Function, expression, specificity, diversity and incompatibility of actinobacteriophage parABS systems

Genome sequences of Microbacterium foliorum phages, BabyYoda and Lynlen

BabyYoda and Lynlen are two cluster EB phages that were discovered at Thiel College using Microbacterium foliorum NRRL B-24224. Both BabyYoda and Lynlen are predicted to be lytic, with siphovirus morphologies, with genome sizes of 41,557 and 41,448 base pairs, respectively.

Diversity of Salmonella enterica phages isolated from chicken farms in Kenya

IMPORTANCE

Non-typhoidal Salmonella enterica infections are one of the leading causes of diarrhoeal diseases that spread to humans from animal sources such as poultry. Hence, keeping poultry farms free of Salmonella is essential for consumer safety and for a better yield of animal products. However, the emergence of antibiotic resistance due to over usage has sped up the search for alternative biocontrol methods such as the use of bacteriophages. Isolation and characterization of novel bacteriophages are key to adapt phage-based biocontrol applications. Here, we isolated and characterized Salmonella phages from samples collected at chicken farms and slaughterhouses in Kenya. The genomic characterization of these phage isolates revealed that they belong to four ICTV (International Committee on Taxonomy of Viruses) phage genera. All these phages are lytic and possibly suitable for biocontrol applications because no lysogenic genes or virulence factors were found in their genomes. Hence, we recommend further studies on these phages for their applications in Salmonella biocontrol.

Genome sequence of cluster A15 Gordonia terrae bacteriophage Nebulosus

The temperate Gordonia phage Nebulosus was isolated from soil on Gordonia terrae and is a siphovirus. The genome is 52,175 bp in length, has 62% GC content, and encodes 96 protein-coding genes. Nebulosus encodes a partitioning system, ParABS, which is likely involved in lysogeny maintenance.

Comparative Genomics of Six Lytic Bacillus subtilis Phages from the Southwest United States

Background

Despite their importance to microbial dynamics involving Bacillus subtilis, we have a limited understanding of the diversity of phages that can lyse this model organism.

Materials and Methods

Phages were isolated from soil samples collected from various sites in the southwest U.S. deserts on a wild B. subtilis strain. Their genomes were assembled, characterized, and bioinformatically compared.

Results

Six Siphoviruses with high nucleotide and amino acid similarity to each other (>80%) but very limited similarity to phages currently in GenBank were isolated. These phages have double-stranded DNA genomes (55,312 to 56,127 bp) with 86-91 putative protein coding genes, and a low GC content. Comparative genomics reveal differences in loci encoding proteins that are putatively involved in bacterial adsorption with evidence for genomic mosaicism and a possible role for small genes.

Conclusions

A comparative approach provides insights into phage evolution, including the role of indels in protein folding.

A Review on Mycobacteriophages: From Classification to Applications

Mycobacterial infections are a group of life-threatening conditions triggered by fast- or slow-growing mycobacteria. Some mycobacteria, such as Mycobacterium tuberculosis, promote the deaths of millions of lives throughout the world annually. The control of mycobacterial infections is influenced by the challenges faced in the diagnosis of these bacteria and the capability of these pathogens to develop resistance against common antibiotics. Detection of mycobacterial infections is always demanding due to the intracellular nature of these pathogens that, along with the lipid-enriched structure of the cell wall, complicates the access to the internal contents of mycobacterial cells. Moreover, recent studies depicted that more than 20% of M. tuberculosis (Mtb) infections are multi-drug resistant (MDR), and only 50% of positive MDR-Mtb cases are responsive to standard treatments. Similarly, the susceptibility of nontuberculosis mycobacteria (NTM) to first-line tuberculosis antibiotics has also declined in recent years. Exploiting mycobacteriophages as viruses that infect mycobacteria has significantly accelerated the diagnosis and treatment of mycobacterial infections. This is because mycobacteriophages, regardless of their cycle type (temperate/lytic), can tackle barriers in the mycobacterial cell wall and make the infected bacteria replicate phage DNA along with their DNA. Although the infectivity of the majority of discovered mycobacteriophages has been evaluated in non-pathogenic M. smegmatis, more research is still ongoing to find mycobacteriophages specific to pathogenic mycobacteria, such as phage DS6A, which has been shown to be able to infect members of the M. tuberculosis complex. Accordingly, this review aimed to introduce some potential mycobacteriophages in the research, specifically those that are infective to the three troublesome mycobacteria, M. tuberculosis, M. avium subsp. paratuberculosis (MAP), and M. abscessus, highlighting their theranostic applications in medicine.

