The push-pull mechanism of bacteriophage Ø29 DNA injection

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

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

AT-specific DNA visualization revisits the directionality of bacteriophage λ DNA ejection

In this study, we specifically visualized DNA molecules at their AT base pairs after in vitro phage ejection. Our AT-specific visualization revealed that either end of the DNA molecule could be ejected first with a nearly 50% probability. This observation challenges the generally accepted theory of Last In First Out (LIFO), which states that the end of the phage λ DNA that enters the capsid last during phage packaging is the first to be ejected, and that both ends of the DNA are unable to move within the extremely condensed phage capsid. To support our observations, we conducted computer simulations that revealed that both ends of the DNA molecule are randomized, resulting in the observed near 50% probability. Additionally, we found that the length of the ejected DNA by LIFO was consistently longer than that by First In First Out (FIFO) during in vitro phage ejection. Our simulations attributed this difference in length to the stiffness difference of the remaining DNA within the phage capsid. In conclusion, this study demonstrates that a DNA molecule within an extremely dense phage capsid exhibits a degree of mobility, allowing it to switch ends during ejection.

Seeing and touching adenovirus: complementary approaches for understanding assembly and disassembly of a complex virion

Understanding adenovirus assembly and disassembly poses many challenges due to the virion complexity. A distinctive feature of adenoviruses is the large amount of virus-encoded proteins packed together with the dsDNA genome. Cryo-electron microscopy (cryo-EM) structures are broadening our understanding of capsid variability along evolution, but little is known about the organization of the non-icosahedral nucleoproteic core and its influence in adenovirus function. Atomic force microscopy (AFM) probes the biomechanics of virus particles, while simultaneously inducing and monitoring their disassembly in real time. Synergistic combination of AFM with EM shows that core proteins play unexpected key roles in maturation and entry, and uncoating dynamics are finely tuned to ensure genome release at the appropriate time and place for successful infection.

Fluctuating nonlinear spring theory: Strength, deformability, and toughness of biological nanoparticles from theoretical reconstruction of force-deformation spectra

We developed the Fluctuating Nonlinear Spring (FNS) model to describe the dynamics of mechanical deformation of biological particles, such as virus capsids. The theory interprets the force-deformation spectra in terms of the "Hertzian stiffness" (non-linear regime of a particle's small-amplitude deformations), elastic constant (large-amplitude elastic deformations), and force range in which the particle's fracture occurs. The FNS theory enables one to quantify the particles' elasticity (Young's moduli for Hertzian and bending deformations), and the limits of their strength (critical forces, fracture toughness) and deformability (critical deformations) as well as the probability distributions of these properties, and to calculate the free energy changes for the particle's Hertzian, elastic, and plastic deformations, and eventual fracture. We applied the FNS theory to describe the protein capsids of bacteriophage P22, Human Adenovirus, and Herpes Simplex virus characterized by deformations before fracture that did not exceed 10-19% of their size. These nanoshells are soft (~1-10-GPa elastic modulus), with low ~50-480-kPa toughness - a regime of material behavior that is not well understood, and with the strength increasing while toughness decreases with their size. The particles' fracture is stochastic, with the average values of critical forces, critical deformations, and fracture toughness comparable with their standard deviations. The FNS theory predicts 0.7-MJ/mol free energy for P22 capsid maturation, and it could be extended to describe uniaxial deformation of cylindrical microtubules and ellipsoidal cellular organelles.

Adenovirus major core protein condenses DNA in clusters and bundles, modulating genome release and capsid internal pressure

Some viruses package dsDNA together with large amounts of positively charged proteins, thought to help condense the genome inside the capsid with no evidence. Further, this role is not clear because these viruses have typically lower packing fractions than viruses encapsidating naked dsDNA. In addition, it has recently been shown that the major adenovirus condensing protein (polypeptide VII) is dispensable for genome encapsidation. Here, we study the morphology and mechanics of adenovirus particles with (Ad5-wt) and without (Ad5-VII-) protein VII. Ad5-VII- particles are stiffer than Ad5-wt, but DNA-counterions revert this difference, indicating that VII screens repulsive DNA-DNA interactions. Consequently, its absence results in increased internal pressure. The core is slightly more ordered in the absence of VII and diffuses faster out of Ad5-VII– than Ad5-wt fractured particles. In Ad5-wt unpacked cores, dsDNA associates in bundles interspersed with VII-DNA clusters. These results indicate that protein VII condenses the adenovirus genome by combining direct clustering and promotion of bridging by other core proteins. This condensation modulates the virion internal pressure and DNA release from disrupted particles, which could be crucial to keep the genome protected inside the semi-disrupted capsid while traveling to the nuclear pore.

A packing for A-form DNA in an icosahedral virus

Studies on viruses infecting archaea living in the most extreme environments continue to show a remarkable diversity of structures, suggesting that the sampling continues to be very sparse. We have used electron cryo-microscopy to study at 3.7-Å resolution the structure of the Sulfolobus polyhedral virus 1 (SPV1), which was originally isolated from a hot, acidic spring in Beppu, Japan. The 2 capsid proteins with variant single jelly-roll folds form pentamers and hexamers which assemble into a T = 43 icosahedral shell. In contrast to tailed icosahedral double-stranded DNA (dsDNA) viruses infecting bacteria and archaea, and herpesviruses infecting animals and humans, where naked DNA is packed under very high pressure due to the repulsion between adjacent layers of DNA, the circular dsDNA in SPV1 is fully covered with a viral protein forming a nucleoprotein filament with attractive interactions between layers. Most strikingly, we have been able to show that the DNA is in an A-form, as it is in the filamentous viruses infecting hyperthermophilic acidophiles. Previous studies have suggested that DNA is in the B-form in bacteriophages, and our study is a direct visualization of the structure of DNA in an icosahedral virus.

The application of atomic force microscopy for viruses and protein shells: Imaging and spectroscopy

Atomic force microscopy (AFM) probes surface-adsorbed samples at the nanoscale by using a sharp stylus of nanometric size located at the end of a micro-cantilever. This technique can also work in a liquid environment and offers unique possibilities to study individual protein assemblies, such as viruses, under conditions that resemble their natural liquid milieu. Here, I show how AFM can be used to explore the topography of viruses and protein cages, including that of structures lacking a well-defined symmetry. AFM is not limited for imaging and allows the manipulation of individual viruses with force spectroscopy approaches, such as single indentation and mechanical fatigue assays. These pushing experiments deform the protein cages to obtain their mechanical information and can be used to monitor the structural changes induced by maturation or the exposure to different biochemical environments, such as pH variation. We discuss how studying capsid rupture and self-healing events offers insight into virus uncoating pathways. On the other hand, pulling tests can provide information about the virus-host interaction established between the viral fibers and the cell membrane.

