISPR-Cas9 system, we report the editing of both modified (ghm-Cytosine) and unmodified (Cytosine) T4 genomes. The modified genome, however, is less susceptible to Cas9 nuclease attack when compared to the unmodified genome. The efficiency of restriction of modified phage infection varied greatly in a spacer-dependent manner, which explains some of the previous contradictory results. We developed a genome editing strategy by codelivering into E. coli a CRISPR-Cas9 plasmid and a donor plasmid containing the desired mutation(s). Single and multiple point mutations, insertions and deletions were introduced into both modified and unmodified genomes. As short as 50-bp homologous flanking arms were sufficient to generate recombinants that can be selected under the pressure of CRISPR-Cas9 nuclease. A 294-bp deletion in RNA ligase gene rnlB produced viable plaques, demonstrating the usefulness of this editing strategy to determine the essentiality of a given gene. These results provide the first demonstration of phage T4 genome editing that might be extended to other phage genomes in nature to create useful recombinants for phage therapy applications.

Aggregation/dispersion transitions of T4 phage triggered by environmental ion availability

Background

Bacteriophage survives in at least two extremes of ionic environments: bacterial host (high ionic-cytosol) and that of soil (low ionic-environmental water). The impact of ionic composition in the micro- and macro-environments has not so far been addressed in phage biology.

Results

Here, we discovered a novel mechanism of aggregation/disaggregation transitions by phage virions. When normal sodium levels in phage media (150 mM) were lowered to 10 mM, advanced imaging by scanning electron microscopy, atomic force microscopy and dynamic light scattering all revealed formation of viral packages, each containing 20–100 virions. When ionic strength was returned from low to high, the aggregated state of phage reversed to a dispersed state, and the change in ionic strength did not substantially affect infectivity of the phage. By providing the direct evidence, that lowering of the sodium ion below the threshold of 20 mM causes rapid aggregation of phage while returning Na⁺ concentration to the values above this threshold causes dispersion of phage, we identified a biophysical mechanism of phage aggregation.

Conclusions

Our results implicate operation of group behavior in phage and suggest a new kind of quorum sensing among its virions that is mediated by ions. Loss of ionic strength may act as a trigger in an evolutionary mechanism to improve the survival of bacteriophage by stimulating aggregation of phage when outside a bacterial host. Reversal of phage aggregation is also a promising breakthrough in biotechnological applications, since we demonstrated here the ability to retain viable virion aggregates on standard micro-filters.

Bacteriophage T4 as a Nanoparticle Platform to Display and Deliver Pathogen Antigens: Construction of an Effective Anthrax Vaccine

Protein-based subunit vaccines represent a safer alternative to the whole pathogen in vaccine development. However, limitations of physiological instability and low immunogenicity of such vaccines demand an efficient delivery system to stimulate robust immune responses. The bacteriophage T4 capsid-based antigen delivery system can robustly elicit both humoral and cellular immune responses without any adjuvant. Therefore, it offers a strong promise as a novel antigen delivery system. Currently Bacillus anthracis, the causative agent of anthrax, is a serious biothreat agent and no FDA-approved anthrax vaccine is available for mass vaccination. Here, we describe a potential anthrax vaccine using a T4 capsid platform to display and deliver the 83 kDa protective antigen, PA, a key component of the anthrax toxin. This T4 vaccine platform might serve as a universal antigen delivery system that can be adapted to develop vaccines against any infectious disease.

Common Evolutionary Origin of Procapsid Proteases, Phage Tail Tubes, and Tubes of Bacterial Type VI Secretion Systems

Many large viruses, including tailed dsDNA bacteriophages and herpesviruses, assemble their capsids via formation of precursors, called procapsids or proheads. The prohead has an internal core, made of scaffolding proteins, and an outer shell, formed by the major capsid protein. The prohead usually contains a protease, which is activated during capsid maturation to destroy the inner core and liberate space for the genome. Here, we report a 2.0 Å resolution structure of the pentameric procapsid protease of bacteriophage T4, gene product (gp)21. The structure corresponds to the enzyme's pre-active state in which its N-terminal region blocks the catalytic center, demonstrating that the activation mechanism involves self-cleavage of nine N-terminal residues. We describe similarities and differences between T4 gp21 and related herpesvirus proteases. We found that gp21 and the herpesvirus proteases have similarity with proteins forming the tubes of phage tails and bacterial type VI secretion systems, suggesting their common evolutionary origin.