Phylogenetic Distribution of WhiB- and Lsr2-Type Regulators in Actinobacteriophage Genomes

Viruses that infect different actinobacterial host species are known as actinobacteriophages. They are composed of highly divergent and mosaic genomes due to frequent gene exchange between their bacterial hosts and related viral species. This is also reflected by the adaptive incorporation of host transcription factors (TFs) into phage regulatory networks. Previous studies discovered Lsr2-type and WhiB-type regulators encoded by actinobacteriophage genomes. However, limited information is available about their distribution, evolution, and impact on host species. In this study, we computationally screened the distribution of known bacterial and phage TFs inside 2951 complete actinobacteriophage genomes and identified 13 different TF domains. Among those, WhiB, Lsr2, MerR, and Cro/CI-like proteins were widespread and found in more than 10% of the analyzed actinobacteriophage genomes. Neighboring genomic context analysis of the whiB and lsr2 loci showed group-specific conservation of gene synteny and potential involvement of these genes in diverse regulatory functions. Both genes were significantly enriched in temperate phages, and the Lsr2-encoding genomes featured an overall lower GC content. Phylogenetic analysis of WhiB and Lsr2 proteins showed the grouping of phage sequences within bacterial clades, suggesting gene acquisition by phages from their bacterial host species or by multiple, independent acquisition events. Overall, our study reports the global distribution of actinobacteriophage regulatory proteins and sheds light on their origin and evolution.

IMPORTANCE Actinobacteriophages are viruses that infect bacterial species of the diverse phylum of Actinobacteria. Phages engage in a close relationship with their bacterial host. This is also reflected by the adoption of genetic material from their host and its incorporation into phage regulatory circuits. In this study, we systematically searched the genomes of actinobacteriophages for the presence of transcription factor domains. We show that proteins belonging to the regulator families of WhiB and Lsr2 belong to the most abundant regulatory proteins encoded by actinobacteriophages. Further phylogenetic analysis shed light on their origin and evolution. Altogether, this study provides an important basis for further experimental investigation of their role in the coordination of the phage life cycle and their interaction with the host regulatory network in this important bacterial phylum.

Actinobacteriophages: Genomics, Dynamics, and Applications

Actinobacteriophages are viruses that infect bacterial hosts in the phylum Actinobacteria. More than 17,000 actinobacteriophages have been described and over 3,000 complete genome sequences reported, resulting from large-scale, high-impact, integrated research-education initiatives such as the Science Education Alliance Phage Hunters Advancing Genomics and Evolutionary Sciences (SEA-PHAGES) program. Their genomic diversity is enormous; actinobacteriophages comprise many architecturally mosaic genomes with distinct DNA sequences. Their genome diversity is driven by the highly dynamic interactions between phages and their hosts, and prophages can confer a variety of systems that defend against attack by genetically distinct phages; phages can neutralize these defense systems by coding for counter-defense proteins. These phages not only provide insights into diverse and dynamic phage populations but also have provided numerous tools for mycobacterial genetics. A case study using a three-phage cocktail to treat a patient with a drug-resistant Mycobacterium abscessus suggests that phages may have considerable potential for the therapeutic treatment of mycobacterial infections.

Phylogenomic Characterization of a Novel Corynebacterium Species Associated with Fatal Diphtheritic Stomatitis in Endangered Yellow-Eyed Penguins

Yellow-eyed penguins, Megadyptes antipodes, are an endangered species that are endemic to New Zealand. Outbreaks of diphtheritic stomatitis have caused significant mortality for this species, especially among young chicks. In this study, we isolated 16 Corynebacterium sp. isolates from the oral cavities of 2- to 14-day-old chicks at a range of infection stages and sequenced the genomes to understand their virulence mechanisms. Phylogenomic and matrix-assisted laser desorption ionization–time of flight (MALDI-TOF) characterization indicate that these strains belong to a novel Corynebacterium species. A simple multiplex PCR-based diagnostic assay has been developed to identify these strains rapidly and reliably. Similar to other corynebacteria, genomic islands and prophages introduced significant diversity among these strains that has potentially led to minor functional variations between the two lineages. Despite the presence of multiple corynebacterial virulence genes and a spaDEF-type pilus gene cluster among these strains, the survival rate was much higher in Galleria mellonella larvae than in those inoculated with Corynebacterium ulcerans NZRM 818 and Corynebacterium pseudotuberculosis NZRM 3004. Therefore, these strains are opportunistic pathogens causing high mortality among young penguin chicks due to a less-developed immune system.