Time-resolved DNA release from an O-antigen-specific Salmonella bacteriophage with a contractile tail

Myoviruses, bacteriophages with T4-like architecture, must contract their tails prior to DNA release. However, quantitative kinetic data on myovirus particle opening are lacking, although they are promising tools in bacteriophage-based antimicrobial strategies directed against Gram-negative hosts. For the first time, we show time-resolved DNA ejection from a bacteriophage with a contractile tail, the multi-O-antigen-specific Salmonella myovirus Det7. DNA release from Det7 was triggered by lipopolysaccharide (LPS) O-antigen receptors and notably slower than in noncontractile-tailed siphoviruses. Det7 showed two individual kinetic steps for tail contraction and particle opening. Our in vitro studies showed that highly specialized tailspike proteins (TSPs) are necessary to attach the particle to LPS. A P22-like TSP confers specificity for the Salmonella Typhimurium O-antigen. Moreover, crystal structure analysis at 1.63 Å resolution confirmed that Det7 recognized the Salmonella Anatum O-antigen via an ϵ15-like TSP, DettilonTSP. DNA ejection triggered by LPS from either host showed similar velocities, so particle opening is thus a process independent of O-antigen composition and the recognizing TSP. In Det7, at permissive temperatures TSPs mediate O-antigen cleavage and couple cell surface binding with DNA ejection, but no irreversible adsorption occurred at low temperatures. This finding was in contrast to short-tailed Salmonella podoviruses, illustrating that tailed phages use common particle-opening mechanisms but have specialized into different infection niches.

Atomic Force Microscopy of Viruses

Atomic force microscopy employs a nanometric tip located at the end of a micro-cantilever to probe surface-mounted samples at nanometer resolution. Because the technique can also work in a liquid environment it offers unique possibilities to study individual viruses under conditions that mimic their natural milieu. Here, we review how AFM imaging can be used to study the surface structure of viruses including that of viruses lacking a well-defined symmetry. Beyond imaging, AFM enables the manipulation of single viruses by force spectroscopy experiments. Pulling experiments can provide information about the early events of virus-host interaction between the viral fibers and the cell membrane receptors. Pushing experiments measure the mechanical response of the viral capsid and its contents and can be used to show how virus maturation and exposure to different pH values change the mechanical response of the viruses and the interaction between the capsid and genome. Finally, we discuss how studying capsid rupture and self-healing events offers insight in virus uncoating pathways.

Biophysical properties of single rotavirus particles account for the functions of protein shells in a multilayered virus

The functions performed by the concentric shells of multilayered dsRNA viruses require specific protein interactions that can be directly explored through their mechanical properties. We studied the stiffness, breaking force, critical strain and mechanical fatigue of individual Triple, Double and Single layered rotavirus (RV) particles. Our results, in combination with Finite Element simulations, demonstrate that the mechanics of the external layer provides the resistance needed to counteract the stringent conditions of extracellular media. Our experiments, in combination with electrostatic analyses, reveal a strong interaction between the two outer layers and how it is suppressed by the removal of calcium ions, a key step for transcription initiation. The intermediate layer presents weak hydrophobic interactions with the inner layer that allow the assembly and favor the conformational dynamics needed for transcription. Our work shows how the biophysical properties of the three shells are finely tuned to produce an infective RV virion.

Structural studies demonstrating a bacteriophage-like replication cycle of the eukaryote-infecting Paramecium bursaria chlorella virus-1

A fundamental stage in viral infection is the internalization of viral genomes in host cells. Although extensively studied, the mechanisms and factors responsible for the genome internalization process remain poorly understood. Here we report our observations, derived from diverse imaging methods on genome internalization of the large dsDNA Paramecium bursaria chlorella virus-1 (PBCV-1). Our studies reveal that early infection stages of this eukaryotic-infecting virus occurs by a bacteriophage-like pathway, whereby PBCV-1 generates a hole in the host cell wall and ejects its dsDNA genome in a linear, base-pair-by-base-pair process, through a membrane tunnel generated by the fusion of the virus internal membrane with the host membrane. Furthermore, our results imply that PBCV-1 DNA condensation that occurs shortly after infection probably plays a role in genome internalization, as hypothesized for the infection of some bacteriophages. The subsequent perforation of the host photosynthetic membranes presumably enables trafficking of viral genomes towards host nuclei. Previous studies established that at late infection stages PBCV-1 generates cytoplasmic organelles, termed viral factories, where viral assembly takes place, a feature characteristic of many large dsDNA viruses that infect eukaryotic organisms. PBCV-1 thus appears to combine a bacteriophage-like mechanism during early infection stages with a eukaryotic-like infection pathway in its late replication cycle.

Although extensively studied, the mechanisms responsible for internalization of viral genomes into their host cells remain unclear. A particularly interesting case of genome release and internalization is provided by the large Paramecium bursaria chlorella virus-1 (PBCV-1), which infects unicellular eukaryotic photosynthetic chlorella cells. In order to release its long dsDNA genome and to enable its translocation to the host nucleus, PBCV-1 must overcome multiple hurdles, including a thick host cell wall and multilayered chloroplast membranes that surround the host cytoplasm. Our observations indicate that these obstacles are dealt with perforations of the host wall, the host cellular membrane, and the host photosynthetic membranes by viral-encoded proteins. Furthermore, our results highlight a bacteriophage-like nature of early PBCV-1 infection stages, thus implying that this virus uniquely combines bacteriophage-like and eukaryotic-like pathways to accomplish its replication cycle.

Atomic force microscopy of virus shells

Microscopes are used to characterize small specimens with the help of probes, such as photons and electrons in optical and electron microscopies, respectively. In atomic force microscopy (AFM) the probe is a nanometric tip located at the end of a microcantilever which palpates the specimen under study as a blind person manages a white cane to explore the surrounding. In this way, AFM allows obtaining nanometric resolution images of individual protein shells, such as viruses, in liquid milieu. Beyond imaging, AFM also enables the manipulation of single protein cages, and the characterization of every physico-chemical property able of inducing any measurable mechanical perturbation to the microcantilever that holds the tip. Here we describe several AFM approaches to study individual protein cages, including imaging and spectroscopic methodologies for extracting mechanical and electrostatic properties. In addition, AFM allows discovering and testing the self-healing capabilities of protein cages because occasionally they may recover fractures induced by the AFM tip. Beyond the protein shells, AFM also is able of exploring the genome inside, obtaining, for instance, the condensation state of dsDNA and measuring its diffusion when the protein cage breaks.

