Bacteriophage P22 SieA-mediated superinfection exclusion

Many temperate phages encode prophage-expressed functions that interfere with superinfection of the host bacterium by external phages. Salmonella phage P22 has four such systems that are expressed from the prophage in a lysogen that are encoded by the c2 (repressor), gtrABC, sieA, and sieB genes. Here we report that the P22-encoded SieA protein is necessary and sufficient for exclusion by the SieA system and that it is an inner membrane protein that blocks DNA injection by P22 and its relatives, but has no effect on infection by other tailed phage types. The P22 virion injects its DNA through the host cell membranes and periplasm via a conduit assembled from three "ejection proteins" after their release from the virion. Phage P22 mutants that overcome the SieA block were isolated, and they have amino acid changes in the C-terminal regions of the gene 16 and 20 encoded ejection proteins. Three different single-amino acid changes in these proteins are required to obtain nearly full resistance to SieA. Hybrid P22 phages that have phage HK620 ejection protein genes are also partially resistant to SieA. There are three sequence types of extant phage-encoded SieA proteins that are less than 30% identical to one another, yet comparison of two of these types found no differences in phage target specificity. Our data strongly suggest a model in which the inner membrane protein SieA interferes with the assembly or function of the periplasmic gp20 and membrane-bound gp16 DNA delivery conduit.IMPORTANCEThe ongoing evolutionary battle between bacteria and the viruses that infect them is a critical feature of bacterial ecology on Earth. Viruses can kill bacteria by infecting them. However, when their chromosomes are integrated into a bacterial genome as a prophage, viruses can also protect the host bacterium by expressing genes whose products defend against infection by other viruses. This defense property is called "superinfection exclusion." A significant fraction of bacteria harbor prophages that encode such protective systems, and there are many different molecular strategies by which superinfection exclusion is mediated. This report is the first to describe the mechanism by which bacteriophage P22 SieA superinfection exclusion protein protects its host bacterium from infection by other P22-like phages. The P22 prophage-encoded inner membrane SieA protein prevents infection by blocking transport of superinfecting phage DNA across the inner membrane during injection.

Bacteriophage P22 SieA mediated superinfection exclusion

Many temperate phages encode prophage-expressed functions that interfere with superinfection of the host bacterium by external phages. Salmonella phage P22 has four such systems that are expressed from the prophage in a lysogen that are encoded by the c2 (repressor), gtrABC, sieA, and sieB genes. Here we report that the P22-encoded SieA protein is the only phage protein required for exclusion by the SieA system, and that it is an inner membrane protein that blocks DNA injection by P22 and its relatives, but has no effect on infection by other tailed phage types. The P22 virion injects its DNA through the host cell membranes and periplasm via a conduit assembled from three "ejection proteins" after their release from the virion. Phage P22 mutants were isolated that overcome the SieA block, and they have amino acid changes in the C-terminal regions of the gene 16 and 20 encoded ejection proteins. Three different single amino acid changes in these proteins are required to obtain nearly full resistance to SieA. Hybrid P22 phages that have phage HK620 ejection protein genes are also partially resistant to SieA. There are three sequence types of extant phage-encoded SieA proteins that are less than 30% identical to one another, yet comparison of two of these types found no differences in target specificity. Our data are consistent with a model in which the inner membrane protein SieA interferes with the assembly or function of the periplasmic gp20 and membrane-bound gp16 DNA delivery conduit.

A Klebsiella pneumoniae NDM-1+ bacteriophage: Adaptive polyvalence and disruption of heterogenous biofilms

Bacteriophage KL-2146 is a lytic virus isolated to infect Klebsiella pneumoniae BAA2146, a pathogen carrying the broad range antibiotic resistance gene New Delhi metallo-betalactamase-1 (NDM-1). Upon complete characterization, the virus is shown to belong to the Drexlerviridae family and is a member of the Webervirus genus located within the (formerly) T1-like cluster of phages. Its double-stranded (dsDNA) genome is 47,844 bp long and is predicted to have 74 protein-coding sequences (CDS). After challenging a variety of K. pneumoniae strains with phage KL-2146, grown on the NDM-1 positive strain BAA-2146, polyvalence was shown for a single antibiotic-sensitive strain, K. pneumoniae 13,883, with a very low initial infection efficiency in liquid culture. However, after one or more cycles of infection in K. pneumoniae 13,883, nearly 100% infection efficiency was achieved, while infection efficiency toward its original host, K. pneumoniae BAA-2146, was decreased. This change in host specificity is reversible upon re-infection of the NDM-1 positive strain (BAA-2146) using phages grown on the NDM-1 negative strain (13883). In biofilm infectivity experiments, the polyvalent nature of KL-2146 was demonstrated with the killing of both the multidrug-resistant K. pneumoniae BAA-2146 and drug-sensitive 13,883 in a multi-strain biofilm. The ability to infect an alternate, antibiotic-sensitive strain makes KL-2146 a useful model for studying phages infecting the NDM-1+ strain, K. pneumoniae BAA-2146. GRAPHICAL ABSTRACT.