IMPORTANCE Yellow-eyed penguins, Megadyptes antipodes, are endangered species with a sharp decline in the numbers of breeding pairs over the last 2 decades. Diphtheritic stomatitis, characterized by a thick fibrinopurulent exudate in the oral cavities and symptoms, including inanition and significant weight loss, is responsible for significant mortality among the young chicks. These chicks are treated with antibiotics, amoxicillin-clavulanic acid or enrofloxacin, but do not always recover from the infection. The pathogen causing these infections and the mechanism of pathogenesis are unclear. This study has identified a novel Corynebacterium species to be associated with diphtheritic stomatitis in yellow-eyed penguins with potential virulence genes that are likely involved in pathogenesis. Importantly, a gene encoding an exotoxin, phospholipase D, is present among these strains. The inactivated form of this enzyme could potentially be used as an effective vaccine to protect these penguins from infection.

The Prophage and Plasmid Mobilome as a Likely Driver of Mycobacterium abscessus Diversity

Mycobacterium abscessus is an emerging pathogen that is often refractory to antibiotic control. Treatment is further complicated by considerable variation among clinical isolates in both their genetic constitution and their clinical manifestations. Here, we show that the prophage and plasmid mobilome is a likely contributor to this variation. Prophages and plasmids are common, abundant, and highly diverse, and code for large repertoires of genes influencing virulence, antibiotic susceptibility, and defense against viral infection. At least 85% of the strains we describe carry one or more prophages, representing at least 17 distinct and diverse sequence "clusters," integrated at 18 different attB locations. The prophages code for 19 distinct configurations of polymorphic toxin and toxin-immunity systems, each with WXG-100 motifs for export through type VII secretion systems. These are located adjacent to attachment junctions, are lysogenically expressed, and are implicated in promoting growth in infected host cells. Although the plethora of prophages and plasmids confounds the understanding of M. abscessus pathogenicity, they also provide an abundance of tools for M. abscessus engineering.IMPORTANCE Mycobacterium abscessus is an important emerging pathogen that is challenging to treat with current antibiotic regimens. There is substantial genomic variation in M. abscessus clinical isolates, but little is known about how this influences pathogenicity and in vivo growth. Much of the genomic variation is likely due to the large and varied mobilome, especially a large and diverse array of prophages and plasmids. The prophages are unrelated to previously characterized phages of mycobacteria and code for a diverse array of genes implicated in both viral defense and in vivo growth. Prophage-encoded polymorphic toxin proteins secreted via the type VII secretion system are common and highly varied and likely contribute to strain-specific pathogenesis.

Bacteria have numerous distinctive groups of phage-plasmids with conserved phage and variable plasmid gene repertoires

Plasmids and temperate phages are key contributors to bacterial evolution. They are usually regarded as very distinct. However, some elements, termed phage–plasmids, are known to be both plasmids and phages, e.g. P1, N15 or SSU5. The number, distribution, relatedness and characteristics of these phage–plasmids are poorly known. Here, we screened for these elements among ca. 2500 phages and 12000 plasmids and identified 780 phage–plasmids across very diverse bacterial phyla. We grouped 92% of them by similarity of gene repertoires to eight defined groups and 18 other broader communities of elements. The existence of these large groups suggests that phage–plasmids are ancient. Their gene repertoires are large, the average element is larger than an average phage or plasmid, and they include slightly more homologs to phages than to plasmids. We analyzed the pangenomes and the genetic organization of each group of phage–plasmids and found the key phage genes to be conserved and co-localized within distinct groups, whereas genes with homologs in plasmids are much more variable and include most accessory genes. Phage–plasmids are a sizeable fraction of the sequenced plasmids (∼7%) and phages (∼5%), and could have key roles in bridging the genetic divide between phages and other mobile genetic elements.