Atomic force microscopy of virus shells

Microscopes are used to characterize small objects with the help of probes that interact with the specimen, such as photons and electrons in optical and electron microscopies, respectively. In atomic force microscopy (AFM), the probe is a nanometric tip located at the end of a microcantilever which palpates the specimen under study just as a blind person manages a walking stick. In this way, AFM allows obtaining nanometric resolution images of individual protein shells, such as viruses, in a liquid milieu. Beyond imaging, AFM also enables not only the manipulation of single protein cages, but also the characterization of every physicochemical property capable of inducing any measurable mechanical perturbation to the microcantilever that holds the tip. In the present revision, we start revising some recipes for adsorbing protein shells on surfaces. Then, we describe several AFM approaches to study individual protein cages, ranging from imaging to spectroscopic methodologies devoted to extracting physical information, such as mechanical and electrostatic properties. We also explain how a convenient combination of AFM and fluorescence methodologies entails monitoring genome release from individual viral shells during mechanical unpacking.

Prohead RNA: a noncoding viral RNA of novel structure and function

Prohead RNA (pRNA) is an essential component of the powerful Φ29-like bacteriophage DNA packaging motor. However, the specific role of this unique RNA in the Φ29 packaging motor remains unknown. This review examines pRNA as a noncoding RNA of novel structure and function. In order to highlight the reasons for exploring the structure and function of pRNA, we (1) provide an overview of Φ29-like bacteriophage and the Φ29 DNA packaging motor, including putative motor mechanisms and structures of its component parts; (2) discuss pRNA structure and possible roles for pRNA in the Φ29 packaging motor; (3) summarize pRNA self-assembly; and (4) describe the prospective therapeutic applications of pRNA. Many questions remain to be answered in order to connect what is currently known about pRNA structure to its novel function in the Φ29 packaging motor. The knowledge gained from studying the structure, function, and sequence variation in pRNA will help develop tools to better navigate the conformational landscapes of RNA. WIREs RNA 2016, 7:428-437. doi: 10.1002/wrna.1330 For further resources related to this article, please visit the WIREs website.

A non-invasive method for studying viral DNA delivery to bacteria reveals key requirements for phage SPP1 DNA entry in Bacillus subtilis cells

Bacteriophages use most frequently a tail apparatus to create a channel across the entire bacterial cell envelope to transfer the viral genome to the host cell cytoplasm, initiating infection. Characterization of this critical step remains a major challenge due to the difficulty to monitor DNA entry in the bacterium and its requirements. In this work we developed a new method to study phage DNA entry that has the potential to be extended to many tailed phages. Its application to study genome delivery of bacteriophage SPP1 into Bacillus subtilis disclosed a key role of the host cell membrane potential in the DNA entry process. An energized B. subtilis membrane and a millimolar concentration of calcium ions are shown to be major requirements for SPP1 DNA entry following the irreversible binding of phage particles to the receptor YueB.

The Global Reciprocal Reprogramming between Mycobacteriophage SWU1 and Mycobacterium Reveals the Molecular Strategy of Subversion and Promotion of Phage Infection

Bacteriophages are the viruses of bacteria, which have contributed extensively to our understanding of life and modern biology. The phage-mediated bacterial growth inhibition represents immense untapped source for novel antimicrobials. Insights into the interaction between mycobacteriophage and Mycobacterium host will inform better utilizing of mycobacteriophage. In this study, RNA sequencing technology (RNA-seq) was used to explore the global response of Mycobacterium smegmatis mc²155 at an early phase of infection with mycobacteriophage SWU1, key host metabolic processes of M. smegmatis mc²155 shut off by SWU1, and the responsible phage proteins. The results of RNA-seq were confirmed by Real-time PCR and functional assay. 1174 genes of M. smegmatis mc²155 (16.9% of the entire encoding capacity) were differentially regulated by phage infection. These genes belong to six functional categories: (i) signal transduction, (ii) cell energetics, (iii) cell wall biosynthesis, (iv) DNA, RNA, and protein biosynthesis, (v) iron uptake, (vi) central metabolism. The transcription patterns of phage SWU1 were also characterized. This study provided the first global glimpse of the reciprocal reprogramming between the mycobacteriophage and Mycobacterium host.

Herpes simplex virus type-1: replication, latency, reactivation and its antiviral targets

Infection by herpes simplex virus type-1 (HSV-1) causes several diseases, ranging from cutaneous, oral and genital infections to fatal encephalitis. Despite the availability of antiviral therapies on the market, their efficacies are incomplete, and new cases of resistant strains arise, mainly in the immunocompromised, but also recently documented in immunocompetent patients. Over the last decades a lot has been discovered about the molecular basis of infection which has been of great benefit to the investigation of new anti-HSV-1 molecules. In this review we summarize replication, latency and reactivation highlighting potential antiviral targets and new molecules described in the past several years in the literature.

An RNA Domain Imparts Specificity and Selectivity to a Viral DNA Packaging Motor

During assembly, double-stranded DNA viruses, including bacteriophages and herpesviruses, utilize a powerful molecular motor to package their genomic DNA into a preformed viral capsid. An integral component of the packaging motor in the Bacillus subtilis bacteriophage ϕ29 is a viral genome-encoded pentameric ring of RNA (prohead RNA [pRNA]). pRNA is a 174-base transcript comprised of two domains, domains I and II. Early studies initially isolated a 120-base form (domain I only) that retains high biological activity in vitro; hence, no function could be assigned to domain II. Here we define a role for this domain in the packaging process. DNA packaging using restriction digests of ϕ29 DNA showed that motors with the 174-base pRNA supported the correct polarity of DNA packaging, selectively packaging the DNA left end. In contrast, motors containing the 120-base pRNA had compromised specificity, packaging both left- and right-end fragments. The presence of domain II also provides selectivity in competition assays with genomes from related phages. Furthermore, motors with the 174-base pRNA were restrictive, in that they packaged only one DNA fragment into the head, whereas motors with the 120-base pRNA packaged several fragments into the head, indicating multiple initiation events. These results show that domain II imparts specificity and stringency to the motor during the packaging initiation events that precede DNA translocation. Heteromeric rings of pRNA demonstrated that one or two copies of domain II were sufficient to impart this selectivity/stringency. Although ϕ29 differs from other double-stranded DNA phages in having an RNA motor component, the function provided by pRNA is carried on the motor protein components in other phages.

