|
Type Species |
(6) |
Virions are about 85 nm in diameter, spherical, with an envelope covered by spikes. The envelope surrounds an icosahedral nucleocapsid which is about 58 nm in diameter. The removal of the nucleocapsid surface protein reveals a polymerase complex which is about 43 nm in diameter (Fig. 1).
Physicochemical and Physical Properties
Virion Mr is about 99 
106; and that of the nucleocapsid is about 40 106. Virion S20w is about 405S. The buoyant density of the virion is 1.27 g/cm3 in CsCl and 1.24 g/cm3 in sucrose. Pseudomonas phage 6, (6) is stable at pH 6-9 and very sensitive to ether, chloroform and detergents.
Virion contains three linear dsRNA segments L (6374 bp), M (4057 bp), and S (2948 bp). The segments have a base composition of 55.2, 56.7, and 55.5% G+C, respectively. Virions contain about 10% RNA. Nucleic acid sequence data are available from GenBank and EMBL.
The genome codes for twelve proteins (Fig. 2). The early proteins P1, P2, P4, and P7 are coded from the L segment and form the viral polymerase complex. The association of protein P8, the NC surface protein, and the viral lytic enzyme, P5, with the polymerase complex forms the NC. These proteins are coded from the genome segment S. Proteins P9, P10, and P11 reside in the envelope. The absorption and fusion complex is formed by proteins P3 and P6. P3 is the spike protein recognizing the receptor, whereas P6 is a membrane protein with membrane fusion activity. P3 is associated with the virion through protein P6. There is so far only one identified nonstructural protein, P12, which is needed in the membrane assembly inside the host cell. Virions are composed of about 70% protein.
Virions contain about 20% phospholipid. This is located in the envelope. There is enough lipid to cover about one-half of the envelope surface area (the rest being protein).
None reported.
Genome Organization and Replication
Virions adsorb to Pseudomonas syringae pili which retract bringing the virion into contact with the host outer membrane. The virus membrane fuses with the host outer membrane and the nucleocapsid associated lytic enzyme locally digests the peptidoglycan. The nucleocapsid enters the cell and the viral polymerase is activated to produce early transcripts. The translated L transcripts produce the early proteins which assemble to polymerase complexes. These package all three positive strand transcripts. Negative strand synthesis takes place inside the polymerase complex. These polymerase complexes transcribe late messages which code the synthesis of late genes. The nucleocapsid surface protein assembles on the polymerase complex and inactivates the transcription. The nucleocapsid acquires the membrane from the host plasma membrane with the aid of a virus specific nonstructural assembly factor. The cell lyses and liberates mature progeny particles (Fig. 3).
6 infects many phytopathogenic Pseudomonas species. In addition, some Pseudomonas pseudoalcaligenes strains are sensitive to this virus.
List of Species Demarcation Criteria in the Genus
Not applicable.
Official virus species names are in italics. Tentative virus species names, alternative names ( ), strains or serotypes are not italicized. Virus name, genome sequence accession numbers [ ], and assigned abbreviation ( ) are:
|
Pseudomonas phage 6 |
[M17461, M17462, M12921] |
(6) |
Tentative Species in the Genus
None reported.
List of Unassigned Viruses in the Family
Recently other 6-type viruses have been added. Their taxonomic status is uncertain.
Phylogenetic Relationships ithin the Family
Not applicable.
In term of genome strategy Pseudomonas phage 6 resembles some of the members of the family Reoviridae. The structure and function of the polymerase particle containing the genome segments is the major similarity. The polymerase particle is also surrounded by two additional layers that are involved in determining host specificity and are crucial in the entry of the polymerase particle to the cell.
Cysto: from Greek kystis, ‘bladder, sack’.
|
|  |