Hybrid Vigor: Importance of Hybrid λ Phages in Early Insights in Molecular Biology

Laboratory-generated hybrids between phage λ and related phages played a seminal role in establishment of the λ model system, which, in turn, served to develop many of the foundational concepts of molecular biology, including gene structure and control. Important λ hybrids with phages 21 and 434 were the earliest of such phages. To understand the biology of these hybrids in full detail, we determined the complete genome sequences of phages 21 and 434. Although both genomes are canonical members of the λ-like phage family, they both carry unsuspected bacterial virulence gene types not previously described in this group of phages. In addition, we determined the sequences of the hybrid phages λ imm21, λ imm434, and λ h434 imm21. These sequences show that the replacements of λ DNA by nonhomologous segments of 21 or 434 DNA occurred through homologous recombination in adjacent sequences that are nearly identical in the parental phages. These five genome sequences correct a number of errors in published sequence fragments of the 21 and 434 genomes, and they point out nine nucleotide differences from Sanger's original λ sequence that are likely present in most extant λ strains in laboratory use today. We discuss the historical importance of these hybrid phages in the development of fundamental tenets of molecular biology and in some of the earliest gene cloning vectors. The 434 and 21 genomes reinforce the conclusion that the genomes of essentially all natural λ-like phages are mosaics of sequence modules from a pool of exchangeable segments.

Genome Sequence of Salmonella enterica Serovar Typhimurium Bacteriophage MG40

We report the complete genome sequence of P22-like Salmonella enterica serovar Typhimurium phage MG40, whose prophage repressor specificity is different from that of other known temperate phages.

The phage L capsid decoration protein has a novel OB-fold and an unusual capsid binding strategy

The major coat proteins of dsDNA tailed phages (order Caudovirales) and herpesviruses form capsids by a mechanism that includes active packaging of the dsDNA genome into a precursor procapsid, followed by expansion and stabilization of the capsid. These viruses have evolved diverse strategies to fortify their capsids, such as non-covalent binding of auxiliary 'decoration' (Dec) proteins. The Dec protein from the P22-like phage L has a highly unusual binding strategy that distinguishes between nearly identical three-fold and quasi-three-fold sites of the icosahedral capsid. Cryo-electron microscopy and three-dimensional image reconstruction were employed to determine the structure of native phage L particles. NMR was used to determine the structure/dynamics of Dec in solution. The NMR structure and the cryo-EM density envelope were combined to build a model of the capsid-bound Dec trimer. Key regions that modulate the binding interface were verified by site-directed mutagenesis.

Cryo-EM Elucidation of the Structure of Bacteriophage P22 Virions after Genome Release

Genome ejection proteins are required to facilitate transport of bacteriophage P22 double-stranded DNA safely through membranes of Salmonella. The structures and locations of all proteins in the context of the mature virion are known, with the exception of three ejection proteins. Furthermore, the changes that occur to the proteins residing in the mature virion upon DNA release are not fully understood. We used cryogenic electron microscopy to obtain what is, to our knowledge, the first asymmetric reconstruction of mature bacteriophage P22 after double-stranded DNA has been extruded from the capsid-a state representative of one step during viral infection. Results of icosahedral and asymmetric reconstructions at estimated resolutions of 7.8 and 12.5 Å resolutions, respectively, are presented. The reconstruction shows tube-like protein density extending from the center of the tail assembly. The portal protein does not revert to the more contracted, procapsid state, but instead maintains an extended and splayed barrel structure. These structural details contribute to our understanding of the molecular mechanism of P22 phage infection and also set the foundation for future exploitation serving engineering purposes.

