Genomic Analysis of Bacteriophages SP6 and K1-5, an Estranged Subgroup of the T7 Supergroup

Single-mode termination of phage transcriptions, disclosing bacterial adaptation for facilitated reinitiations

Bacterial and bacteriophage RNA polymerases (RNAPs) have divergently evolved and share the RNA hairpin-dependent intrinsic termination of transcription. Here, we examined phage T7, T3 and SP6 RNAP terminations utilizing the single-molecule fluorescence assays we had developed for bacterial terminations. We discovered the phage termination mode or outcome is virtually single with decomposing termination. Therein, RNAP is displaced forward along DNA and departs both RNA and DNA for one-step decomposition, three-dimensional diffusion and reinitiation at any promoter. This phage displacement-mediated decomposing termination is much slower than readthrough and appears homologous with the bacterial one. However, the phage sole mode of termination contrasts with the bacterial dual mode, where both decomposing and recycling terminations occur compatibly at any single hairpin- or Rho-dependent terminator. In the bacterial recycling termination, RNA is sheared from RNA·DNA hybrid, and RNAP remains bound to DNA for one-dimensional diffusion, which enables facilitated recycling for reinitiation at the nearest promoter located downstream or upstream in the sense or antisense orientation. Aligning with proximity of most terminators to adjacent promoters in bacterial genomes, the shearing-mediated recycling termination could be bacterial adaptation for the facilitated reinitiations repeated at a promoter for accelerated expression and coupled at adjoining promoters for coordinated regulation.

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.

pH modulates the role of SP6 RNA polymerase in transcription process: an in silico study

SP6 RNA polymerase (SP6 RNAP) is an essential enzyme for the transcription process in SP6 bacteriophage. SP6 RNAP plays a vital role in mRNA vaccine designing technology and other translational biotechnology research due to the high specificity towards its promoter. The self-replicating performance also put this polymerase to study extensively. Despite of the reports emphasizing the function of this enzyme, a detailed structural and functional understanding of RNA polymerase is not reported so far. Here, we report the first-ever information about SP6RNAP structure and its effect on promoter binding at different pH environments using molecular docking and molecular dynamics simulation (MDS) study. We also report the changes in polymerase conformations in different pH conditions using in-silico approach. The docking study was also performed for SP6 RNAP with SP6 promoter at different pH environments using the in-silico docking tools and conducted the MDS study for complexes. MM/PBSA and per residue energy contribution has been performed at three different pH environments. The structural aspects confirmed that the pH 7.9 state favors the polymerase functional activity in the transcription process which was in the range reported using transcription assay. This polymerase's unique features may play its emerging role as an efficient transcription factor in translational biological research.Communicated by Ramaswamy H. Sarma.

Three novel Erwinia billingiae phages isolated from organic waste represent three new genera

Despite the ecological significance of viral communities, phages remain insufficiently studied. Current genomic databases lack high-quality phage genome sequences linked to specific bacteria. Bacteria of the genus Erwinia are known to colonize the phyllosphere of plants, both as commensals and as pathogens. We isolated three Erwinia billingiae phages-Zoomie, Pecta, and Snitter-from organic household waste. Based on sequence similarity to their closest relatives, we propose that they represent three new genera: "Pectavirus" within the family Zobellviridae, "Snittervirus" in the subfamily Tempevirinae, family Drexlerviridae, and "Zoomievirus" within the family Autographiviridae, which, together with the genus Limelightvirus, may constitute a new subfamily.

Microencapsulation of Bacteriophages for the Delivery to and Modulation of the Human Gut Microbiota through Milk and Cereal Products

There is a bidirectional interaction between the gut microbiota and human health status. Disturbance of the microbiota increases the risk of pathogen infections and other diseases. The use of bacteriophages as antibacterial therapy or prophylaxis is intended to counteract intestinal disorders. To deliver bacteriophages unharmed into the gut, they must be protected from acidic conditions in the stomach. Therefore, an encapsulation method based on in situ complexation of alginate (2%), calcium ions (0.5%), and milk proteins (1%) by spray drying was investigated. Powdered capsules with particle sizes of ~10 µm and bacteriophage K5 titers of ~108 plaque forming units (pfu) g−1 were obtained. They protected the bacteriophages from acid (pH 2.5) in the stomach for 2 h and released them within 30 min under intestinal conditions (in vitro). There was no loss of viability during storage over two months (4 °C). Instead of consuming bacteriophage capsules in pure form (i.e., as powder/tablets), they could be inserted into food matrices, as exemplary shown in this study using cereal cookies as a semi-solid food matrix. By consuming bacteriophages in combination with probiotic organisms (e.g., via yoghurt with cereal cookies), probiotics could directly repopulate the niches generated by bacteriophages and, thus, contribute to a healthier life.

λ Recombineering Used to Engineer the Genome of Phage T7

Bacteriophage T7 and T7-like bacteriophages are valuable genetic models for lytic phage biology that have heretofore been intractable with in vivo genetic engineering methods. This manuscript describes that the presence of λ Red recombination proteins makes in vivo recombineering of T7 possible, so that single base changes and whole gene replacements on the T7 genome can be made. Red recombination functions also increase the efficiency of T7 genome DNA transfection of cells by ~100-fold. Likewise, Red function enables two other T7-like bacteriophages that do not normally propagate in E. coli to be recovered following genome transfection. These results constitute major technical advances in the speed and efficiency of bacteriophage T7 engineering and will aid in the rapid development of new phage variants for a variety of applications.