IMPORTANCE

During virus assembly, genome packaging involves the delivery of newly synthesized viral nucleic acid into a protein shell. In the double-stranded DNA phages and herpesviruses, this is accomplished by a powerful molecular motor that translocates the viral DNA into a preformed viral shell. A key event in DNA packaging is recognition of the viral DNA among other nucleic acids in the host cell. Commonly, a DNA-binding protein mediates the interaction of viral DNA with the motor/head shell. Here we show that for the bacteriophage ϕ29, this essential step of genome recognition is mediated by a viral genome-encoded RNA rather than a protein. A domain of the prohead RNA (pRNA) imparts specificity and stringency to the motor by ensuring the correct orientation of DNA packaging and restricting initiation to a single event. Since this assembly step is unique to the virus, DNA packaging is a novel target for the development of antiviral drugs.

The actin-like MreB cytoskeleton organizes viral DNA replication in bacteria

Little is known about the organization or proteins involved in membrane-associated replication of prokaryotic genomes. Here we show that the actin-like MreB cytoskeleton of the distantly related bacteria Escherichia coli and Bacillus subtilis is required for efficient viral DNA replication. Detailed analyses of B. subtilis phage ϕ29 showed that the MreB cytoskeleton plays a crucial role in organizing phage DNA replication at the membrane. Thus, phage double-stranded DNA and components of the ϕ29 replication machinery localize in peripheral helix-like structures in a cytoskeleton-dependent way. Importantly, we show that MreB interacts directly with the ϕ29 membrane-protein p16.7, responsible for attaching viral DNA at the cell membrane. Altogether, the results reveal another function for the MreB cytoskeleton and describe a mechanism by which viral DNA replication is organized at the bacterial membrane.

Adenosine triphosphatases of thermophilic archaeal double-stranded DNA viruses

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

A viral packaging motor varies its DNA rotation and step size to preserve subunit coordination as the capsid fills

Multimeric, ring-shaped molecular motors rely on the coordinated action of their subunits to perform crucial biological functions. During these tasks, motors often change their operation in response to regulatory signals. Here, we investigate a viral packaging machine as it fills the capsid with DNA and encounters increasing internal pressure. We find that the motor rotates the DNA during packaging and that the rotation per base pair increases with filling. This change accompanies a reduction in the motor's step size. We propose that these adjustments preserve motor coordination by allowing one subunit to make periodic, specific, and regulatory contacts with the DNA. At high filling, we also observe the downregulation of the ATP-binding rate and the emergence of long-lived pauses, suggesting a throttling-down mechanism employed by the motor near the completion of packaging. This study illustrates how a biological motor adjusts its operation in response to changing conditions, while remaining highly coordinated.

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

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

New insights in the ϕ29 terminal protein DNA-binding and host nucleoid localization functions

Protein-primed DNA replication constitutes a strategy to initiate viral DNA synthesis in a variety of prokaryotic and eukaryotic organisms. Although the main function of viral terminal proteins (TPs) is to provide a free hydroxyl group to start initiation of DNA replication, there are compelling evidences that TPs can also play other biological roles. In the case of Bacillus subtilis bacteriophage ϕ29, the N-terminal domain of the TP organizes viral DNA replication at the bacterial nucleoid being essential for an efficient phage DNA replication, and it contains a nuclear localization signal (NLS) that is functional in eukaryotes. Here we provide information about the structural properties of the ϕ29 TP N-terminal domain, which possesses sequence-independent DNA-binding capacity, and dissect the amino acid residues important for its biological function. By mutating all the basic residues of the TP N-terminal domain we identify the amino acids responsible for its interaction with the B. subtilis genome, establishing a correlation between the capacity of DNA-binding and nucleoid localization of the protein. Significantly, these residues are important to recruit the DNA polymerase at the bacterial nucleoid and, subsequently, for an efficient phage DNA replication.

Herpes virus genome, the pressure is on

Herpes simplex virus type 1 (HSV-1) packages its micrometers-long double-stranded DNA genome into a nanometer-scale protein shell, termed the capsid. Upon confinement within the capsid, neighboring DNA strands experience repulsive electrostatic and hydration forces as well as bending stress associated with the tight curvature required of packaged DNA. By osmotically suppressing DNA release from HSV-1 capsids, we provide the first experimental evidence of a high internal pressure of tens of atmospheres within a eukaryotic human virus, resulting from the confined genome. Furthermore, the ejection is progressively suppressed by increasing external osmotic pressures, which reveals that internal pressure is capable of powering ejection of the entire genome from the viral capsid. Despite billions of years of evolution separating eukaryotic viruses and bacteriophages, pressure-driven DNA ejection has been conserved. This suggests it is a key mechanism for viral infection and thus presents a new target for antiviral therapies.

Mapping in vitro local material properties of intact and disrupted virions at high resolution using multi-harmonic atomic force microscopy

Understanding the relationships between viral material properties (stiffness, strength, charge density, adhesion, hydration, viscosity, etc.), structure (protein sub-units, genome, surface receptors, appendages), and functions (self-assembly, stability, disassembly, infection) is of significant importance in physical virology and nanomedicine. Conventional Atomic Force Microscopy (AFM) methods have measured a single physical property such as the stiffness of the entire virus from nano-indentation at a few points which severely limits the study of structure-property-function relationships. We present an in vitro dynamic AFM technique operating in the intermittent contact regime which synthesizes anharmonic Lorentz-force excited AFM cantilevers to map quantitatively at nanometer resolution the local electro-mechanical force gradient, adhesion, and hydration layer viscosity within individual φ29 virions. Furthermore, the changes in material properties over the entire φ29 virion provoked by the local disruption of its shell are studied, providing evidence of bacteriophage depressurization. The technique significantly generalizes recent multi-harmonic theory (A. Raman, et al., Nat. Nanotechnol., 2011, 6, 809-814) and enables high-resolution in vitro quantitative mapping of multiple material properties within weakly bonded viruses and nanoparticles with complex structure that otherwise cannot be observed using standard AFM techniques.