Genes affecting progression of bacteriophage P22 infection in Salmonella identified by transposon and single gene deletion screens

Bacteriophages rely on their hosts for replication, and many host genes critically determine either viral progeny production or host success via phage resistance. A random insertion transposon library of 240,000 mutants in Salmonella enterica serovar Typhimurium was used to monitor effects of individual bacterial gene disruptions on bacteriophage P22 lytic infection. These experiments revealed candidate host genes that alter the timing of phage P22 propagation. Using a False Discovery Rate of < 0.1, mutations in 235 host genes either blocked or delayed progression of P22 lytic infection, including many genes for which this role was previously unknown. Mutations in 77 genes reduced the survival time of host DNA after infection, including mutations in genes for enterobacterial common antigen (ECA) synthesis and osmoregulated periplasmic glucan (OPG). We also screened over 2000 Salmonella single gene deletion mutants to identify genes that impacted either plaque formation or culture growth rates. The gene encoding the periplasmic membrane protein YajC was newly found to be essential for P22 infection. Targeted mutagenesis of yajC shows that an essentially full-length protein is required for function, and potassium efflux measurements demonstrated that YajC is critical for phage DNA ejection across the cytoplasmic membrane.

The genome sequence of Escherichia coli tailed phage D6 and the diversity of Enterobacteriales circular plasmid prophages

The temperate Escherichia coli bacteriophage D6 can exist as a circular plasmid prophage, and we report here its 91,159bp complete genome sequence. It is a distant relative of the well-studied phage P1, but it is sufficiently different that it typifies a previously undescribed tailed phage type or cluster. Examination of the database of bacterial genome sequences revealed that phage P1 and D6 prophage plasmids are common in the Enterobacteriales, and in addition, previously described Salmonella phage SSU5 represents a different type of temperate tailed phage with a circular plasmid prophage that is also very common in this host order. This analysis also discovered additional divergent clusters of putative circular plasmid prophages within the two larger P1 and SSU5 groups (superclusters) that inhabit the Enterobacteriales as well as bacteria in several other orders in the Gamma-proteobacteria class. Very few of these sequences are annotated as putative prophages.

Complete genome sequence of lytic bacteriophage RG-2014 that infects the multidrug resistant bacterium Delftia tsuruhatensis ARB-1

A lytic bacteriophage RG-2014 infecting a biofilm forming multidrug resistant bacterium Delftia tsuruhatensis strain ARB-1 as its host was isolated from a full-scale municipal wastewater treatment plant. Lytic phage RG-2014 was isolated for developing phage based therapeutic approaches against Delftia tsuruhatensis strain ARB-1. The strain ARB-1 belongs to the Comamonadaceae family of the Betaproteobacteria class. RG-2014 was characterized for its type, burst size, latent and eclipse time periods of 150 ± 9 PFU/cell, 10-min, <5-min, respectively. The phage was found to be a dsDNA virus belonging to the Podoviridae family. It has an isometric icosahedrally shaped capsid with a diameter of 85 nm. The complete genome of the isolated phage was sequenced and determined to be 73.8 kbp in length with a G + C content of 59.9%. Significant similarities in gene homology and order were observed between Delftia phage RG-2014 and the E. coli phage N4 indicating that it is a member of the N4-like phage group.

Plasmid diversity and phylogenetic consistency in the Lyme disease agent Borrelia burgdorferi

Background

Bacteria from the genus Borrelia are known to harbor numerous linear and circular plasmids. We report here a comparative analysis of the nucleotide sequences of 236 plasmids present in fourteen independent isolates of the Lyme disease agent B. burgdorferi.

Results

We have sequenced the genomes of 14 B. burgdorferi sensu stricto isolates that carry a total of 236 plasmids. These individual isolates carry between seven and 23 plasmids. Their chromosomes, the cp26 and cp32 circular plasmids, as well as the lp54 linear plasmid, are quite evolutionarily stable; however, the remaining plasmids have undergone numerous non-homologous and often duplicative recombination events. We identify 32 different putative plasmid compatibility types among the 236 plasmids, of which 15 are (usually) circular and 17 are linear. Because of past rearrangements, any given gene, even though it might be universally present in these isolates, is often found on different linear plasmid compatibility types in different isolates. For example, the arp gene and the vls cassette region are present on plasmids of four and five different compatibility types, respectively, in different isolates. A majority of the plasmid types have more than one organizationally different subtype, and the number of such variants ranges from one to eight among the 18 linear plasmid types. In spite of this substantial organizational diversity, the plasmids are not so variable that every isolate has a novel version of every plasmid (i.e., there appears to be a limited number of extant plasmid subtypes).