Isolation and Characterization of Bacteriophages That Infect Citrobacter rodentium, a Model Pathogen for Intestinal Diseases

Enteropathogenic Escherichia coli (EPEC) is a major pathogen for diarrheal diseases among children. Antibiotics, when used appropriately, are effective; however, their overuse and misuse have led to the rise of antibiotic resistance worldwide. Thus, there are renewed efforts into the development of phage therapy as an alternative antibacterial therapy. Because EPEC in vivo models have shortcomings, a surrogate is used to study the mouse pathogen Citrobacter rodentium in animal models. In this study, two new phages CrRp3 and CrRp10, which infect C. rodentium, were isolated and characterized. CrRp3 was found to be a new species within the genus Vectrevirus, and CrRp10 is a new strain within the species Escherichia virus Ime09, in the genus Tequatrovirus. Both phages appear to have independently evolved from E. coli phages, rather than other Citrobacter spp. phages. Neither phage strain carries known genes associated with bacterial virulence, antibiotic resistance, or lysogeny. CrRp3 is more potent, having a 24-fold faster adsorption rate and shorter lytic cycle when compared to the same properties of CrRp10. However, a lysis curve analysis revealed that CrRp10 prevented growth of C. rodentium for 18 h, whereas resistance developed against CrRp3 within 9 h. We also show that hypoxic (5% oxygen) conditions decreased CrRp3 ability to control bacterial densities in culture. In contrast, low oxygen conditions did not affect CrRp10 ability to replicate on C. rodentium. Together, CrRp10 is likely to be the better candidate for future phage therapy investigations.

Engineering of receptor-binding proteins in bacteriophages and phage tail-like bacteriocins

Bacteriophages and phage tail-like bacteriocins (PTLBs) rely on receptor-binding proteins (RBPs) located in tail fibers or spikes for an initial and specific interaction with susceptible bacteria. Bacteriophages kill bacteria through a lytic, replicative cycle, whereas PTLBs kill the target through membrane depolarization in a single hit mechanism. Extensive efforts in the engineering of RBPs of both phages and PTLBs have been undertaken to obtain a greater understanding of the structural organization of RBPs. In addition, a major goal of engineering RBPs of phages and PTLBs is the production of antibacterials with a customized spectrum. Swapping of the RBP of phages and PTLBs results in a shift in activity spectrum in accordance with the spectrum of the new RBP. The engineering of strictly virulent phages with new RBPs required significant technical advances in the past decades, whereas the engineering of RBPs of PTLBs relied on the traditional molecular techniques used for the manipulation of bacteria and was thus relatively straightforward. While phages and PTLBs share their potential for specificity tuning, specific features of phages such as their lytic killing mechanism, their self-replicative nature and thus different pharmacokinetics and their potential to co-evolve are clear differentiators compared with PTLBs in terms of their antibacterial use.

Antibiotic Therapy Using Phage Depolymerases: Robustness Across a Range of Conditions

Phage-derived depolymerases directed against bacterial capsules are showing therapeutic promise in various animal models of infection. However, individual animal model studies are often constrained by use of highly specific protocols, such that results may not generalize to even slight modifications. Here we explore the robustness of depolymerase therapies shown to succeed in a previous study of mice. Treatment success rates were reduced by treatment delay, more so for some enzymes than others: K1- and K5 capsule-degrading enzymes retained partial efficacy on delay, while K30 depolymerase did not. Phage were superior to enzymes under delayed treatment only for K1. Route of administration (intramuscular versus intraperitoneal) mattered for success of K1E, possibly for K1F, not for K1H depolymerase. Significantly, K1 capsule-degrading enzymes proved highly successful when using immune-suppressed, leukopenic mice, even with delayed treatment. Evolution of bacteria resistant to K1-degrading enzymes did not thwart therapeutic success in leukopenic mice, likely because resistant bacteria were avirulent. In combination with previous studies these results continue to support the efficacy of depolymerases as antibacterial agents in vivo, but system-specific details are becoming evident.

Isolation and characterization of a group of new Proteus bacteriophages

Four lytic Proteus bacteriophages, PM75, PM85, PM93, and PM116, which are active against multi-drug-resistant strains of P. mirabilis, were isolated from cattle and poultry samples. According to electron microscopy data, all of the investigated phages belonged to the family Podoviridae. They all demonstrated lytic activity against sensitive strains of P. mirabilis, and three of the phages, PM85, PM93, and PM116, are potential candidates for use in antibacterial treatment. The genomes and putative proteins of bacteriophages PM85, PM93, and PM116 were similar to those of Proteus phage vB_PmiP_Pm5460 [KP890822], and the investigated phages formed a distinct clade within the genus Sp6virus, subfamily Autographivirinae. The genome sequence of phage PM75 was similar to that of a previously described Proteus phage, PM16 [KF319020], and both of them demonstrated low nucleotide sequence identity to the genomes of the other most similar phages, namely, Vibrio phage VP93, Pantoea phage LIMElight, and KP34-like bacteriophages. According to cluster analysis of the complete genome sequences and phylogenetic analysis of the proteins essential for their life cycle, phages PM75 and PM16 are distinct from other similar phages from the phiKMV supergroup and should be recognized as constituting a new genus, "Pm16virus", within the subfamily Autographivirinae.

A Genome Comparison of T7-like Podoviruses That Infect Caulobacter crescentus

Bacteriophages remain an understudied component of bacterial communities. Therefore, our laboratory has initiated an effort to isolate large numbers of bacteriophages that infect Caulobacter crescentus to provide an estimate of the diversity of bacteriophages that infect this common environmental bacterium. The majority of the new isolates are phicbkviruses, a genus of giant viruses that appear to be Caulobacter specific. However, we have also isolated several Podoviruses with icosahedral heads and small tails. One of these Podoviruses, designated Lullwater, is similar to two previously isolated Caulobacter phages, Cd1 and Percy. All three have genomes that are approximately 45 kb and contain approximately 30 genes. The gene order is conserved among the three genomes with one of the genes coding for a DNA polymerase that has homology to the family of T7 DNA polymerases. Phylogenetic trees based on either the DNA polymerase or the RNA polymerase amino acid sequences suggests that the three phages represent a new branch of the T7virus tree. Based on these similarities, we concluded that Cd1, Lullwater, and Percy comprise a new group in the T7virus genus.

