10 nm upon adsorption or after chloroform treatment (Fig. 1).
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Type Species |
(PRD1) |
Virions are icosahedra, have no envelope, and measure about 63 nm in diameter (Fig. 1). Capsids have apical spikes which are 20 nm long for Bacillus phage AP50 (AP50) particles. The capsid of Enterobacteria phage PRD1 (PRD1) is constructed of 240 major capsid protein (P3) trimers that form a pseudo T = 25 lattice. Protein P3 contains two beta barrels and form very tight trimers. The spikes are formed of three protein species (P2, P5, P31) and are used for receptor recognition. The capsid protein (CP) encloses an inner lipoprotein vesicle that is formed of approximately equal amounts of protein and lipid. About 15 virus specific proteins are associated with the vesicle. The DNA is coiled within the latter. Virions are normally tail-less, but produce tail-like tubes of about 60 10 nm upon adsorption or after chloroform treatment (Fig. 1).
Physicochemical and Physical Properties
Virion Mr is about 70 106; S20w is 357-416S; buoyant density in CsCl is about 1.29 g/cm3. Virions are usually stable at pH 5-8. Bacillus phage NS11 (NS11) has a pH optimum of 3.5. Infectivity is sensitive to ether, chloroform, and detergents.
Virions contain a single molecule of linear dsDNA, 14.7-15.7 kbp in size (Mr 9.2-9.9 106). The DNA mass corresponds to 14-15% of particle weight. The complete nucleotide sequences of phage PRD1 (14,925 bp) and those of five closely related phages (Enterobacteria phage PR3; Enterobacteria phage PR4; Enterobacteria phage PR5; Enterobacteria phage PR772, and Enterobacteria phage L17) have been determined. The 5
-ends of the genome carry covalently linked proteins.
PRD1 proteins and their functions are listed in Table 1. Tectiviruses infecting members of the genus Bacillus contain at least 6 proteins (Mr 12.4-63 103). Their major CP has a Mr of 43-48 103 and suggested to be present in about 920 copies. Amino acid sequence data are available.
Virions contain about 15% lipids by weight (5-6 species). Lipids constitute about 60% of the inner vesicle. In PRD1, lipids form a bilayer and seem to be in a liquid crystalline phase. Phospholipid contents (56% phosphatidylethanolamine and 37% phosphatidyl-glycerol, 5% cardiolipin) are higher than in the host, but vary depending on the host strain. The fatty acid composition is identical to that of the host.
Not known.
Genome Organization and Replication
The linear genome has inverted terminal repeats. Replication is protein-primed, proceeds by strand displacement, and can start at both ends of the genome because the inverted terminal repeats contain sites for the initiation of replication (Fig. 2). After DNA entry, replication and transcription, capsid proteins polymerize in the cytoplasm, whereas membrane-associated proteins are inserted into the host plasma membrane. With the help of nonstructural virion-encoded assembly factors and the coat-forming proteins, a virus-specific lipoprotein vesicle is translocated to the interior of the cell where it forms an empty capsid and where DNA packaging takes place. Mature virions are released by lysis of the host cell (Fig. 3).
Phages are virulent. The tail-like tube probably acts as a DNA injection device. PRD1 and its relatives adsorb to IncP type plasmid encoded DNA transfer complex and multiply in Gram-negative bacteria (enterobacteria, Acinetobacter, Pseudomonas, Vibrio, possibly others). Phage NS11 is specific for the genus Alicyclobacillus, phages AP50 and six other isolates are specific for Bacillus, Thermus phage P37-14 (P37-14) and about 50 isolates are specific for Thermus and occur in volcanic hot springs.
List of Species Demarcation Criteria in the Genus
In the absence of comparative DNA data, four species are distinguished on the basis of host range and growth conditions. Phage NS11 is acidophilic and thermophilic, the Thermus phages are thermophilic. PRD1 and the Bacillus tectiviruses are mesophilic.
Official virus species names are in italics. Tentative virus species names, alternative names ( ), strains or serotypes are not italicized. Virus names, genome sequence accession numbers [ ], and assigned abbreviations ( ) are:
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Bacillus phage AP50 |
(AP50) | |
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Bacillus phage NS11 |
(NS11) | |
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Enterobacteria phage PRD1 |
[69077] |
(PRD1) |
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Enterobacteria phage PR3 |
(PR3) | |
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Enterobacteria phage PR4 |
(PR4) | |
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Enterobacteria phage PR5 |
(PR5) | |
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Enterobacteria phage PR772 |
(PR772) | |
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Enterobacteria phage L17 |
(L17) | |
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Thermus phage P37-14 |
(P37-14) |
Tentative Species in the Genus
None reported.
List of Unassigned Viruses in the Family
Vibrio phage 06N-58P (may be a corticovirus).
Phylogenetic Relationships within the Family
No phylogenetic tree yet completed, the enterobacteria phages sequenced show 95% similarity.
Tectiviruses have similarities to tailed phages (capsid size, “tail”) and corticoviruses (capsid size, inner component, presence of lipids). They differ from tailed phages by the presence of an inner vesicle and the transitory nature of their “tail” and from corticoviruses by DNA conformation and the ability to produce a tail-like tube.
Surprisingly, PRD1, and presumably other tectiviruses, has many features in common with adenoviruses. These include: (1) a linear genome, (2) genomic inverted terminal repeats, (3) protein-primed initiation of replication, (4) type B (E. coli Pol II) DNA polymerase, (5) trimeric capsid protein that contains two beta barrels and forms hexons, (6) arrangement of the capsid protein in a pseudo T = 25 lattice, and (7) receptor-binding spikes at capsid vertices.
Tecti: from Latin tectus, “covered”.
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