Mycobacteriophages as Potential Therapeutic Agents against Drug-Resistant Tuberculosis

The current emergence of multi-, extensively-, extremely-, and total-drug resistant strains of Mycobacterium tuberculosis poses a major health, social, and economic threat, and stresses the need to develop new therapeutic strategies. The notion of phage therapy against bacteria has been around for more than a century and, although its implementation was abandoned after the introduction of drugs, it is now making a comeback and gaining renewed interest in Western medicine as an alternative to treat drug-resistant pathogens. Mycobacteriophages are genetically diverse viruses that specifically infect mycobacterial hosts, including members of the M. tuberculosis complex. This review describes general features of mycobacteriophages and their mechanisms of killing M. tuberculosis, as well as their advantages and limitations as therapeutic and prophylactic agents against drug-resistant M. tuberculosis strains. This review also discusses the role of human lung micro-environments in shaping the availability of mycobacteriophage receptors on the M. tuberculosis cell envelope surface, the risk of potential development of bacterial resistance to mycobacteriophages, and the interactions with the mammalian host immune system. Finally, it summarizes the knowledge gaps and defines key questions to be addressed regarding the clinical application of phage therapy for the treatment of drug-resistant tuberculosis.

Characterization and spontaneous induction of urinary tract Streptococcus anginosus prophages

Streptococcus anginosus is an often overlooked and understudied emerging pathogen inhabiting many areas of the human body. Through our sequencing of S. anginosus strains isolated from the female bladder microbiota, we detected numerous prophage sequences. Bioinformatic analysis of these sequences identified 17 distinct groups of S. anginosus prophages. The majority of these phages exhibit no sequence homology to previously characterized temperate or virulent phage sequences, indicating an unexplored diversity of Streptococcus phages. By culturing these bacterial isolates, we confirmed that the prophages of five of these groups are capable of induction. One of these putative phages was imaged, the first such evidence of an S. anginosus virus-like particle; it exhibits morphological characteristics of siphoviruses.

Protein-Mediated and RNA-Based Origins of Replication of Extrachromosomal Mycobacterial Prophages

Bacteriophages are the most abundant biological entities in the biosphere and are a source of uncharacterized biological mechanisms and genetic tools. Here, we identify segments of phage genomes that are used for stable extrachromosomal replication in the prophage state. Autonomous replication of some of these phages requires a RepA-like protein, although most lack repA and use RNA-based systems for replication initiation. We describe a suite of plasmids based on these prophage replication functions that vary in copy number, stability, host range, and compatibility. These plasmids expand the toolbox available for genetic manipulation of Mycobacterium and other Actinobacteria, including Gordonia terrae.

Temperate bacteriophages are common and establish lysogens of their bacterial hosts in which the prophage is stably inherited. It is typical for such prophages to be integrated into the bacterial chromosome, but extrachromosomally replicating prophages have been described also, with the best characterized being the Escherichia coli phage P1 system. Among the large collection of sequenced mycobacteriophages, more than half are temperate or predicted to be temperate, most of which code for a tyrosine or serine integrase that promotes site-specific prophage integration. However, within the large group of 621 cluster A temperate phages, ∼20% lack an integration cassette, which is replaced with a parABS partitioning system. A subset of these phages carry genes coding for a RepA-like protein (RepA phages), which we show here is necessary and sufficient for autonomous extrachromosomal replication. The non-RepA phages appear to replicate using an RNA-based system, as a parABS-proximal region expressing a noncoding RNA is required for replication. Both RepA and non-RepA phage-based plasmids replicate at one or two copies per cell, transform both Mycobacterium smegmatis and Mycobacterium tuberculosis, and are compatible with pAL5000-derived oriM and integration-proficient plasmid vectors. Characterization of these phage-based plasmids offers insights into the variability of lysogenic maintenance systems and provides a large suite of plasmids for actinobacterial genetics that vary in stability, copy number, compatibility, and host range.

Evolution of Superinfection Immunity in Cluster A Mycobacteriophages

Many aspects regarding superinfection, immunity, virulence, and the evolution of immune specificities are poorly understood due to the lack of large collections of isolated and sequenced phages with a spectrum of genetic diversity. Using a genetically diverse collection of Cluster A phages, we show that the classical and relatively straightforward patterns of homoimmunity, heteroimmunity, and virulence result from interactions between homotypic and heterotypic phages at the extreme edges of an evolutionary continuum of immune specificities. Genetic interactions between mesotypic phages result in more complex mesoimmunity phenotypes and virulence profiles. These results highlight that the evolution of immune specificities can be shaped by homotypic and mesotypic interactions and may be more dynamic than previously considered.