Conclusions

Although there have been many past recombination events, both homologous and nonhomologous, among the plasmids, particular organizational variants of these plasmids correlate with particular chromosomal genotypes, suggesting that there has not been rapid horizontal transfer of whole linear plasmids among B. burgdorferi lineages. We argue that plasmid rearrangements are essentially non-revertable and are present at a frequency of only about 0.65% that of single nucleotide changes, making rearrangement-derived novel junctions (mosaic boundaries) ideal phylogenetic markers in the study of B. burgdorferi population structure and plasmid evolution and exchange.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3553-5) contains supplementary material, which is available to authorized users.

Three-dimensional reconstructions of the bacteriophage CUS-3 virion reveal a conserved coat protein I-domain but a distinct tailspike receptor-binding domain

CUS-3 is a short-tailed, dsDNA bacteriophage that infects serotype K1 Escherichia coli. We report icosahedrally averaged and asymmetric, three-dimensional, cryo-electron microscopic reconstructions of the CUS-3 virion. Its coat protein structure adopts the "HK97-fold" shared by other tailed phages and is quite similar to that in phages P22 and Sf6 despite only weak amino acid sequence similarity. In addition, these coat proteins share a unique extra external domain ("I-domain"), suggesting that the group of P22-like phages has evolved over a very long time period without acquiring a new coat protein gene from another phage group. On the other hand, the morphology of the CUS-3 tailspike differs significantly from that of P22 or Sf6, but is similar to the tailspike of phage K1F, a member of the extremely distantly related T7 group of phages. We conclude that CUS-3 obtained its tailspike gene from a distantly related phage quite recently.

OmpA and OmpC are critical host factors for bacteriophage Sf6 entry in Shigella

Despite being essential for successful infection, the molecular cues involved in host recognition and genome transfer of viruses are not completely understood. Bacterial outer membrane proteins A and C co-purify in lipid vesicles with bacteriophage Sf6, implicating both outer membrane proteins as potential host receptors. We determined that outer membrane proteins A and C mediate Sf6 infection by dramatically increasing its rate and efficiency. We performed a combination of in vivo studies with three omp null mutants of Shigella flexneri, including classic phage plaque assays and time-lapse fluorescence microscopy to monitor genome ejection at the single virion level. Cryo-electron tomography of phage 'infecting' outer membrane vesicles shows the tail needle contacting and indenting the outer membrane. Lastly, in vitro ejection studies reveal that lipopolysaccharide and outer membrane proteins are both required for Sf6 genome release. We conclude that Sf6 phage entry utilizes either outer membrane proteins A or C, with outer membrane protein A being the preferred receptor.

The tip of the tail needle affects the rate of DNA delivery by bacteriophage P22

The P22-like bacteriophages have short tails. Their virions bind to their polysaccharide receptors through six trimeric tailspike proteins that surround the tail tip. These short tails also have a trimeric needle protein that extends beyond the tailspikes from the center of the tail tip, in a position that suggests that it should make first contact with the host’s outer membrane during the infection process. The base of the needle serves as a plug that keeps the DNA in the virion, but role of the needle during adsorption and DNA injection is not well understood. Among the P22-like phages are needle types with two completely different C-terminal distal tip domains. In the phage Sf6-type needle, unlike the other P22-type needle, the distal tip folds into a “knob” with a TNF-like fold, similar to the fiber knobs of bacteriophage PRD1 and Adenovirus. The phage HS1 knob is very similar to that of Sf6, and we report here its crystal structure which, like the Sf6 knob, contains three bound L-glutamate molecules. A chimeric P22 phage with a tail needle that contains the HS1 terminal knob efficiently infects the P22 host, Salmonella enterica, suggesting the knob does not confer host specificity. Likewise, mutations that should abrogate the binding of L-glutamate to the needle do not appear to affect virion function, but several different other genetic changes to the tip of the needle slow down potassium release from the host during infection. These findings suggest that the needle plays a role in phage P22 DNA delivery by controlling the kinetics of DNA ejection into the host.

Function and horizontal transfer of the small terminase subunit of the tailed bacteriophage Sf6 DNA packaging nanomotor

Bacteriophage Sf6 DNA packaging series initiate at many locations across a 2kbp region. Our in vivo studies show that Sf6 small terminase subunit (TerS) protein recognizes a specific packaging (pac) site near the center of this region, that this site lies within the portion of the Sf6 gene that encodes the DNA-binding domain of TerS protein, that this domain of the TerS protein is responsible for the imprecision in Sf6 packaging initiation, and that the DNA-binding domain of TerS must be covalently attached to the domain that interacts with the rest of the packaging motor. The TerS DNA-binding domain is self-contained in that it apparently does not interact closely with the rest of the motor and it binds to a recognition site that lies within the DNA that encodes the domain. This arrangement has allowed the horizontal exchange of terS genes among phages to be very successful.