Langevin dynamics simulation of DNA ejection from a phage

We have performed Langevin dynamics simulations of a coarse-grained model of ejection of dsDNA from Φ29 phage. Our simulation results show significant variations in the local ejection speed, consistent with experimental observations reported in the literature for both in vivo and in vitro systems. In efforts to understand the origin of such variations in the local speed of ejection, we have investigated the correlations between the local ejection kinetics and the packaged structures created at various motor forces and chain flexibility. At lower motor forces, the packaged DNA length is shorter with better organization. On the other hand, at higher motor forces typical of realistic situations, the DNA organization inside the capsid suffers from significant orientational disorder, but yet with long orientational correlation times. This in turn leads to lack of registry between the direction of the DNA segments just to be ejected and the direction of exit. As a result, a significant amount of momentum transfer is required locally for successful exit. Consequently, the DNA ejection temporarily slows down exhibiting pauses. This slowing down occurs at random times during the ejection process, completely determined by the particular starting conformation created by prescribed motor forces. In order to augment our inference, we have additionally investigated the ejection of chains with deliberately changed persistence length. For less inflexible chains, the demand on the occurrence of large momentum transfer for successful ejection is weaker, resulting in more uniform ejection kinetics. While being consistent with experimental observations, our results show the nonergodic nature of the ejection kinetics and call for better theoretical models to portray the kinetics of genome ejection from phages.

Effects of pulling forces, osmotic pressure, condensing agents and viscosity on the thermodynamics and kinetics of DNA ejection from bacteriophages to bacterial cells: a computational study

In this work, we report on simulations of double-stranded DNA (dsDNA) ejection from bacteriophage φ29 into a bacterial cell. The ejection was studied with a coarse-grained model, in which viral dsDNA was represented by beads on a torsion-less string. The bacteriophage's capsid and the bacterial cell were defined by sets of spherical constraints. To account for the effects of the viscous medium inside the bacterial cell, the simulations were carried out using a Langevin dynamics protocol. Our simplest simulations (involving constant viscosity and no external biasing forces) produced results compatible with the push-pull model of DNA ejection, with an ejection rate significantly higher in the first part of ejection than in the latter parts. Additionally, we performed more complicated simulations, in which we included additional factors such as external forces, osmotic pressure, condensing agents and ejection-dependent viscosity. The effects of these factors (independently and in combination) on the thermodynamics and kinetics of DNA ejection were studied. We found that, in general, the dependence of ejection forces and ejection rates on the amount of DNA ejected becomes more complex if the ejection is modeled with a broader, more realistic set of parameters and influences (such as variation in the solvent's viscosity and the application of an external force). However, certain combinations of factors and numerical parameters led to the opposition of some ejection-driving and ejection-inhibiting influences, ultimately causing an apparent simplification of the ejection profiles.

Popping the cork: mechanisms of phage genome ejection

Sixty years after Hershey and Chase showed that nucleic acid is the major component of phage particles that is ejected into cells, we still do not fully understand how the process occurs. Advances in electron microscopy have revealed the structure of the condensed DNA confined in a phage capsid, and the mechanisms and energetics of packaging a phage genome are beginning to be better understood. Condensing DNA subjects it to high osmotic pressure, which has been suggested to provide the driving force for its ejection during infection. However, forces internal to a phage capsid cannot, alone, cause complete genome ejection into cells. Here, we describe the structure of the DNA inside mature phages and summarize the current models of genome ejection, both in vitro and in vivo.

Mechanical properties of viruses

Structural biology techniques have greatly contributed to unveil the relationships between structure, properties and functions of viruses. In recent years, classic structural approaches are being complemented by single-molecule techniques such as atomic force microscopy and optical tweezers to study physical properties and functions of viral particles that are not accessible to classic structural techniques. Among these features are mechanical properties such as stiffness, intrinsic elasticity, tensile strength and material fatigue. The field of virus mechanics is contributing to materials science by investigating some physical parameters of "soft" biological matter and biological nano-objects. Virus mechanics studies are also starting to unveil the biological implications of physical properties of viruses. Growing evidence indicate that viruses are subjected to internal and external forces, and that they may have adapted to withstand and even use those forces. This chapter describes what is known on the mechanical properties of virus particles, their structural determinants, and possible biological implications, of which several examples are provided.

"Push through one-way valve" mechanism of viral DNA packaging

Double-stranded (ds)DNA viruses package their genomic DNA into a procapsid using a force-generating nanomotor powered by ATP hydrolysis. Viral DNA packaging motors are mainly composed of the connector channel and two DNA packaging enzymes. In 1998, it was proposed that viral DNA packaging motors exercise a mechanism similar to the action of AAA+ ATPases that assemble into ring-shaped oligomers, often hexamers, with a central channel (Guo et al. Molecular Cell, 2:149). This chapter focuses on the most recent findings in the bacteriophage ϕ29 DNA packaging nanomotor to address this intriguing notion. Almost all dsDNA viruses are composed entirely of protein, but in the unique case of ϕ29, packaging RNA (pRNA) plays an intermediate role in the packaging process. Evidence revealed that DNA packaging is accomplished via a "push through one-way valve" mechanism. The ATPase gp16 pushes dsDNA through the connector channel section by section into the procapsid. The dodecameric connector channel functions as a one-way valve that only allows dsDNA to enter but not exit the procapsid during DNA packaging. Although the roles of the ATPase gp16 and the motor connector channel are separate and independent, pRNA bridges these two components to ensure the coordination of an integrated motor. ATP induces a conformational change in gp16, leading to its stronger binding to dsDNA. Furthermore, ATP hydrolysis led to the departure of dsDNA from the ATPase/dsDNA complex, an action used to push dsDNA through the connector channel. It was found unexpectedly that by mutating the basic lysine rings of the connector channel or by changing the pH did not measurably impair DNA translocation or affect the one-way traffic property of the channel, suggesting that the positive charges in the lysine ring are not essential in gearing the dsDNA. The motor channel exercises three discrete, reversible, and controllable steps of gating, with each step altering the channel size by 31% to control the direction of translocation of dsDNA. Many DNA packaging models have been contingent upon the number of base pairs packaged per ATP relative to helical turns for B-type DNA. Both 2 and 2.5 bp per ATP have been used to argue for four, five, or six discrete steps of DNA translocation. The "push through one-way valve" mechanism renews the perception of dsDNA packaging energy calculations and provides insight into the discrepancy between 2 and 2.5 bp per ATP. Application of the DNA packaging motor in nanotechnology and nanomedicine is also addressed. Comparison with nine other DNA packaging models revealed that the "push through one-way valve" is the most agreeable mechanism to interpret most of the findings that led to historical models. The application of viral DNA packaging motors is also discussed.