Therapeutic Application of Phage Capsule Depolymerases against K1, K5, and K30 Capsulated E. coli in Mice

Capsule depolymerase enzymes offer a promising class of new antibiotics. In vivo studies are encouraging but it is unclear how well this type of phage product will generalize in therapeutics, or whether different depolymerases against the same capsule function similarly. Here, in vivo efficacy was tested using cloned bacteriophage depolymerases against Escherichia coli strains with three different capsule types: K1, K5, and K30. When treating infections with the cognate capsule type in a mouse thigh model, the previously studied K1E depolymerase rescued poorly, whereas K1F, K1H, K5, and K30 depolymerases rescued well. K30 gp41 was identified as the catalytically active protein. In contrast to the in vivo studies, K1E enzyme actively degraded K1 capsule polysaccharide in vitro and sensitized K1 bacteria to serum killing. The only in vitro correlate of poor K1E performance in vivo was that the purified enzyme did not form the expected trimer. K1E appeared as an 18-mer which might limit its in vivo distribution. Overall, depolymerases were easily identified, cloned from phage genomes, and as purified proteins they proved generally effective.

Dual host specificity of phage SP6 is facilitated by tailspike rotation

Bacteriophage SP6 exhibits dual-host adsorption specificity. The SP6 tailspikes are recognized as important in host range determination but the mechanisms underlying dual host specificity are unknown. Cryo-electron tomography and sub-tomogram classification were used to analyze the SP6 virion with a particular focus on the interaction of tailspikes with host membranes. The SP6 tail is surrounded by six V-shaped structures that interconnect in forming a hand-over-hand hexameric garland. Each V-shaped structure consists of two trimeric tailspike proteins: gp46 and gp47, connected through the adaptor protein gp37. SP6 infection of Salmonella enterica serovars Typhimurium and Newport results in distinguishable changes in tailspike orientation, providing the first direct demonstration how tailspikes can confer dual host adsorption specificity. SP6 also infects S. Typhimurium strains lacking O antigen; in these infections tailspikes have no apparent specific role and the phage tail must therefore interact with a distinct host receptor to allow infection.

Bacteriophage-encoded virion-associated enzymes to overcome the carbohydrate barriers during the infection process

Bacteriophages are bacterial viruses that infect the host after successful receptor recognition and adsorption to the cell surface. The irreversible adherence followed by genome material ejection into host cell cytoplasm must be preceded by the passage of diverse carbohydrate barriers such as capsule polysaccharides (CPSs), O-polysaccharide chains of lipopolysaccharide (LPS) molecules, extracellular polysaccharides (EPSs) forming biofilm matrix, and peptidoglycan (PG) layers. For that purpose, bacteriophages are equipped with various virion-associated carbohydrate active enzymes, termed polysaccharide depolymerases and lysins, that recognize, bind, and degrade the polysaccharide compounds. We discuss the existing diversity in structural locations, variable architectures, enzymatic specificities, and evolutionary aspects of polysaccharide depolymerases and virion-associated lysins (VALs) and illustrate how these aspects can correlate with the host spectrum. In addition, we present methods that can be used for activity determination and the application potential of these enzymes as antibacterials, antivirulence agents, and diagnostic tools.

Bacteriophage SP6 encodes a second tailspike protein that recognizes Salmonella enterica serogroups C2 and C3

SP6 is a salmonella phage closely related to coliphage K1-5. K1-5 is notable in that it encodes two polysaccharide-degrading tailspike proteins, an endosialidase that allows it to infect E. coli K1, and a lyase that enables it to infect K5 strains. SP6 is similar to K1-5 except that it encodes a P22-like endorhamnosidase tailspike, gp46, allowing it to infect group B Salmonella. We show here that SP6 can also infect Salmonella serogroups C₂ and C₃ and that a mutation in a putative second tailspike, gp47, eliminates this specificity. Gene 47 was fused to the coding region of the N-terminal portion of the Pseudomonas aeruginosa R2 pyocin tail fiber and expressed in trans such that the fusion protein becomes incorporated into pyocin particles. These pyocins, termed AvR2-SP47, killed serogroups C₂ and C₃Salmonella. We conclude that SP6 encodes two tail proteins providing it a broad host range among Salmonella enterica.

Contributions of P2- and P22-like prophages to understanding the enormous diversity and abundance of tailed bacteriophages

We identified 9371 tailed phage prophages of 20 known types in reported complete genome sequences of 3298 bacteria in the Salmonella genus. These include 4758 P2 type and 744 P22 type prophages. The latter prophage types were found in the genome sequences of 127 and 24 bacterial host genera, increasing the known host ranges of phages in these groups by 114 and 20 genera, respectively. These prophage nucleotide sequences displayed much more diversity than was previously known from the 48 P2 and 24 P22 type authentic phages whose genomes have been sequenced. More detailed analysis of these prophage sequences indicated that major capsid protein (MCP) gene exchange between tailed phage clusters or types is extremely rare and that P22 prophage-encoded tailspikes correspond perfectly with their hosts' surface polysaccharide structure; thus, MCP and tailspike sequences accurately predict tailed phage type (and thus lifestyle) and host cell surface polysaccharide structure, respectively.