Unraveling the role of the C-terminal helix turn helix of the coat-binding domain of bacteriophage P22 scaffolding protein

Many viruses encode scaffolding and coat proteins that co-assemble to form procapsids, which are transient precursor structures leading to progeny virions. In bacteriophage P22, the association of scaffolding and coat proteins is mediated mainly by ionic interactions. The coat protein-binding domain of scaffolding protein is a helix turn helix structure near the C terminus with a high number of charged surface residues. Residues Arg-293 and Lys-296 are particularly important for coat protein binding. The two helices contact each other through hydrophobic side chains. In this study, substitution of the residues of the interface between the helices, and the residues in the β-turn, by aspartic acid was used examine the importance of the conformation of the domain in coat binding. These replacements strongly affected the ability of the scaffolding protein to interact with coat protein. The severity of the defect in the association of scaffolding protein to coat protein was dependent on location, with substitutions at residues in the turn and helix 2 causing the most significant effects. Substituting aspartic acid for hydrophobic interface residues dramatically perturbs the stability of the structure, but similar substitutions in the turn had much less effect on the integrity of this domain, as determined by circular dichroism. We propose that the binding of scaffolding protein to coat protein is dependent on angle of the β-turn and the orientation of the charged surface on helix 2. Surprisingly, formation of the highly complex procapsid structure depends on a relatively simple interaction.

Genome stability of Lyme disease spirochetes: comparative genomics of Borrelia burgdorferi plasmids

Lyme disease is the most common tick-borne human illness in North America. In order to understand the molecular pathogenesis, natural diversity, population structure and epizootic spread of the North American Lyme agent, Borrelia burgdorferi sensu stricto, a much better understanding of the natural diversity of its genome will be required. Towards this end we present a comparative analysis of the nucleotide sequences of the numerous plasmids of B. burgdorferi isolates B31, N40, JD1 and 297. These strains were chosen because they include the three most commonly studied laboratory strains, and because they represent different major genetic lineages and so are informative regarding the genetic diversity and evolution of this organism. A unique feature of Borrelia genomes is that they carry a large number of linear and circular plasmids, and this work shows that strains N40, JD1, 297 and B31 carry related but non-identical sets of 16, 20, 19 and 21 plasmids, respectively, that comprise 33–40% of their genomes. We deduce that there are at least 28 plasmid compatibility types among the four strains. The B. burgdorferi ∼900 Kbp linear chromosomes are evolutionarily exceptionally stable, except for a short ≤20 Kbp plasmid-like section at the right end. A few of the plasmids, including the linear lp54 and circular cp26, are also very stable. We show here that the other plasmids, especially the linear ones, are considerably more variable. Nearly all of the linear plasmids have undergone one or more substantial inter-plasmid rearrangements since their last common ancestor. In spite of these rearrangements and differences in plasmid contents, the overall gene complement of the different isolates has remained relatively constant.

Structural evolution of the P22-like phages: comparison of Sf6 and P22 procapsid and virion architectures

Coat proteins of tailed, dsDNA phages and in herpesviruses include a conserved core similar to the bacteriophage HK97 subunit. This core is often embellished with other domains such as the telokin Ig-like domain of phage P22. Eighty-six P22-like phages and prophages with sequenced genomes share a similar set of virion assembly genes and, based on comparisons of twelve viral assembly proteins (structural and assembly/packaging chaperones), these phages are classified into three groups (P22-like, Sf6-like, and CUS-3-like). We used cryo-electron microscopy and 3D image reconstruction to determine the structures of Sf6 procapsids and virions (~7Å resolution), and the structure of the entire, asymmetric Sf6 virion (16-Å resolution). The Sf6 coat protein is similar to that of P22 yet it has differences in the telokin domain and in its overall quaternary organization. Thermal stability and agarose gel experiments show that Sf6 virions are slightly less stable than those of P22. Finally, bacterial host outer membrane proteins A and C were identified in lipid vesicles that co-purify with Sf6 particles, but are not components of the capsid.