Direct measurement of phage phi29 stiffness provides evidence of internal pressure

Using AFM nanoindentation experiments, DNA-full phi29 phage capsids are shown to be stiffer than when empty. The presence of counterions softens full viruses in a reversible manner, indicating that pressure originates from the confined DNA. A finite element analysis of the experiments provides an estimate of the pressure of ∼40 atm inside the capsid, which is similar to theoretical predictions.

Disclosing the in vivo organization of a viral histone-like protein in Bacillus subtilis mediated by its capacity to recognize the viral genome

Organization of replicating prokaryotic genomes requires architectural elements that, similarly to eukaryotic systems, induce topological changes such as DNA supercoiling. Bacteriophage 29 protein p6 has been described as a histone-like protein that compacts the viral genome by forming a nucleoprotein complex and plays a key role in the initiation of protein-primed DNA replication. In this work, we analyze the subcellular localization of protein p6 by immunofluorescence microscopy and show that, at early infection stages, it localizes in a peripheral helix-like configuration. Later, at middle infection stages, protein p6 is recruited to the bacterial nucleoid. This migrating process is shown to depend on the synthesis of components of the 29 DNA replication machinery (i.e., terminal protein and DNA polymerase) needed for the replication of viral DNA, which is required to recruit the bulk of protein p6. Importantly, the double-stranded DNA-binding capacity of protein p6 is essential for its relocalization at the nucleoid. Altogether, the results disclose the in vivo organization of a viral histone-like protein in bacteria.

Short noncontractile tail machines: adsorption and DNA delivery by podoviruses

Tailed dsDNA bacteriophage virions bind to susceptible cells with the tips of their tails and then deliver their DNA through the tail into the cells to initiate infection. This chapter discusses what is known about this process in the short-tailed phages (Podoviridae). Their short tails require that many of these virions adsorb to the outer layers of the cell and work their way down to the outer membrane surface before releasing their DNA. Interestingly, the receptor-binding protein of many short-tailed phages (and some with long tails) has an enzymatic activity that cleaves their polysaccharide receptors. Reversible adsorption and irreversible adsorption to primary and secondary receptors are discussed, including how sequence divergence in tail fiber and tailspike proteins leads to different host specificities. Upon reaching the outer membrane of Gram-negative cells, some podoviral tail machines release virion proteins into the cell that help the DNA efficiently traverse the outer layers of the cell and/or prepare the cell cytoplasm for phage genome arrival. Podoviruses utilize several rather different variations on this theme. The virion DNA is then released into the cell; the energetics of this process is discussed. Phages like T7 and N4 deliver their DNA relatively slowly, using enzymes to pull the genome into the cell. At least in part this mechanism ensures that genes in late-entering DNA are not expressed at early times. On the other hand, phages like P22 probably deliver their DNA more rapidly so that it can be circularized before the cascade of gene expression begins.

Three reversible and controllable discrete steps of channel gating of a viral DNA packaging motor

The channel of the viral DNA packaging motor allows dsDNA to enter the protein procapsid shell during maturation and to exit during infection. We recently showed that the bacteriophage phi29 DNA packaging motor exercises a one-way traffic property using a channel as a valve for dsDNA translocation. This raises a question of how dsDNA is ejected during infection if the channel only allows the dsDNA to travel inward. We proposed that DNA forward or reverse travel is controlled by conformational changes of the channel. Here we reported our direct observation that the channel indeed exercises conformational changes by single channel recording at a single-molecule level. The changes were induced by high electrical voltage, or by affinity binding to the C-terminal wider end located within the capsid. Novel enough, the conformational change of the purified connector channel exhibited three discrete gating steps, with a size reduction of 32% for each step. We investigated the role of the terminal and internal loop of the channel in gating by different mutants. The step-wise conformational change of the channel was also reversible and controllable, making it an ideal nano-valve for constructing a nanomachine with potential applications in nanobiotechnology and nanomedicine.

Built-in mechanical stress in viral shells

Mechanical properties of biological molecular aggregates are essential to their function. A remarkable example are double-stranded DNA viruses such as the φ29 bacteriophage, that not only has to withstand pressures of tens of atmospheres exerted by the confined DNA, but also uses this stored elastic energy during DNA translocation into the host. Here we show that empty prolated φ29 bacteriophage proheads exhibit an intriguing anisotropic stiffness which behaves counterintuitively different from standard continuum elasticity predictions. By using atomic force microscopy, we find that the φ29 shells are approximately two-times stiffer along the short than along the long axis. This result can be attributed to the existence of a residual stress, a hypothesis that we confirm by coarse-grained simulations. This built-in stress of the virus prohead could be a strategy to provide extra mechanical strength to withstand the DNA compaction during and after packing and a variety of extracellular conditions, such as osmotic shocks or dehydration.

Salt-dependent DNA-DNA spacings in intact bacteriophage λ reflect relative importance of DNA self-repulsion and bending energies

Using solution synchrotron x-ray scattering, we measure the variation of DNA-DNA d spacings in bacteriophage λ with mono-, di-, and polyvalent salt concentrations, for wild-type [48.5×10(3) base pairs (bp)] and short-genome-mutant (37.8 kbp) strains. From the decrease in d spacings with increasing salt, we deduce the relative contributions of DNA self-repulsion and bending to the energetics of packaged phage genomes. We quantify the DNA-DNA interaction energies within the intact phage by combining the measured d spacings in the capsid with measurements of osmotic pressure in DNA assemblies under the same salt conditions in bulk solution. In the commonly used Tris-Mg buffer, the DNA-DNA interaction energies inside the phage capsids are shown to be about 1kT/bp, an order of magnitude larger than the bending energies.