Genomics of Three New Bacteriophages Useful in the Biocontrol of Salmonella

Non-typhoid Salmonella is the principal pathogen related to food-borne diseases throughout the world. Widespread antibiotic resistance has adversely affected human health and has encouraged the search for alternative antimicrobial agents. The advances in bacteriophage therapy highlight their use in controlling a broad spectrum of food-borne pathogens. One requirement for the use of bacteriophages as antibacterials is the characterization of their genomes. In this work, complete genome sequencing and molecular analyses were carried out for three new virulent Salmonella-specific bacteriophages (UAB_Phi20, UAB_Phi78, and UAB_Phi87) able to infect a broad range of Salmonella strains. Sequence analysis of the genomes of UAB_Phi20, UAB_Phi78, and UAB_Phi87 bacteriophages did not evidence the presence of known virulence-associated and antibiotic resistance genes, and potential immunoreactive food allergens. The UAB_Phi20 genome comprised 41,809 base pairs with 80 open reading frames (ORFs); 24 of them with assigned function. Genome sequence showed a high homology of UAB_Phi20 with Salmonella bacteriophage P22 and other P22likeviruses genus of the Podoviridae family, including ST64T and ST104. The DNA of UAB_Phi78 contained 44,110 bp including direct terminal repeats (DTR) of 179 bp and 58 putative ORFs were predicted and 20 were assigned function. This bacteriophage was assigned to the SP6likeviruses genus of the Podoviridae family based on its high similarity not only with SP6 but also with the K1-5, K1E, and K1F bacteriophages, all of which infect Escherichia coli. The UAB_Phi87 genome sequence consisted of 87,669 bp with terminal direct repeats of 608 bp; although 148 ORFs were identified, putative functions could be assigned to only 29 of them. Sequence comparisons revealed the mosaic structure of UAB_Phi87 and its high similarity with bacteriophages Felix O1 and wV8 of E. coli with respect to genetic content and functional organization. Phylogenetic analysis of large terminase subunits confirms their packaging strategies and grouping to the different phage genus type. All these studies are necessary for the development and the use of an efficient cocktail with commercial applications in bacteriophage therapy against Salmonella.

Complete Genomic and Lysis-Cassette Characterization of the Novel Phage, KBNP1315, which Infects Avian Pathogenic Escherichia coli (APEC)

Avian pathogenic Escherichia coli (APEC) is a major pathogen that causes avian colibacillosis and is associated with severe economic losses in the chicken-farming industry. Here, bacteriophage KBNP1315, infecting APEC strain KBP1315, was genomically and functionally characterized. The evolutionary relationships of KBNP1315 were analyzed at the genomic level using gene (protein)-sharing networks, the Markov clustering (MCL) algorithm, and comparative genomics. Our network analysis showed that KBNP1315 was connected to 30 members of the Autographivirinae subfamily, which comprises the SP6-, T7-, P60-, phiKMV-, GAP227- and KP34-related groups. Network decomposition suggested that KBNP1315 belongs to the SP6-like phages, but our comparison of putative encoded proteins revealed that key proteins of KBNP1315, including the tail spike protein and endolysin, had relative low levels of amino acid sequence similarity with other members of the SP6-like phages. Thus KBNP1315 may only be distantly related to the SP6-like phages, and (based on the difference in endolysin) its lysis mechanism may differ from theirs. To characterize the lytic functions of the holin and endolysin proteins from KBNP1315, we expressed these proteins individually or simultaneously in E. coli BL21 (DE3) competent cell. Interestingly, the expressed endolysin was secreted into the periplasm and caused a high degree of host cell lysis that was dose-dependently delayed/blocked by NaN₃-mediated inhibition of the SecA pathway. The expressed holin triggered only a moderate inhibition of cell growth, whereas coexpression of holin and endolysin enhanced the lytic effect of endolysin. Together, these results revealed that KBNP1315 appears to use a pin-holin/signal-arrest-release (SAR) endolysin pathway to trigger host cell lysis.

It seems like only yesterday

I spent my childhood and adolescence in North and South Carolina, attended Duke University, and then entered Duke Medical School. One year in the laboratory of George Schwert in the biochemistry department kindled my interest in biochemistry. After one year of residency on the medical service of Duke Hospital, chaired by Eugene Stead, I joined the group of Arthur Kornberg at Stanford Medical School as a postdoctoral fellow. Two years later I accepted a faculty position at Harvard Medical School, where I remain today. During these 50 years, together with an outstanding group of students, postdoctoral fellows, and collaborators, I have pursued studies on DNA replication. I have experienced the excitement of discovering a number of important enzymes in DNA replication that, in turn, triggered an interest in the dynamics of a replisome. My associations with industry have been stimulating and fostered new friendships. I could not have chosen a better career.

Salmonella phages and prophages: genomics, taxonomy, and applied aspects

Since this book was originally published in 2007 there has been a significant increase in the number of Salmonella bacteriophages, particularly lytic virus, and Salmonella strains which have been fully sequenced. In addition, new insights into phage taxonomy have resulted in new phage genera, some of which have been recognized by the International Committee of Taxonomy of Viruses (ICTV). The properties of each of these genera are discussed, along with the role of phage as agents of genetic exchange, as therapeutic agents, and their involvement in phage typing.

Improvements to pairwise sequence comparison (PASC): a genome-based web tool for virus classification

The number of viral genome sequences in the public databases is increasing dramatically, and these sequences are playing an important role in virus classification. Pairwise sequence comparison is a sequence-based virus classification method. A program using this method calculates the pairwise identities of virus sequences within a virus family and displays their distribution, and visual analysis helps to determine demarcations at different taxonomic levels such as strain, species, genus and subfamily. Subsequent comparison of new sequences against existing ones allows viruses from which the new sequences were derived to be classified. Although this method cannot be used as the only criterion for virus classification in some cases, it is a quantitative method and has many advantages over conventional virus classification methods. It has been applied to several virus families, and there is an increasing interest in using this method for other virus families/groups. The Pairwise Sequence Comparison (PASC) classification tool was created at the National Center for Biotechnology Information. The tool's database stores pairwise identities for complete genomes/segments of 56 virus families/groups. Data in the system are updated every day to reflect changes in virus taxonomy and additions of new virus sequences to the public database. The web interface of the tool ( http://www.ncbi.nlm.nih.gov/sutils/pasc/ ) makes it easy to navigate and perform analyses. Multiple new viral genome sequences can be tested simultaneously with this system to suggest the taxonomic position of virus isolates in a specific family. PASC eliminates potential discrepancies in the results caused by different algorithms and/or different data used by researchers.