Walker-A Motif Acts to Coordinate ATP Hydrolysis with Motor Output in Viral DNA Packaging

During the assembly of many viruses, a powerful ATP-driven motor translocates DNA into a preformed procapsid. A Walker-A "P-loop" motif is proposed to coordinate ATP binding and hydrolysis with DNA translocation. We use genetic, biochemical, and biophysical techniques to survey the roles of P-loop residues in bacteriophage lambda motor function. We identify 55 point mutations that reduce virus yield to below detectable levels in a highly sensitive genetic complementation assay and 33 that cause varying reductions in yield. Most changes in the predicted conserved residues K76, R79, G81, and S83 produce no detectable yield. Biochemical analyses show that R79A and S83A mutant proteins fold, assemble, and display genome maturation activity similar to wild-type (WT) but exhibit little ATPase or DNA packaging activity. Kinetic DNA cleavage and ATPase measurements implicate R79 in motor ring assembly on DNA, supporting recent structural models that locate the P-loop at the interface between motor subunits. Single-molecule measurements detect no translocation for K76A and K76R, while G81A and S83A exhibit strong impairments, consistent with their predicted roles in ATP binding. We identify eight residue changes spanning A78-K84 that yield impaired translocation phenotypes and show that Walker-A residues play important roles in determining motor velocity, pausing, and processivity. The efficiency of initiation of packaging correlates strongly with motor velocity. Frequent pausing and slipping caused by changes A78V and R79K suggest that these residues are important for ATP alignment and coupling of ATP binding to DNA gripping. Our findings support recent structural models implicating the P-loop arginine in ATP hydrolysis and mechanochemical coupling.

Exclusion of small terminase mediated DNA threading models for genome packaging in bacteriophage T4

Tailed bacteriophages and herpes viruses use powerful molecular machines to package their genomes. The packaging machine consists of three components: portal, motor (large terminase; TerL) and regulator (small terminase; TerS). Portal, a dodecamer, and motor, a pentamer, form two concentric rings at the special five-fold vertex of the icosahedral capsid. Powered by ATPase, the motor ratchets DNA into the capsid through the portal channel. TerS is essential for packaging, particularly for genome recognition, but its mechanism is unknown and controversial. Structures of gear-shaped TerS rings inspired models that invoke DNA threading through the central channel. Here, we report that mutations of basic residues that line phage T4 TerS (gp16) channel do not disrupt DNA binding. Even deletion of the entire channel helix retained DNA binding and produced progeny phage in vivo. On the other hand, large oligomers of TerS (11-mers/12-mers), but not small oligomers (trimers to hexamers), bind DNA. These results suggest that TerS oligomerization creates a large outer surface, which, but not the interior of the channel, is critical for function, probably to wrap viral genome around the ring during packaging initiation. Hence, models involving TerS-mediated DNA threading may be excluded as an essential mechanism for viral genome packaging.

Highly Effective Soluble and Bacteriophage T4 Nanoparticle Plague Vaccines Against Yersinia pestis

Plague caused by Yersinia pestis is an ancient disease, responsible for millions of deaths in human history. Unfortunately, there is no FDA-approved vaccine available. Recombinant subunit vaccines based on two major antigens, Caf 1 (F1) and LcrV (V), have been under investigation and showed promise. However, there are two main problems associated with these vaccines. First, the Yersinia capsular protein F1 has high propensity to aggregate, particularly when expressed in heterologous systems such as Escherichia coli, thus affecting vaccine quality and efficacy. Second, the subunit vaccines do not induce adequate cell-mediated immune responses that also appear to be essential for optimal protection against plague. We have developed two basic approaches, structure-based immunogen design and phage T4 nanoparticle delivery, to construct new plague vaccines that may overcome these problems. First, by engineering F1 protein, we generated a monomeric and soluble F1V mutant (F1mutV) which has similar immunogenicity as wild-type F1V. The NH2-terminal β-strand of F1 was transplanted to the COOH-terminus and the sequence flanking the β-strand was duplicated to retain a key CD4(+) T cell epitope. Second, we generated a nanoparticle plague vaccine that can induce balanced antibody- and cell-mediated immune responses. This was done by arraying the F1mutV on phage T4 via the small outer capsid (Soc) protein which binds to T4 capsid at nanomolar affinity. Preparation of these vaccines is described in detail and we hope that these would be considered as candidates for licensing a next-generation plague vaccine.

Highly efficient integration of the viral portal proteins from different types of phages into planar bilayers for the black lipid membrane analysis

