Ff phages
| Ff phages | |
|---|---|
| Virus classification | |
| Group: | Group II (ssDNA) |
| Family: | Inoviridae |
| Genus: | Inovirus |
The Ff phages are filamentous phages that infect gram negative bacteria bearing the F episome. The term is used to refer to the closely related group of phages including the f1, fd and M13 phages. The If1 and Ike phage are also closely related to this group.
Theses phages undergo a non-lytic lifecycle, where infected bacteria are not killed but continuously release viral particles. This is in contrast to the lytic and lysogenic cycles of other phages where the host is killed when virions are released. The Ff phages have a circular ssDNA genome wrapped in a few thousand copies of the major coat protein to give a rod shaped viral particle that is capped on either end with two different sets of proteins. Only one end binds to the F pilus and can inject the DNA into the host cell.
Ff phages need the host cell to be Hfr or F+. When a virion binds to the sex pilus of a host by protein 3 in its beaded end, the pilus retracts and the virus fuses with the host membrane. Several host proteins are involved in this step and the genome of the virus is released into the bacterial cytoplasm.
The single stranded DNA of the virus is converted into double stranded DNA by the host machinery. This form is called the replicative form (RF). The strand which came from the virus (VS) is nicked by the viral protein p2 (pII). Host DNA replication machinery then replicates the VS in a rolling circle form of replication by extending the 3' end of the nick, pushing out the 5' end as a single strand. P2 chops off the strand each time its extension completes a full circle and ligates the single stranded copy into a circle. These ssDNA circles are converted to dsDNA like the original viral genome and the RFs accumulate in the cell. The RFs are transcribed and viral proteins accumulate. The RFs are passed on indefinitely to the daughter cells as plasmids.
When sufficient copies of the RF and a viral protein p5 (pV / g5p) have formed, new ssDNA viral genomes get bound by p5 by interaction with a specific sequence on the DNA called the packing signal. This prevents its conversion to dsDNA. The interaction between the DNA and the p5 is weak except at the packing signal. The p5-DNA complex binds to a complex of viral and host proteins on the membrane. As the DNA is stripped of the p5, it is extruded across the membrane and wrapped in a helical casing of p8.
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