Complete nucleotide sequence of Klebsiella phage P13 and prediction of an EPS depolymerase gene

The complete genome of Klebsiella phage P13 was sequenced and analyzed. Bacteriophage P13 has a double-stranded linear DNA with a length of 45,976 bp and a G+C content of 51.7 %, which is slightly lower than that of Klebsiella pneumoniae KCTC 2242. The codon biases of phage P13 are very similar to those of SP6-like phages and K. pneumoniae KCTC 2242. Bioinformatics analysis shows that the phage P13 genome has 282 open reading frames (ORFs) that are greater than 100 bp in length, and 50 of these ORFs were identified as predicted genes with an average length of 833 bp. Among these genes, 41 show homology to known proteins in the GenBank database. The functions of the 24 putative proteins were investigated, and 13 of these were found to be highly conserved. According to the homology analysis of the 50 predicted genes and the whole genome, phage P13 is homologous to SP6-like phages. Furthermore, the morphological characteristics of phage P13 suggest that it belongs to the SP6-like viral genus of the Podoviridae subfamily Autographivirinae. Two hypothetical genes encoding an extracellular polysaccharide depolymerase were predicted using PSI-BLAST. This analysis serves as groundwork for further research and application of the enzyme.

Complete genome sequence of 285P, a novel T7-like polyvalent E. coli bacteriophage

Bacteriophages are considered potential biological agents for the control of infectious diseases and environmental disinfection. Here, we describe a novel T7-like polyvalent Escherichia coli bacteriophage, designated "285P," which can lyse several strains of E. coli. The genome, which consists of 39,270 base pairs with a G+C content of 48.73 %, was sequenced and annotated. Forty-three potential open reading frames were identified using bioinformatics tools. Based on whole-genome sequence comparison, phage 285P was identified as a novel strain of subgroup T7. It showed strongest sequence similarity to Kluyvera phage Kvp1. The phylogenetic analyses of both non-structural proteins (endonuclease gp3, amidase gp3.5, DNA primase/helicase gp4, DNA polymerase gp5, and exonuclease gp6) and structural protein (tail fiber protein gp17) led to the identification of 285P as T7-like phage. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and matrix-assisted laser desorption/ionization time-of-flight mass spectrometric analyses verified the annotation of the structural proteins (major capsid protein gp10a, tail protein gp12, and tail fiber protein gp17).

ArdA proteins from different mobile genetic elements can bind to the EcoKI Type I DNA methyltransferase of E. coli K12

Anti-restriction and anti-modification (anti-RM) is the ability to prevent cleavage by DNA restriction–modification (RM) systems of foreign DNA entering a new bacterial host. The evolutionary consequence of anti-RM is the enhanced dissemination of mobile genetic elements. Homologues of ArdA anti-RM proteins are encoded by genes present in many mobile genetic elements such as conjugative plasmids and transposons within bacterial genomes. The ArdA proteins cause anti-RM by mimicking the DNA structure bound by Type I RM enzymes. We have investigated ArdA proteins from the genomes of Enterococcus faecalis V583, Staphylococcus aureus Mu50 and Bacteroides fragilis NCTC 9343, and compared them to the ArdA protein expressed by the conjugative transposon Tn916. We find that despite having very different structural stability and secondary structure content, they can all bind to the EcoKI methyltransferase, a core component of the EcoKI Type I RM system. This finding indicates that the less structured ArdA proteins become fully folded upon binding. The ability of ArdA from diverse mobile elements to inhibit Type I RM systems from other bacteria suggests that they are an advantage for transfer not only between closely-related bacteria but also between more distantly related bacterial species.

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Diverse ArdA proteins all target the EcoKI Type I DNA modification enzyme.

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ArdA proteins have variable secondary structure content.

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ArdA all bind equally well to EcoKI despite stability variations.

Safety analysis of a Russian phage cocktail: from metagenomic analysis to oral application in healthy human subjects

Phage therapy has a long tradition in Eastern Europe, where preparations are comprised of complex phage cocktails whose compositions have not been described. We investigated the composition of a phage cocktail from the Russian pharmaceutical company Microgen targeting Escherichia coli/Proteus infections. Electron microscopy identified six phage types, with numerically T7-like phages dominating over T4-like phages. A metagenomic approach using taxonomical classification, reference mapping and de novo assembly identified 18 distinct phage types, including 7 genera of Podoviridae, 2 established and 2 proposed genera of Myoviridae, and 2 genera of Siphoviridae. De novo assembly yielded 7 contigs greater than 30 kb, including a 147-kb Myovirus genome and a 42-kb genome of a potentially new phage. Bioinformatic analysis did not reveal undesired genes and a small human volunteer trial did not associate adverse effects with oral phage exposure.

Complete genome sequence of a polyvalent bacteriophage, phiKP26, active on Salmonella and Escherichia coli

Bacteriophages are viruses that specifically infect and lyse prokaryotic cells and therefore might be used as biocontrol agents. However, it is necessary to acquire genomic information to predict and understand the phage's characteristics for the efficient and safe use of bacteriophages as biocontrol agents against bacterial pathogens. In this study, the complete genome sequence of a novel enterobacteriophage, phiKP26, was determined by pyrosequencing. Genomic analysis of phiKP26 revealed a genome size of 47,285 bp with an overall G + C content of 44.3 %. Seventy-eight open reading frames (ORFs) in the phiKP26 genome were grouped into the modules of replication, DNA packaging, morphogenesis, cell lysis and absence of genes related to virulence and lysogeny.