Temperate phages encode an immunity system to control lytic gene expression during lysogeny. This gene regulatory circuit consists of multiple interacting genetic elements, and although it is essential for controlling phage growth, it is subject to conflicting evolutionary pressures. During superinfection of a lysogen, the prophage’s circuit interacts with the superinfecting phage’s circuit and prevents lytic growth if the two circuits are closely related. The circuitry is advantageous since it provides the prophage with a defense mechanism, but the circuitry is also disadvantageous since it limits the phage’s host range during superinfection. Evolutionarily related phages have divergent, orthogonal immunity systems that no longer interact and are heteroimmune, but we do not understand how immunity systems evolve new specificities. Here, we use a group of Cluster A mycobacteriophages that exhibit a spectrum of genetic diversity to examine how immunity system evolution impacts superinfection immunity. We show that phages with mesotypic (i.e., genetically related but distinct) immunity systems exhibit asymmetric and incomplete superinfection phenotypes. They form complex immunity networks instead of well-defined immunity groups, and mutations conferring escape (i.e., virulence) from homotypic or mesotypic immunity have various escape specificities. Thus, virulence and the evolution of new immune specificities are shaped by interactions with homotypic and mesotypic immunity systems.

More Evidence of Collusion: a New Prophage-Mediated Viral Defense System Encoded by Mycobacteriophage Sbash

The arms race between bacteria and their bacteriophages profoundly influences microbial evolution. With an estimated 10²³ phage infections occurring per second, there is strong selection for both bacterial survival and phage coevolution for continued propagation. Many phage resistance systems, including restriction-modification systems, clustered regularly interspaced short palindromic repeat-Cas (CRISPR-Cas) systems, a variety of abortive infection systems, and many others that are not yet mechanistically defined, have been described. Temperate bacteriophages are common and form stable lysogens that are immune to superinfection by the same or closely related phages. However, temperate phages collude with their hosts to confer defense against genomically distinct phages, to the mutual benefit of the bacterial host and the prophage. Prophage-mediated viral systems have been described in Mycobacterium phages and Pseudomonas phages but are predicted to be widespread throughout the microbial world. Here we describe a new viral defense system in which the mycobacteriophage Sbash prophage colludes with its Mycobacterium smegmatis host to confer highly specific defense against infection by the unrelated mycobacteriophage Crossroads. Sbash genes 30 and 31 are lysogenically expressed and are necessary and sufficient to confer defense against Crossroads but do not defend against any of the closely related phages grouped in subcluster L2. The mapping of Crossroads defense escape mutants shows that genes 132 and 141 are involved in recognition by the Sbash defense system and are proposed to activate a loss in membrane potential mediated by Sbash gp30 and gp31.IMPORTANCE Viral infection is an ongoing challenge to bacterial survival, and there is strong selection for development or acquisition of defense systems that promote survival when bacteria are attacked by bacteriophages. Temperate phages play central roles in these dynamics through lysogenic expression of genes that defend against phage attack, including those unrelated to the prophage. Few prophage-mediated viral defense systems have been characterized, but they are likely widespread both in phage genomes and in the prophages integrated in bacterial chromosomes.

Mycobacteriophage CRB2 defines a new subcluster in mycobacteriophage classification

Mycobacteriophages are viruses -mostly temperates- that infect Mycobacterium smegmatis and sometimes Mycobacterium tuberculosis. Mycobacteriophages are grouped in clusters on the basis of the overall nucleotide sequence homology, being further divided in subclusters as more mycobacteriophage genomes are sequenced and annotated. As part of our on-going screening for novel isolates, we herein report the bioinformatics analysis of CRB2, a mycobacteriophage belonging into the Siphoviridae family that propagates at 30°C. CRB2 has a 72,217 bp genome with a 69.78% GC content that belongs to Cluster B; nucleotide comparison with other B cluster members positions CRB2 as the sole member of a new subcluster, B9, being mycobacteriophage Saguaro (belonging into subcluster B7) its closest relative. Sequencing and annotation of 14 mycobacteriophages isolated by our group has yielded six cluster A members, a singleton, four of the five members of subcluster B6, one of the three reported members of subcluster G4, and CRB2 which defines subcluster B9. Considering the massive mycobacteriophage search performed in USA and the relatively rarity of our phages, we propose that factors other than size of the sampling determine the variability of mycobacteriophage distribution, and thus a world-wide concerted mining would most likely bring extremely rare and yet undiscovered mycobacteriophages.