The planar lipid bilayer technology is a technique that yields incredibly useful structural function information about a single channel protein. It is also currently actively utilized as a powerful platform using biological protein nanopores for the development of single-molecule nanopore sensing technology, as well as ultrafast DNA sequencing technology. The portal protein, GP10, from the bacteriophage Φ29 was the first phage portal protein shown to be successfully inserted into planar bilayer membranes, thereby it may inspire more researchers to apply the techniques to portal proteins from the other bacteriophages. However, the technology is far from perfect since the insertion of the channel proteins into planar bilayer membranes is not only technically difficult but also time-consuming. For the fusion of phage portal proteins, vesicles are typically needed to be reconstituted with the portal proteins to form proteoliposomes. However, most of the phage portal proteins have low solubility, and may self-aggregate during the preparation of the proteoliposomes. Furthermore, the fusion of the formed proteoliposomes is sporadic, unpredictable and varied from person to person. Due to the lack of experimental consistency between labs, the results from different methodologies reported for generating fusible proteoliposomes are highly variable. In this research, we propose a new method for the preparation of the fusible proteoliposomes containing portal proteins from bacteriophages, to circumvent the problems aforementioned. Compared to the conventional methods, this method was able to avoid the protein aggregation issues during the vesicle preparation by eliminating the need for detergents and the subsequent time-consuming step for detergent removal. The proteoliposomes prepared by the method were shown to be more efficiently and rapidly inserted into planar bilayer membranes bathed in different conducting buffer solutions including those with nonelectrolytes such as glycerol and PEG. In addition, the method of forming proteoliposomes has significantly extended the shelf life of the proteoliposomes. To further explore its potentials, we have successfully applied the method to the insertion of a mutant portal protein, GP20, from T4 bacteriophage, a hydrophobic portal protein that has not been explored using the planar lipid bilayer membrane technique. The results suggest that this method could be used to prepare proteoliposomes formed by hydrophobic portal proteins from other bacteriophages.

Mechanisms of DNA Packaging by Large Double-Stranded DNA Viruses

Translocation of viral double-stranded DNA (dsDNA) into the icosahedral prohead shell is catalyzed by TerL, a motor protein that has ATPase, endonuclease, and translocase activities. TerL, following endonucleolytic cleavage of immature viral DNA concatemer recognized by TerS, assembles into a pentameric ring motor on the prohead's portal vertex and uses ATP hydrolysis energy for DNA translocation. TerL's N-terminal ATPase is connected by a hinge to the C-terminal endonuclease. Inchworm models propose that modest domain motions accompanying ATP hydrolysis are amplified, through changes in electrostatic interactions, into larger movements of the C-terminal domain bound to DNA. In phage ϕ29, four of the five TerL subunits sequentially hydrolyze ATP, each powering translocation of 2.5 bp. After one viral genome is encapsidated, the internal pressure signals termination of packaging and ejection of the motor. Current focus is on the structures of packaging complexes and the dynamics of TerL during DNA packaging, endonuclease regulation, and motor mechanics.

A novel DNA-binding protein from Campylobacter jejuni bacteriophage NCTC12673

We previously suggested that the double-stranded genomic DNA of Campylobacter jejuni bacteriophage NCTC12673 was complexed with proteins. Mass spectrometry of peptides obtained from tryptic digests of purified phage DNA indicated that phage protein Gp001 co-purified with the DNA. Gp001 is an acidic protein that lacks any obvious homology or conserved domains found in known DNA-binding proteins. The DNA-binding ability of recombinant Gp001 was examined using an electrophoretic mobility shift assay. Slow DNA-Gp001 complex formation was observed at pH 5.5, but not at neutral or basic pH. This nucleoprotein complex had difficulty entering agarose gels used in the assay while proteinase K pretreatment released the DNA from the complex. No mobility shift was observed when the DNA was immediately subjected to electrophoresis after mixing with Gp001, even if both components were separately pre-incubated at pH 5.5. The complexed DNA was unable to transform chemically competent Escherichia coli cells and was less susceptible to degradation by nucleases. The formation of Gp001-DNA complexes at low pH may provide a mechanism for maintaining DNA integrity while the phage pursues its host through the gastrointestinal tract. Also, this feature can potentially be used to improve DNA delivery protocols applied in gene therapy.

Cryo-EM structure of the bacteriophage T4 portal protein assembly at near-atomic resolution

The structure and assembly of bacteriophage T4 has been extensively studied. However, the detailed structure of the portal protein remained unknown. Here we report the structure of the bacteriophage T4 portal assembly, gene product 20 (gp20), determined by cryo-electron microscopy (cryo-EM) to 3.6 Å resolution. In addition, analysis of a 10 Å resolution cryo-EM map of an empty prolate T4 head shows how the dodecameric portal assembly interacts with the capsid protein gp23 at the special pentameric vertex. The gp20 structure also verifies that the portal assembly is required for initiating head assembly, for attachment of the packaging motor, and for participation in DNA packaging. Comparison of the Myoviridae T4 portal structure with the known portal structures of φ29, SPP1 and P22, representing Podo- and Siphoviridae, shows that the portal structure probably dates back to a time when self-replicating microorganisms were being established on Earth.

Tailed bacteriophages translocate the genome into and out of the capsid through a portal protein assembly located between the phage s head and tail. Here Sun et al. provide a cryo-EM structure of the bacteriophage T4 portal protein assembly, suggesting the functions and evolution of the portal structure.