Identification of EPS-degrading activity within the tail spikes of the novel Pseudomonas putida phage AF

We report the study of phage AF, the first member of the canonical lambdoid phage group infecting Pseudomonas putida. Its 42.6 kb genome is related to the "epsilon15-like viruses" and the "BPP-1-like viruses", a clade of bacteriophages shaped by extensive horizontal gene transfer. The AF virions display exopolysaccharide (EPS)-degrading activity, which originates from the action of the C-terminal domain of the tail spike (Gp19). This protein shows high similarity to the tail spike of the T7-like P. putida-infecting phage φ15. These unrelated phages have an identical host spectrum and EPS degradation characteristics, designating the C-terminal part of Gp19 as sole determinant for these functions. While intact AF particles have biofilm-degrading properties, Gp19 and non-infectious AF particles do not, emphasizing the role of phage amplification in biofilm degradation.

Evolutionary principles and synthetic biology: avoiding a molecular tragedy of the commons with an engineered phage

Background

In prior work, adding a gene to phage T7 that degraded the host K1 capsule facilitated growth when plated on capsulated hosts. However, the transgenic protein (an endosialidase) is expressed as an exoenzyme, released from the cell at lysis but unattached to the phage particle. There is thus the possibility that the gene will be subject to a tragedy of the commons and be selected against, if the enzyme benefits other genomes.

Results

This evolutionary perspective was supported in short term experiments. The genome carrying the endosialidase gene was favored on a capsulated host if grown in physical isolation of control genomes (lacking the gene) but was selected against otherwise.

Conclusions

These results challenge efforts to engineer phages with exoenzymes that degrade biofilm polymers. If biofilms do not facilitate spatially structured phage growth, the transgenic enzymes may be rapidly eliminated from the phage population after release in the environment, even if the transgene benefits overall phage growth on the biofilm.

Multiple genetic pathways to similar fitness limits during viral adaptation to a new host

The gain in fitness during adaptation depends on the supply of beneficial mutations. Despite a good theoretical understanding of how evolution proceeds for a defined set of mutations, there is little understanding of constraints on net fitness-whether fitness will reach a limit despite ongoing selection and mutation, and if there is a limit, what determines it. Here, the dsDNA bacteriophage SP6, a virus of Salmonella, was adapted to Escherichia coli K-12. From an isolate capable of modest growth on E. coli, four lines were adapted for rapid growth by protocols differing in use of mutagen, propagation method, and duration, but using the same media, temperature, and a continual excess of the novel host. Nucleotide changes underlying those adaptations differed greatly in number and identity, but the four lines achieved similar absolute fitness at the end, an increase of more than 4000-fold phage descendants per hour. Thus, the fitness landscape allows multiple genetic paths to the same approximate fitness limit. The existence and causes of fitness limits have ramifications to genome engineering, vaccine design, and "lethal mutagenesis" treatments to cure viral infections.

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.

Genomic and proteomic characterization of the broad-host-range Salmonella phage PVP-SE1: creation of a new phage genus

(Bacterio)phage PVP-SE1, isolated from a German wastewater plant, presents a high potential value as a biocontrol agent and as a diagnostic tool, even compared to the well-studied typing phage Felix 01, due to its broad lytic spectrum against different Salmonella strains. Sequence analysis of its genome (145,964 bp) shows it to be terminally redundant and circularly permuted. Its G+C content, 45.6 mol%, is lower than that of its hosts (50 to 54 mol%). We found a total of 244 open reading frames (ORFs), representing 91.6% of the coding capacity of the genome. Approximately 46% of encoded proteins are unique to this phage, and 22.1% of the proteins could be functionally assigned. This myovirus encodes a large number of tRNAs (n=24), reflecting its lytic capacity and evolution through different hosts. Tandem mass spectrometric analysis using electron spray ionization revealed 25 structural proteins as part of the mature phage particle. The genome sequence was found to share homology with 140 proteins of the Escherichia coli bacteriophage rV5. Both phages are unrelated to any other known virus, which suggests that an "rV5-like virus" genus should be created within the Myoviridae to contain these two phages.

Evolution of mosaically related tailed bacteriophage genomes seen through the lens of phage P22 virion assembly

The mosaic composition of the genomes of dsDNA tailed bacteriophages (Caudovirales) is well known. Observations of this mosaicism have generally come from comparisons of small numbers of often rather distantly related phages, and little is known about the frequency or detailed nature of the processes that generate this kind of diversity. Here we review and examine the mosaicism within fifty-seven clusters of virion assembly genes from bacteriophage P22 and its "close" relatives. We compare these orthologous gene clusters, discuss their surprising diversity and document horizontal exchange of genetic information between subgroups of the P22-like phages as well as between these phages and other phage types. We also point out apparent restrictions in the locations of mosaic sequence boundaries in this gene cluster. The relatively large sample size and the fact that phage P22 virion structure and assembly are exceptionally well understood make the conclusions especially informative and convincing.

Isolation and characterisation of KP34--a novel φKMV-like bacteriophage for Klebsiella pneumoniae

Bacteriophage KP34 is a novel virus belonging to the subfamily Autographivirinae lytic for extended-spectrum β-lactamase-producing Klebsiella pneumoniae strains. Its biological features, morphology, susceptibility to chemical and physical agents, burst size, host specificity and activity spectrum were determined. As a potential antibacterial agent used in therapy, KP34 molecular features including genome sequence and protein composition were examined. Phylogenetic analyses and clustering of KP34 phage genome sequences revealed its clear relationships with "phiKMV-like viruses". Simultaneously, whole-genome analyses permitted clustering and classification of all phages, with completely sequenced genomes, belonging to the Podoviridae.