Mycobacteriophages

Mycobacteriophages are viruses that infect mycobacterial hosts. A large number of mycobacteriophages have been isolated and genomically characterized, providing insights into viral diversity and evolution, as well as fueling development of tools for mycobacterial genetics. Mycobacteriophages have intimate relationships with their hosts and provide insights into the genetics and physiology of the mycobacteria and tools for potential clinical applications such as drug development, diagnosis, vaccines, and potentially therapy.

Expression and evolutionary patterns of mycobacteriophage D29 and its temperate close relatives

Background

Mycobacteriophages are viruses that infect Mycobacterium hosts. A large collection of phages known to infect the same bacterial host strain – Mycobacterium smegmatis mc²155 – exhibit substantial diversity and characteristically mosaic architectures. The well-studied lytic mycobacteriophage D29 appears to be a deletion derivative of a putative temperate parent, although its parent has yet to be identified.

Results

Here we describe three newly-isolated temperate phages – Kerberos, Pomar16 and StarStuff – that are related to D29, and are predicted to be very close relatives of its putative temperate parent, revealing the repressor and additional genes that are lost in D29. Transcriptional profiles show the patterns of both lysogenic and lytic gene expression and identify highly-expressed, abundant, stable, small non-coding transcripts made from the P_left early lytic promoter, and which are toxic to M. smegmatis.

Conclusions

Comparative genomics of phages D29, Kerberos, Pomar16 and StarStuff provide insights into bacteriophage evolution, and comparative transcriptomics identifies the pattern of lysogenic and lytic expression with unusual features including highly expressed, small, non-coding RNAs.

Electronic supplementary material

The online version of this article (10.1186/s12866-017-1131-2) contains supplementary material, which is available to authorized users.

Complete Genome Sequences of Cluster A Mycobacteriophages BobSwaget, Fred313, KADY, Lokk, MyraDee, Stagni, and StepMih

Seven mycobacteriophages from distinct geographical locations were isolated, using Mycobacterium smegmatis mc²155 as the host, and then purified and sequenced. All of the genomes are related to cluster A mycobacteriophages, BobSwaget and Lokk in subcluster A2; Fred313, KADY, Stagni, and StepMih in subcluster A3; and MyraDee in subcluster A18, the first phage to be assigned to that subcluster.

Bacteriophages of Gordonia spp. Display a Spectrum of Diversity and Genetic Relationships

The global bacteriophage population is large, dynamic, old, and highly diverse genetically. Many phages are tailed and contain double-stranded DNA, but these remain poorly characterized genomically. A collection of over 1,000 phages infecting Mycobacterium smegmatis reveals the diversity of phages of a common bacterial host, but their relationships to phages of phylogenetically proximal hosts are not known. Comparative sequence analysis of 79 phages isolated on Gordonia shows these also to be diverse and that the phages can be grouped into 14 clusters of related genomes, with an additional 14 phages that are “singletons” with no closely related genomes. One group of six phages is closely related to Cluster A mycobacteriophages, but the other Gordonia phages are distant relatives and share only 10% of their genes with the mycobacteriophages. The Gordonia phage genomes vary in genome length (17.1 to 103.4 kb), percentage of GC content (47 to 68.8%), and genome architecture and contain a variety of features not seen in other phage genomes. Like the mycobacteriophages, the highly mosaic Gordonia phages demonstrate a spectrum of genetic relationships. We show this is a general property of bacteriophages and suggest that any barriers to genetic exchange are soft and readily violable.

Despite the numerical dominance of bacteriophages in the biosphere, there is a dearth of complete genomic sequences. Current genomic information reveals that phages are highly diverse genomically and have mosaic architectures formed by extensive horizontal genetic exchange. Comparative analysis of 79 phages of Gordonia shows them to not only be highly diverse, but to present a spectrum of relatedness. Most are distantly related to phages of the phylogenetically proximal host Mycobacterium smegmatis, although one group of Gordonia phages is more closely related to mycobacteriophages than to the other Gordonia phages. Phage genome sequence space remains largely unexplored, but further isolation and genomic comparison of phages targeted at related groups of hosts promise to reveal pathways of bacteriophage evolution.