Dynamics of bacteriophage genome ejection in vitro and in vivo

Bacteriophages, phages for short, are viruses of bacteria. The majority of phages contain a double-stranded DNA genome packaged in a capsid at a density of ∼500 mg ml(-1). This high density requires substantial compression of the normal B-form helix, leading to the conjecture that DNA in mature phage virions is under significant pressure, and that pressure is used to eject the DNA during infection. A large number of theoretical, computer simulation and in vitro experimental studies surrounding this conjecture have revealed many--though often isolated and/or contradictory--aspects of packaged DNA. This prompts us to present a unified view of the statistical physics and thermodynamics of DNA packaged in phage capsids. We argue that the DNA in a mature phage is in a (meta)stable state, wherein electrostatic self-repulsion is balanced by curvature stress due to confinement in the capsid. We show that in addition to the osmotic pressure associated with the packaged DNA and its counterions, there are four different pressures within the capsid: pressure on the DNA, hydrostatic pressure, the pressure experienced by the capsid and the pressure associated with the chemical potential of DNA ejection. Significantly, we analyze the mechanism of force transmission in the packaged DNA and demonstrate that the pressure on DNA is not important for ejection. We derive equations showing a strong hydrostatic pressure difference across the capsid shell. We propose that when a phage is triggered to eject by interaction with its receptor in vitro, the (thermodynamic) incentive of water molecules to enter the phage capsid flushes the DNA out of the capsid. In vivo, the difference between the osmotic pressures in the bacterial cell cytoplasm and the culture medium similarly results in a water flow that drags the DNA out of the capsid and into the bacterial cell.

Viral terminal protein directs early organization of phage DNA replication at the bacterial nucleoid

The mechanism leading to protein-primed DNA replication has been studied extensively in vitro. However, little is known about the in vivo organization of the proteins involved in this fundamental process. Here we show that the terminal proteins (TPs) of phages ϕ29 and PRD1, infecting the distantly related bacteria Bacillus subtilis and Escherichia coli, respectively, associate with the host bacterial nucleoid independently of other viral-encoded proteins. Analyses of phage ϕ29 revealed that the TP N-terminal domain (residues 1-73) possesses sequence-independent DNA-binding capacity and is responsible for its nucleoid association. Importantly, we show that in the absence of the TP N-terminal domain the efficiency of ϕ29 DNA replication is severely affected. Moreover, the TP recruits the phage DNA polymerase to the bacterial nucleoid, and both proteins later are redistributed to enlarged helix-like structures in an MreB cytoskeleton-dependent way. These data disclose a key function for the TP in vivo: organizing the early viral DNA replication machinery at the cell nucleoid.

Initial Events Associated with Virus PBCV-1 Infection of Chlorella NC64A

Chlorella viruses (or chloroviruses) are very large, plaque-forming viruses. The viruses are multilayered structures containing a large double-stranded DNA genome, a lipid bilayered membrane, and an outer icosahedral capsid shell. The viruses replicate in certain isolates of the coccal green alga, Chlorella. Sequence analysis of the 330-kbp genome of Paramecium bursaria Chlorella virus 1 (PBCV-1), the prototype of the virus family Phycodnaviridae, reveals <365 protein-encoding genes and 11 tRNA genes. Products of about 40% of these genes resemble proteins of known function, including many that are unexpected for a virus. Among these is a virus-encoded protein, called Kcv, which forms a functional K(+) channel. This chapter focuses on the initial steps in virus infection and provides a plausible role for the function of the viral K(+) channel in lowering the turgor pressure of the host. This step appears to be a prerequisite for delivery of the viral genome into the host.

The force acting on a polymer partially confined in a tube

We consider the force acting on a polymer part of whose length is configurationally confined in a tube and the rest of which is free. This situation arises in many different physical contexts, including a flexible synthetic polymer partially confined in a nanopore and a stiff viral genome partially ejected from its capsid. In both cases the force acting to pull the chain molecule out of its confinement is argued to be constant once a few persistence lengths are "free"/"outside". We present Brownian dynamics simulations that confirm the constancy of the force for different chain lengths and illustrate the dependence of the force on the strength of tube confinement. Experimental results are reported for genome ejection from viral capsids, from which we estimate the pulling force to be a few tenths of a piconewton.

Polarized DNA ejection from the herpesvirus capsid

Ejection of DNA from the capsid is an early step in infection by all herpesviruses. Ejection or DNA uncoating occurs after a parental capsid has entered the host cell cytoplasm, migrated to the nucleus, and bound to a nuclear pore. DNA exits the capsid through the portal vertex and proceeds by way of the nuclear pore complex into the nucleoplasm where it is transcribed and replicated. Here, we describe use of an in vitro uncoating system to determine which genome end exits first from the herpes simplex virus 1 capsid. Purified DNA-containing capsids were bound to a solid surface and warmed under conditions in which some, but not all, of the DNA was ejected. Restriction endonuclease digestion was then used to identify the genomic origin of the ejected DNA. The results support the view that the S segment end exits the capsid first. Preferential release at the S end demonstrates that herpesvirus DNA uncoating conforms to the paradigm in double-stranded DNA bacteriophage where the last end packaged is the first to be ejected. Release of herpes simplex virus 1 DNA beginning at the S end causes the first gene to enter the host cell nucleus to be alpha4, a transcription factor required for expression of early genes.

Shared catalysis in virus entry and bacterial cell wall depolymerization

Bacterial virus entry and cell wall depolymerization require the breakdown of peptidoglycan (PG), the peptide-cross-linked polysaccharide matrix that surrounds bacterial cells. Structural studies of lysostaphin, a PG lytic enzyme (autolysin), have suggested that residues in the active site facilitate hydrolysis, but a clear mechanism for this reaction has remained unsolved. The active-site residues and a structural pattern of beta-sheets are conserved among lysostaphin homologs (such as LytM of Staphylococcus aureus) and the C-terminal domain of gene product 13 (gp13), a protein at the tail tip of the Bacillus subtilis bacteriophage varphi29. gp13 activity on PG and muropeptides was assayed using high-performance liquid chromatography, and gp13 was found to be a d,d-endopeptidase that cleaved the peptide cross-link. Computational modeling of the B. subtilis cross-linked peptide into the gp13 active site suggested that Asp195 may facilitate scissile-bond activation and that His247 is oriented to mediate nucleophile generation. To our knowledge, this is the first model of a Zn(2)(+) metallopeptidase and its substrate. Residue Asp195 of gp13 was found to be critical for Zn(2)(+) binding and catalysis by substitution mutagenesis with Ala or Cys. Circular dichroism and particle-induced X-ray emission spectroscopy showed that the general protein folding and Zn(2)(+) binding were maintained in the Cys mutant but reduced in the Ala mutant. These findings together support a model in which the Asp195 and His247 in gp13 and homologous residues in the LytM and lysostaphin active sites facilitate hydrolysis of the peptide substrate that cross-links PG. Thus, these autolysins and phage-entry enzymes have a shared chemical mechanism of action.