The use of genomic signature distance between bacteriophages and their hosts displays evolutionary relationships and phage growth cycle determination

BACKGROUND

Bacteriophage classification is mainly based on morphological traits and genome characteristics combined with host information and in some cases on phage growth lifestyle. A lack of molecular tools can impede more precise studies on phylogenetic relationships or even a taxonomic classification. The use of methods to analyze genome sequences without the requirement for homology has allowed advances in classification.

RESULTS

Here, we proposed to use genome sequence signature to characterize bacteriophages and to compare them to their host genome signature in order to obtain host-phage relationships and information on their lifestyle. We analyze the host-phage relationships in the four most representative groups of Caudoviridae, the dsDNA group of phages. We demonstrate that the use of phage genomic signature and its comparison with that of the host allows a grouping of phages and is also able to predict the host-phage relationships (lytic vs. temperate).

CONCLUSIONS

We can thus condense, in relatively simple figures, this phage information dispersed over many publications.

Complete genomic sequence of bacteriophage felix o1

Bacteriophage O1 is a Myoviridae A1 group member used historically for identifying Salmonella. Sequencing revealed a single, linear, 86,155-base-pair genome with 39% average G+C content, 131 open reading frames, and 22 tRNAs. Closest protein homologs occur in Erwinia amylovora phage φEa21-4 and Escherichia coli phage wV8. Proteomic analysis indentified structural proteins: Gp23, Gp36 (major tail protein), Gp49, Gp53, Gp54, Gp55, Gp57, Gp58 (major capsid protein), Gp59, Gp63, Gp64, Gp67, Gp68, Gp69, Gp73, Gp74 and Gp77 (tail fiber). Based on phage-host codon differences, 7 tRNAs could affect translation rate during infection. Introns, holin-lysin cassettes, bacterial toxin homologs and host RNA polymerase-modifying genes were absent.

A tale of tails: Sialidase is key to success in a model of phage therapy against K1-capsulated Escherichia coli

Prior studies treating mice infected with Escherichia coli O18:K1:H7 observed that phages requiring the K1 capsule for infection (K1-dep) were superior to capsule-independent (K1-ind) phages. We show that three K1-ind phages all have low fitness when grown on cells in serum whereas fitnesses of four K1-dep phages were high. The difference is serum-specific, as fitnesses in broth overlapped. Sialidase activity was associated with all K1-dep virions tested but no K1-ind virions, a phenotype supported by sequence analyses. Adding endosialidase to cells infected with K1-ind phage increased fitness in serum by enhancing productive infection after adsorption. We propose that virion sialidase activity is the primary determinant of high fitness on cells grown in serum, and thus in a mammalian host. Although the benefit of sialidase is specific to K1-capsulated bacteria, this study may provide a scientific rationale for selecting phages for therapeutic use in many systemic infections.

Extensive DNA mimicry by the ArdA anti-restriction protein and its role in the spread of antibiotic resistance

The ardA gene, found in many prokaryotes including important pathogenic species, allows associated mobile genetic elements to evade the ubiquitous Type I DNA restriction systems and thereby assist the spread of resistance genes in bacterial populations. As such, ardA contributes to a major healthcare problem. We have solved the structure of the ArdA protein from the conjugative transposon Tn916 and find that it has a novel extremely elongated curved cylindrical structure with defined helical grooves. The high density of aspartate and glutamate residues on the surface follow a helical pattern and the whole protein mimics a 42-base pair stretch of B-form DNA making ArdA by far the largest DNA mimic known. Each monomer of this dimeric structure comprises three alpha–beta domains, each with a different fold. These domains have the same fold as previously determined proteins possessing entirely different functions. This DNA mimicry explains how ArdA can bind and inhibit the Type I restriction enzymes and we demonstrate that 6 different ardA from pathogenic bacteria can function in Escherichia coli hosting a range of different Type I restriction systems.

Layers of evolvability in a bacteriophage life history trait

Functional redundancy in genomes arises from genes with overlapping functions, allowing phenotypes to persist after gene knockouts. Evolutionary redundancy or evolvability of a genome is one step removed, in that functional redundancy is absent but the genome has the potential to evolve to restore a lost phenotype. Exploring the extent to which this recovery alters gene networks can illuminate how functional gene interactions change through time. Here, the evolvability of lysis was studied in bacteriophage T7, revealing hidden functional interactions. Lysis is the destruction of host cell wall and membranes that releases progeny and is therefore essential for phage propagation. In most phages, lysis is mediated by a two-component genetic module: a muralytic enzyme that degrades the bacterial cell wall (endolysin) and a holin that permeabilizes the inner membrane to allow the endolysin access to the cell wall. T7 carries one known holin, one endolysin, and a second muralytic enzyme that plays little role in lysis by wild-type phage. If the primary endolysin is deleted, the second muralytic enzyme evolves to restore normal lysis after selection for faster growth. Here, a second level of evolutionary redundancy was revealed. When the second muralytic enzyme was prevented from adapting in a genome lacking the primary endolysin, the phage reevolved lysis de novo in the absence of any known muralytic enzymes by changes in multiple genes outside the original lysis module. This second level of redundancy proved to be evolutionarily inferior to the first, and both result in a lower fitness and slower lysis than wild-type T7. Deletion of the holin gene delayed lysis time modestly; fitness was restored by compensatory substitutions in genes that lack known roles in lysis of the wild type.

