Pseudomonas phage Φ6
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|Pseudomonas phage Φ6|
|Group:||Group III (dsRNA)|
|Species:||Pseudomonas phage Φ6|
Φ6 (Phi 6) is the best-studied bacteriophage of the virus family Cystoviridae. It infects Pseudomonas bacteria (typically plant-pathogenic P. syringae). It has a three-part, segmented, double-stranded RNA genome, totalling ~13.5 kb in length. Φ6 and its relatives have a lipid membrane around their nucleocapsid, a rare trait among bacteriophages. It is a lytic phage, though under certain circumstances has been observed to display a delay in lysis which may be described as a "carrier state".
Life cycle of Φ6
Φ6 typically attaches to the Type IV pilus of P. syringae with its attachment protein, P3. It is thought that the cell then retracts its pilus, pulling the phage toward the bacterium. Fusion of the viral envelope with the bacterial outer membrane is facilitated by the phage protein, P6. The muralytic (peptidoglycan-digesting) enzyme, P5, then digests a portion of the cell wall, and the nucleocapsid enters the cell coated with the bacterial outer membrane.
A copy of the sense strand of the large genome segment (6374 bases) is then synthesized (transcription) on the vertices of the capsid, with the RNA-dependent RNA polymerase, P2, and released into the host cell cytosol. The four proteins translated from the large segment spontaneously assemble into procapsids, which then package a large segment sense strand, polymarizing its complement during entry through the P2 polymerase-containing vertices. While the large segment is being translated (expressed) and synthesized (replicated), the parental phage releases copies of the sense strands of the medium segment (4061 bases) and small segment (2948 bases) into the cytosol. They are translated, and packaged into the procapsids in order: medium then small. The filled capsids are then coated with the nucleocapsid protein P8, and then outer membrane proteins somehow attract bacterial inner membrane, which then envelopes the nucleocapsid.
The lytic protein, P5, is contained between the P8 nucleocapsid shell and the viral envelope. The completed phage progeny remain in the cytosol until sufficient levels of the lytic protein P5 degrade the host cell wall. The cytosol then bursts forth, disrupting the outer membrane, releasing the phage. The bacterium is killed by this lysis.
RNA-dependent RNA polymerase of Φ6
RNA-dependent RNA polymerases (RdRPs) are critical components in the life cycle of double-stranded RNA (dsRNA) viruses. However, it is not fully understood how these important enzymes function during viral replication. Expression and characterization of the purified recombinant RdRP of Φ6 is the first direct demonstration of RdRP activity catalyzed by a single protein from a dsRNA virus. The recombinant Φ6 RdRP is highly active in vitro, possesses RNA replication and transcription activities, and is capable of using both homologous and heterologous RNA molecules as templates. The crystal structure of the Φ6 polymerase, solved in complex with a number of ligands, provides insights towards understanding the mechanism of primer-independent initiation of RNA-dependent RNA polymerization. Interestingly, this RNA polymerase appears to operate without a sigma factor/subunit. The purified Φ6 RdRP displays processive elongation in vitro and self-assembles along with polymerase complex proteins into subviral particles that are fully functional.
Φ6 has been studied as a model to understand how segmented RNA viruses package their genomes, its structure has been studied by scientists interested in lipid-containing bacteriophages, and it has been used as a model organism to test evolutionary theory such as Muller's ratchet. Phage Φ6 has been used extensively in additional phage experimental evolution studies.
- Koivunen et al. (2008). "Structure-Function Insights Into the RNA-Dependent RNA Polymerase of the dsRNA Bacteriophage Φ6". Segmented Double-stranded RNA Viruses: Structure and Molecular Biology. Caister Academic Press. ISBN 978-1-904455-21-9.