Bacteriophage evolution differs by host, lifestyle and genome

Bacteriophages play key roles in microbial evolution^1,2, marine nutrient cycling³ and human disease⁴. Phages are genetically diverse, and their genome architectures are characteristically mosaic, driven by horizontal gene transfer with other phages and host genomes⁵. As a consequence, phage evolution is complex and their genomes are composed of genes with distinct and varied evolutionary histories^6,7. However, there are conflicting perspectives on the roles of mosaicism and the extent to which it generates a spectrum of genome diversity⁸ or genetically discrete populations^9,10. Here, we show that bacteriophages evolve within two general evolutionary modes that differ in the extent of horizontal gene transfer by an order of magnitude. Temperate phages distribute into high and low gene flux modes, whereas lytic phages share only the lower gene flux mode. The evolutionary modes are also a function of the bacterial host and different proportions of temperate and lytic phages are distributed in either mode depending on the host phylum. Groups of genetically related phages fall into either the high or low gene flux modes, suggesting there are genetic as well as ecological drivers of horizontal gene transfer rates. Consequently, genome mosaicism varies depending on the host, lifestyle and genetic constitution of phages.

Vibrio Phage KVP40 Encodes a Functional NAD+ Salvage Pathway

The genome of T4-type Vibrio bacteriophage KVP40 has five genes predicted to encode proteins of pyridine nucleotide metabolism, of which two, nadV and natV, would suffice for an NAD⁺ salvage pathway. NadV is an apparent nicotinamide phosphoribosyltransferase (NAmPRTase), and NatV is an apparent bifunctional nicotinamide mononucleotide adenylyltransferase (NMNATase) and nicotinamide-adenine dinucleotide pyrophosphatase (Nudix hydrolase). Genes encoding the predicted salvage pathway were cloned and expressed in Escherichia coli, the proteins were purified, and their enzymatic properties were examined. KVP40 NadV NAmPRTase is active in vitro, and a clone complements a Salmonella mutant defective in both the bacterial de novo and salvage pathways. Similar to other NAmPRTases, the KVP40 enzyme displayed ATPase activity indicative of energy coupling in the reaction mechanism. The NatV NMNATase activity was measured in a coupled reaction system demonstrating NAD⁺ biosynthesis from nicotinamide, phosphoribosyl pyrophosphate, and ATP. The NatV Nudix hydrolase domain was also shown to be active, with preferred substrates of ADP-ribose, NAD⁺, and NADH. Expression analysis using reverse transcription-quantitative PCR (qRT-PCR) and enzyme assays of infected Vibrio parahaemolyticus cells demonstrated nadV and natV transcription during the early and delayed-early periods of infection when other KVP40 genes of nucleotide precursor metabolism are expressed. The distribution and phylogeny of NadV and NatV proteins among several large double-stranded DNA (dsDNA) myophages, and also those from some very large siphophages, suggest broad relevance of pyridine nucleotide scavenging in virus-infected cells. NAD⁺ biosynthesis presents another important metabolic resource control point by large, rapidly replicating dsDNA bacteriophages.IMPORTANCE T4-type bacteriophages enhance DNA precursor synthesis through reductive reactions that use NADH/NADPH as the electron donor and NAD⁺ for ADP-ribosylation of proteins involved in transcribing and translating the phage genome. We show here that phage KVP40 encodes a functional pyridine nucleotide scavenging pathway that is expressed during the metabolic period of the infection cycle. The pathway is conserved in other large, dsDNA phages in which the two genes, nadV and natV, share an evolutionary history in their respective phage-host group.

Genome Sequence of Cluster W Mycobacteriophage Taptic

The Taptic genome is the first to be annotated from the W cluster of mycobacteriophages infecting Mycobacterium smegmatis mc²155. All 92 predicted open reading frames (ORFs) and a single tRNA specifying glycine (tRNA-gly) are transcribed rightward. Many functionally uncharacterized ORFs appear to be W cluster specific, as nucleotide similarity is shared only with other W cluster genomes.