Decoding bacteriophage P22 assembly: identification of two charged residues in scaffolding protein responsible for coat protein interaction

Proper assembly of viruses must occur through specific interactions between capsid proteins. Many double-stranded DNA viruses and bacteriophages require internal scaffolding proteins to assemble their coat proteins into icosahedral capsids. The 303 amino acid bacteriophage P22 scaffolding protein is mostly helical, and its C-terminal helix-turn-helix (HTH) domain binds to the coat protein during virion assembly, directing the formation of an intermediate structure called the procapsid. The interaction between coat and scaffolding protein HTH domain is electrostatic, but the amino acids that form the protein-protein interface have yet to be described. In the present study, we used alanine scanning mutagenesis of charged surface residues of the C-terminal HTH domain of scaffolding protein. We have determined that P22 scaffolding protein residues R293 and K296 are crucial for binding to coat protein and that the neighboring charges are not essential but do modulate the affinity between the two proteins.

Determining DNA packaging strategy by analysis of the termini of the chromosomes in tailed-bacteriophage virions

Tailed-bacteriophage virions contain a single linear dsDNA chromosome which can range in size from about 18 to 500 kbp across the known tailed-phage types. These linear chromosomes can have one of several known types of termini as follows: cohesive ends (5'- or 3'-single-strand extensions), circularly permuted direct terminal repeats, short or long exact direct terminal repeats, terminal host DNA sequences, or covalently bound terminal proteins. These different types of ends reflect differing DNA replication strategies and especially differing terminase actions during DNA packaging. In general, complete genome sequence determination does not by itself elucidate the nature of these ends, so directed experimental analysis is usually required to understand the nature of the virion chromosome ends. This chapter discusses these methods.

Genomic analysis of bacteriophage epsilon 34 of Salmonella enterica serovar Anatum (15+)

Background

The presence of prophages has been an important variable in genetic exchange and divergence in most bacteria. This study reports the determination of the genomic sequence of Salmonella phage ε³⁴, a temperate bacteriophage that was important in the early study of prophages that modify their hosts' cell surface and is of a type (P22-like) that is common in Salmonella genomes.

Results

The sequence shows that ε³⁴ is a mosaically related member of the P22 branch of the lambdoid phages. Its sequence is compared with the known P22-like phages and several related but previously unanalyzed prophage sequences in reported bacterial genome sequences.

Conclusion

These comparisons indicate that there has been little if any genetic exchange within the procapsid assembly gene cluster with P22-like E. coli/Shigella phages that are have orthologous but divergent genes in this region. Presumably this observation reflects the fact that virion assembly proteins interact intimately and divergent proteins can no longer interact. On the other hand, non-assembly genes in the "ant moron" appear to be in a state of rapid flux, and regulatory genes outside the assembly gene cluster have clearly enjoyed numerous and recent horizontal exchanges with phages outside the P22-like group. The present analysis also shows that ε³⁴ harbors a gtrABC gene cluster which should encode the enzymatic machinery to chemically modify the host O antigen polysaccharide, thus explaining its ability to alter its host's serotype. A comprehensive comparative analysis of the known phage gtrABC gene clusters shows that they are highly mobile, having been exchanged even between phage types, and that most "bacterial" gtrABC genes lie in prophages that vary from being largely intact to highly degraded. Clearly, temperate phages are very major contributors to the O-antigen serotype of their Salmonella hosts.

Subunit conformations and assembly states of a DNA-translocating motor: the terminase of bacteriophage P22

Bacteriophage P22, a podovirus infecting strains of Salmonella typhimurium, packages a 42-kbp genome using a headful mechanism. DNA translocation is accomplished by the phage terminase, a powerful molecular motor consisting of large and small subunits. Although many of the structural proteins of the P22 virion have been well characterized, little is known about the terminase subunits and their molecular mechanism of DNA translocation. We report here structural and assembly properties of ectopically expressed and highly purified terminase large and small subunits. The large subunit (gp2), which contains the nuclease and ATPase activities of terminase, exists as a stable monomer with an alpha/beta fold. The small subunit (gp3), which recognizes DNA for packaging and may regulate gp2 activity, exhibits a highly alpha-helical secondary structure and self-associates to form a stable oligomeric ring in solution. For wild-type gp3, the ring contains nine subunits, as demonstrated by hydrodynamic measurements, electron microscopy, and native mass spectrometry. We have also characterized a gp3 mutant (Ala 112-->Thr) that forms a 10-subunit ring, despite a subunit fold indistinguishable from wild type. Both the nonameric and decameric gp3 rings exhibit nonspecific DNA-binding activity, and gp2 is able to bind strongly to the DNA/gp3 complex but not to DNA alone. We propose a scheme for the roles of P22 terminase large and small subunits in the recruitment and packaging of viral DNA and discuss the model in relation to proposals for terminase-driven DNA translocation in other phages.