Preparation of a phage DNA fragment library for whole genome shotgun sequencing

The most efficient method to determine the genomic sequence of a dsDNA phage is to use a whole genome shotgun approach (WGSA). Preparation of a library where each genomic fragment has an equal chance of being represented is critical to the success of the WGSA. For many phages, there are regions of the genome likely to be under-represented in the shotgun library, which results in more gaps in the shotgun assembly than predicted by the Poisson distribution. However, as phage genomes are relatively small, this increased number of gaps does not present an insurmountable impediment to using the WGSA. This chapter will focus on construction of a high-quality random library and sequence analysis of this library in a 96-well format. Techniques are described for the mechanical fragmentation of genomic DNA into 2 kb average size fragments, preparation of the fragmented DNA for shotgun cloning, and advice on the choice of cloning vector for library preparation. Protocols for deepwell block culture, plasmid isolation, and sequencing in 96-well format are given. The rationale for determining the total number of random clones from a library to sequence for a 50 and 150 kb genome is explained. The steps involved in going from hundreds of shotgun sequencing traces to generating contigs will be outlined as well as how to close gaps in the sequence by primer walking on phage DNA and PCR-generated templates. Finally, examples will be given of how biological information about the phage genomic termini can be derived by analysis of the organization of individual clones in the shotgun sequence assembly. Specific examples are given for the circularly permuted termini of pac type phages, the direct terminal repeats found in most T7-like phages, variable host DNA at either end as in the Mu-like phages, and the 5' and 3' overhanging ends of cos type phages. The end result of these steps is the entire DNA sequence of a novel phage, ready for gene prediction.

Phage proteomics: applications of mass spectrometry

Current techniques in mass spectrometry (MS) allow sensitive and accurate identification of proteins thanks to the in silico availability of these protein sequences within databases. This chapter provides a short overview of MS techniques used in the identification of phage structural proteins and focuses on an electron spray peptide ionization (ESI-MS/MS) approach to identify the phage structural proteome in a comprehensive and systematic ways. Such analyses provide an experimental examination of structural proteins and confirm genome-based gene predictions.

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 freshwater cyanophage Pf-WMP3 Infecting cyanobacterium Phormidium foveolarum: the conserved elements for a phage

Cyanophages are ecologically abundant, genetically diverse in aquatic environments, and affect the population and evolutionary trajectories of their hosts. After reporting the cyanophage Pf-WMP4 genome (Liu et al. in Virology 366:28-39, 2007), we hereby present a related cyanophage, Pf-WMP3, which also infects the freshwater cyanobacterium Phormidium foveolarum. The Pf-WMP3 genome contains 43,249 bp with 234 bp direct terminal repeats. The overall genome organization and core genes of the two phages are comparable to those of the T7 supergroup phages. Compared with Pf-WMP4, cyanophage Pf-WMP3 has diverged extensively at the DNA level; however, they are closely related at the protein level and genome architecture. The left arm genes for the two phages, which mainly encode the DNA replication machinery, are not conserved in the gene order. Whereas the right arm genes of the two phages coding for structural proteins show high similarity in amino acid sequences and modular architecture, indicating that they have retained similar development strategies. The differences in similarity levels between the left and right arm genes suggest that the structural genes are the most conserved elements for a phage.

Diversity among the tailed-bacteriophages that infect the Enterobacteriaceae

Complete genome sequences have been determined for 73 tailed-phages that infect members of the bacterial Enterobacteriaceae family. Biological criteria such as genome size, gene organization and gene orientation were used to place these phages into categories. There are 13 such categories, some of which are themselves extremely diverse. The relationships between and within these categories are discussed with an emphasis on the head assembly genes. Although some of them are clearly homologues, suggesting a very ancient origin, there is little evidence for exchange of individual head genes between these phage categories. More recent horizontal exchange of phage tail fiber and early proteins between the categories occurs, but is probably not extremely rapid.

Unifying classical and molecular taxonomic classification: analysis of the Podoviridae using BLASTP-based tools

We defined phage genera by measuring genome relationships by the numbers of shared homologous/orthologous proteins. We used BLAST-based tools (CoreExtractor.vbs and CoreGenes) to analyze 55 fully sequenced bacteriophage genomes from the NCBI and EBI databases. This approach was first applied to the T7-related phages. Using a cut-off score of 40% homologous proteins, we identified three genera within the T7-related phages, redefined the phi29-related phages, and introduced five novel genera. The T7- and phi29-related phages were given subfamily status and named "Autographivirinae" and "Picovirinae", respectively. Our results confirm and refine the ICTV phage classification, enable elimination of errors in public databases, and provide a straightforward tool for the molecular classification of new phage genomes.

Structure of the receptor-binding protein of bacteriophage det7: a podoviral tail spike in a myovirus

A new Salmonella enterica phage, Det7, was isolated from sewage and shown by electron microscopy to belong to the Myoviridae morphogroup of bacteriophages. Det7 contains a 75-kDa protein with 50% overall sequence identity to the tail spike endorhamnosidase of podovirus P22. Adsorption of myoviruses to their bacterial hosts is normally mediated by long and short tail fibers attached to a contractile tail, whereas podoviruses do not contain fibers but attach to host cells through stubby tail spikes attached to a very short, noncontractile tail. The amino-terminal 150 residues of the Det7 protein lack homology to the P22 tail spike and are probably responsible for binding to the base plate of the myoviral tail. Det7 tail spike lacking this putative particle-binding domain was purified from Escherichia coli, and well-diffracting crystals of the protein were obtained. The structure, determined by molecular replacement and refined at a 1.6-A resolution, is very similar to that of bacteriophage P22 tail spike. Fluorescence titrations with an octasaccharide suggest Det7 tail spike to bind its receptor lipopolysaccharide somewhat less tightly than the P22 tail spike. The Det7 tail spike is even more resistant to thermal unfolding than the already exceptionally stable homologue from P22. Folding and assembly of both trimeric proteins are equally temperature sensitive and equally slow. Despite the close structural, biochemical, and sequence similarities between both proteins, the Det7 tail spike lacks both carboxy-terminal cysteines previously proposed to form a transient disulfide during P22 tail spike assembly. Our data suggest receptor-binding module exchange between podoviruses and myoviruses in the course of bacteriophage evolution.