DNA poised for release in bacteriophage phi29

We present here the first asymmetric, three-dimensional reconstruction of a tailed dsDNA virus, the mature bacteriophage phi29, at subnanometer resolution. This structure reveals the rich detail of the asymmetric interactions and conformational dynamics of the phi29 protein and DNA components, and provides novel insight into the mechanics of virus assembly. For example, the dodecameric head-tail connector protein undergoes significant rearrangement upon assembly into the virion. Specific interactions occur between the tightly packed dsDNA and the proteins of the head and tail. Of particular interest and novelty, an approximately 60A diameter toroid of dsDNA was observed in the connector-lower collar cavity. The extreme deformation that occurs over a small stretch of DNA is likely a consequence of the high pressure of the packaged genome. This toroid structure may help retain the DNA inside the capsid prior to its injection into the bacterial host.

Biophysics of viral infectivity: matching genome length with capsid size

In this review, we discuss recent advances in biophysical virology, presenting experimental and theoretical studies on the physical properties of viruses. We focus on the double-stranded (ds) DNA bacteriophages as model systems for all of the dsDNA viruses both prokaryotic and eukaryotic. Recent studies demonstrate that the DNA packaged into a viral capsid is highly pressurized, which provides a force for the first step of passive injection of viral DNA into a bacterial cell. Moreover, specific studies on capsid strength show a strong correlation between genome length, and capsid size and robustness. The implications of these newly appreciated physical properties of a viral particle with respect to the infection process are discussed.

In vivo DNA binding of bacteriophage GA-1 protein p6

Bacteriophage GA-1 infects Bacillus sp. strain G1R and has a linear double-stranded DNA genome with a terminal protein covalently linked to its 5' ends. GA-1 protein p6 is very abundant in infected cells and binds DNA with no sequence specificity. We show here that it binds in vivo to the whole viral genome, as detected by cross-linking, chromatin immunoprecipitation, and real-time PCR analyses, and has the characteristics of a histone-like protein. Binding to DNA of GA-1 protein p6 shows little supercoiling dependency, in contrast to the ortholog protein of the evolutionary related Bacillus subtilis phage phi29. This feature is a property of the protein rather than the DNA or the cellular background, since phi29 protein p6 shows supercoiling-dependent binding to GA-1 DNA in Bacillus sp. strain G1R. GA-1 DNA replication is impaired in the presence of the gyrase inhibitors novobiocin and nalidixic acid, which indicates that, although noncovalently closed, the viral genome is topologically constrained in vivo. GA-1 protein p6 is also able to bind phi29 DNA in B. subtilis cells; however, as expected, the binding is less supercoiling dependent than the one observed with the phi29 protein p6. In addition, the nucleoprotein complex formed is not functional, since it is not able to transcomplement the DNA replication deficiency of a phi29 sus6 mutant. Furthermore, we took advantage of phi29 protein p6 binding to GA-1 DNA to find that the viral DNA ejection mechanism seems to take place, as in the case of phi29, with a right to left polarity in a two-step, push-pull process.

Pressure built by DNA packing inside virions: enough to drive DNA ejection in vitro, largely insufficient for delivery into the bacterial cytoplasm

Tailed bacteriophage particles carry DNA highly pressurized inside the capsid. Challenge with their receptor promotes release of viral DNA. We show that addition of the osmolyte polyethylene glycol (PEG) has two distinct effects in bacteriophage SPP1 DNA ejection. One effect is to inhibit the trigger for DNA ejection. The other effect is to exert an osmotic pressure that controls the extent of DNA released in phages that initiate ejection. We carried out independent measurements of each effect, which is an essential requirement for their quantitative study. The fraction of phages that do not eject increased linearly with the external osmotic pressure. In the remaining phage particles ejection stopped after a defined amount of DNA was reached inside the capsid. Direct measurement of the size of non-ejected DNA by gel electrophoresis at different PEG concentrations in the latter sub-population allowed determination of the external osmotic pressure that balances the force powering DNA exit (47 atm for SPP1 wild-type). DNA exit stops when the ejection force mainly due to repulsion between DNA strands inside the SPP1 capsid equalizes the force resisting DNA insertion into the PEG solution. Considering the turgor pressure in the Bacillus subtilis cytoplasm the energy stored in the tight phage DNA packing is only sufficient to power entry of the first 17% of the SPP1 chromosome into the cell, the remaining 83% requiring application of additional force for internalization.

Portal motor velocity and internal force resisting viral DNA packaging in bacteriophage phi29

During the assembly of many viruses, a powerful molecular motor compacts the genome into a preassembled capsid. Here, we present measurements of viral DNA packaging in bacteriophage phi29 using an improved optical tweezers method that allows DNA translocation to be measured from initiation to completion. This method allowed us to study the previously uncharacterized early stages of packaging and facilitated more accurate measurement of the length of DNA packaged. We measured the motor velocity versus load at near-zero filling and developed a ramped DNA stretching technique that allowed us to measure the velocity versus capsid filling at near-zero load. These measurements reveal that the motor can generate significantly higher velocities and forces than detected previously. Toward the end of packaging, the internal force resisting DNA confinement rises steeply, consistent with the trend predicted by many theoretical models. However, the force rises to a higher magnitude, particularly during the early stages of packaging, than predicted by models that assume coaxial inverse spooling of the DNA. This finding suggests that the DNA is not arranged in that conformation during the early stages of packaging and indicates that internal force is available to drive complete genome ejection in vitro. The maximum force exceeds 100 pN, which is about one-half that predicted to rupture the capsid shell.

Viral capsids: mechanical characteristics, genome packaging and delivery mechanisms

The main functions of viral capsids are to protect, transport and deliver their genome. The mechanical properties of capsids are supposed to be adapted to these tasks. Bacteriophage capsids also need to withstand the high pressures the DNA is exerting onto it as a result of the DNA packaging and its consequent confinement within the capsid. It is proposed that this pressure helps driving the genome into the host, but other mechanisms also seem to play an important role in ejection. DNA packaging and ejection strategies are obviously dependent on the mechanical properties of the capsid. This review focuses on the mechanical properties of viral capsids in general and the elucidation of the biophysical aspects of genome packaging mechanisms and genome delivery processes of double-stranded DNA bacteriophages in particular.