Highly discriminatory binding of capsid-cementing proteins in bacteriophage L

Cementing proteins that bind to the virion surface have been described in double-stranded DNA viruses such as herpesvirus, adenovirus, and numerous bacteriophages. The three-dimensional structure of bacteriophage L determined by electron cryo-microscopy reveals binding modes of two cementing proteins-one, called Dec, encoded by phage gene orf134 and the other by an as yet unidentified gene. These two proteins form homotrimers and bind at the quasi 3-fold axes nearest the icosahedral 2-fold axes and at the icosahedral 3-fold vertices, respectively. They do not bind at the quasi 3-fold axes near the icosahedral 5-fold vertices. These observations indicate precise recognition of the two cementing proteins at a subset of the quasi equivalent sites on the phage capsid. Sequence analysis shows striking similarity between the C-terminal portion of phage L Dec protein and five regions in the long tail fiber of a T4-like phage, suggesting functional parallelism between them.

The structure of an infectious P22 virion shows the signal for headful DNA packaging

Bacteriophages, herpesviruses, and other large double-stranded DNA (dsDNA) viruses contain molecular machines that pump DNA into preassembled procapsids, generating internal capsid pressures exceeding, by 10-fold, that of bottled champagne. A 17 angstrom resolution asymmetric reconstruction of the infectious P22 virion reveals that tightly spooled DNA about the portal dodecamer forces a conformation that is significantly different from that observed in isolated portals assembled from ectopically expressed protein. We propose that the tight dsDNA spooling activates the switch that signals the headful chromosome packing density to the particle exterior.

Nucleotide sequence of the head assembly gene cluster of bacteriophage L and decoration protein characterization

The temperate Salmonella enterica bacteriophage L is a close relative of the very well studied bacteriophage P22. In this study we show that the L procapsid assembly and DNA packaging genes, which encode terminase, portal, scaffold, and coat proteins, are extremely close relatives of the homologous P22 genes (96.3 to 99.1% identity in encoded amino acid sequence). However, we also identify an L gene, dec, which is not present in the P22 genome and which encodes a protein (Dec) that is present on the surface of L virions in about 150 to 180 molecules/virion. We also show that the Dec protein is a trimer in solution and that it binds to P22 virions in numbers similar to those for L virions. Its binding dramatically stabilizes P22 virions against disruption by a magnesium ion chelating agent. Dec protein binds to P22 coat protein shells that have expanded naturally in vivo or by sodium dodecyl sulfate treatment in vitro but does not bind to unexpanded procapsid shells. Finally, analysis of phage L restriction site locations and a number of patches of nucleotide sequence suggest that phages ST64T and L are extremely close relatives, perhaps the two closest relatives that have been independently isolated to date among the lambdoid phages.

The generalized transducing Salmonella bacteriophage ES18: complete genome sequence and DNA packaging strategy

The generalized transducing double-stranded DNA bacteriophage ES18 has an icosahedral head and a long noncontractile tail, and it infects both rough and smooth Salmonella enterica strains. We report here the complete 46,900-bp genome nucleotide sequence and provide an analysis of the sequence. Its 79 genes and their organization clearly show that ES18 is a member of the lambda-like (lambdoid) phage group; however, it contains a novel set of genes that program assembly of the virion head. Most of its integration-excision, immunity, Nin region, and lysis genes are nearly identical to those of the short-tailed Salmonella phage P22, while other early genes are nearly identical to Escherichia coli phages lambda and HK97, S. enterica phage ST64T, or a Shigella flexneri prophage. Some of the ES18 late genes are novel, while others are most closely related to phages HK97, lambda, or N15. Thus, the ES18 genome is mosaically related to other lambdoid phages, as is typical for all group members. Analysis of virion DNA showed that it is circularly permuted and about 10% terminally redundant and that initiation of DNA packaging series occurs across an approximately 1-kbp region rather than at a precise location on the genome. This supports a model in which ES18 terminase can move substantial distances along the DNA between recognition and cleavage of DNA destined to be packaged. Bioinformatic analysis of large terminase subunits shows that the different functional classes of phage-encoded terminases can usually be predicted from their amino acid sequence.

The chromosome of Shigella flexneri bacteriophage Sf6: complete nucleotide sequence, genetic mosaicism, and DNA packaging

Shigella flexneri temperate bacteriophage Sf6 is of interest in part because its prophage expresses the oac gene that alters the antigenic properties of the surface O-antigen polysaccharide of its host bacterium. We have determined the complete sequence of its 39,044 bp genome. The sequence shows that Sf6 is a member of the canonical lambdoid phage group, and like other phages of this type has a highly mosaic genome. It has chromosomal regions that encode proteins >80% identical with at least 15 different previously characterized lambdoid phages and prophages, but 43% of the genome, including the virion assembly genes, is homologous to the genome of one phage, HK620. An analysis of the nucleotide differences between Sf6 and HK620 indicates that even these similar regions are highly mosaic. This mosaicism suggests ways in which the virion structural proteins might interact with each other. The Sf6 early operons are arranged like a typical lambdoid phage, with "boundary sequences" often found between functional modules in the "metabolic" genome domain. By virtue of high degree of similarity in the encoding genes and their DNA target sites, we predict that the integrase, early transcription anti-terminator, CI and Cro repressors, and CII protein of Sf6 have DNA binding specificities very similar to the homologous proteins encoded by phages HK620, lambda, 434 and P22, respectively. The late operon contains two tRNA genes. The Sf6 terminase genes are unusual. Analysis of in vivo initiation of the DNA packaging series showed that the Sf6 apparatus that recognizes DNA for packaging appears to cleave DNA for initiation of packaging series at many sites within a large region of about 1800 bp that includes a possible pac site. This is unlike previously characterized phage packaging mechanisms.

The pKO2 linear plasmid prophage of Klebsiella oxytoca

Temperate bacteriophages with plasmid prophages are uncommon in nature, and of these only phages N15 and PY54 are known to have a linear plasmid prophage with closed hairpin telomeres. We report here the complete nucleotide sequence of the 51,601-bp Klebsiella oxytoca linear plasmid pKO2, and we demonstrate experimentally that it is also a prophage. We call this bacteriophage phiKO2. An analysis of the 64 predicted phiKO2 genes indicate that it is a fairly close relative of phage N15; they share a mosaic relationship that is typical of different members of double-stranded DNA tailed-phage groups. Although the head, tail shaft, and lysis genes are not recognizably homologous between these phages, other genes such as the plasmid partitioning, replicase, prophage repressor, and protelomerase genes (and their putative targets) are so similar that we predict that they must have nearly identical DNA binding specificities. The phiKO2 virion is unusual in that its phage lambda-like tails have an exceptionally long (3,433 amino acids) central tip tail fiber protein. The phiKO2 genome also carries putative homologues of bacterial dinI and umuD genes, both of which are involved in the host SOS response. We show that these divergently transcribed genes are regulated by LexA protein binding to a single target site that overlaps both promoters.

Bidirectional replication from an internal ori site of the linear N15 plasmid prophage

The prophage of coliphage N15 is not integrated into the chromosome but exists as a linear plasmid molecule with covalently closed hairpin ends (telomeres). Upon infection the injected phage DNA circularizes via its cohesive ends. Then, a phage-encoded enzyme, protelomerase, cuts the circle and forms the hairpin telomeres. N15 protelomerase acts as a telomere-resolving enzyme during prophage DNA replication. We characterized the N15 replicon and found that replication of circular N15 miniplasmids requires only the repA gene, which encodes a multidomain protein homologous to replication proteins of bacterial plasmids replicated by a theta-mechanism. Replication of a linear N15 miniplasmid also requires the protelomerase gene and telomere regions. N15 prophage replication is initiated at an internal ori site located within repA and proceeds bidirectionally. Electron microscopy data suggest that after duplication of the left telomere, protelomerase cuts this site generating Y-shaped molecules. Full replication of the molecule and subsequent resolution of the right telomere then results in two linear plasmid molecules. N15 prophage replication thus appears to follow a mechanism that is distinct from that employed by eukaryotic replicons with this type of telomere and suggests the possibility of evolutionarily independent appearances of prokaryotic and eukaryotic replicons with covalently closed telomeres.

Corrected sequence of the bacteriophage p22 genome

We report the first accurate genome sequence for bacteriophage P22, correcting a 0.14% error rate in previously determined sequences. DNA sequencing technology is now good enough that genomes of important model systems like P22 can be sequenced with essentially 100% accuracy with minimal investment of time and resources.