Group Group II (ssDNA) Genus Inovirus | Family Inoviridae Rank Species | |
Similar M13 bacteriophage, T7 phage, Phi X 174, T3 phage, Bacteriophage Qβ | ||
Bacteriophage f1 is structurally classified as a class I filamentous phage, and is closely related to the other Ff phages, such as M13 and phage fd.
Contents
In the following article, genes will be written in italics and their associated proteins in Roman.
Morphology
Phage f1 is a filamentous (rod-shaped) ssDNA phage, with a molecular mass of about 1.6×107 Da; by weight it is 11.3 percent DNA. The flexible phages are about 850 nm long and 4.3 or 6.3 nm wide. The thousands of identical major coat proteins that make up the sheath are arranged in a fishcale-like α-helix.
The ends of the filament are sealed with protein caps; the blunt end contains 3–5 copies of each VII and IX, while the terminally extruded bead-like end contains about 5 copies each of III and VI.
Genome Organization
f1 has a circular, 6407 bp genome of ss-DNA.
There are 11 genes encoded on the genome; two are overlapping in-frame genes. Five of the encoded proteins make up the viron, three are needed for synthesis and the rest are for assembly. The genes are generally referred to by Roman numerals I-XI, and are in the order II(X), V, VII, IX, VIII, III, VI, I(XI), IV, intergenic region (IG or IGR). The IGR contains the packing signal (PS) as well as sequences that dictate termination, nicking for replication, and the binding of II and IHF.
Adsorption and Injection
f1 is male specific; it only infects Hfr or F+ strains of E. coli.
Several host proteins are required for infection and depolymerization of the coat proteins of the phage. A domain of III binds to the primary receptor, the tip of the F pilus, and retraction of the pilus by an unknown mechanism brings another domain of III close enough to the membrane to bind the coreceptor, host tolA.
The removal of the cap proteins and release of the ssDNA into the cytoplasm are both mediated by the interactions of tolA and III. A hydrophobic portion of III inserts itself into the inner membrane, fastening the phage particle to the host and distributing the coat proteins into the host membrane. It is the degradation of the major coat protein that causes the genome to be released into the cytoplasm of the host.
DNA replication
The ssDNA genome is replicated and translated with host enzymes after it is inserted into the cytoplasm. RNA polymerase, DNA polymerase III, and host gyrase form a supercoiled RF template used for replication.
Phage II specifically cuts the (+) strand at the origin, and host Rep helicase aids rolling circle replication. II is necessary for ligation of the newly isolated (+) strand, and more host enzymes are necessary for translation.
Phage proteins can be detected in the supernatant about 10 minutes after infection (37°).
Packaging
DNA-V complexes form in the hundreds and prevent further copies of the complement strand from being made while also "pre-packaging" and stabilize the ssDNA into an almost linear form. About 1500 copies of V form a flexible, semi-enclosed left-handed helix that encapsulate the DNA. The ssDNA does not interact with itself inside the capsule, and remains untwisted and unpaired except for the extremely stable packing signal. The PS exists as an imperfect hairpin and is responsible for the attachment and orientation of the genome in the phage. Proper packing can be accomplished if and only if the PS is present; even after the genome has been covered it remains partially exposed at the blunt end of the complex.
Lysis
This phage does not lyse its host, but can secrete many copies of itself throughout the life of the host cell. The cell can continue to divide even while infected, and cell metabolism is minimally affected by the phage. The phages are secreted as they are assembled, and once the DNA-V complex is formed all further assembly and secretion steps take place in or at a membrane. The proteins depolymerized upon phage entry can be reused for the packaging of new phage, and as with newly synthesized coat proteins, the proteins remain as integral membrane proteins until needed.
The host-encoded thioredoxin is the only known host protein required for assembly of nascent phage and likely confers processivity. The presence of a properly presented ssDNA in an adhesion zone with thioredoxin and the coat proteins allows for the elongation and secretion of phages. The phage is elongated by continually removing dimers of V and replacing them with the major coat protein. The phage coat consists of about 2700 copies of VIII. The end of the DNA signals the cell to add the end cap proteins, which are likely already associated with VIII.
As with most filamentous phages, there is no defined limit on the amount of DNA that can be packaged into the phage coat. Excessive copies of the phage genome can be packaged, called "polyphage"; although these are due to improper termination of extrusion about 5% of the phage produced in the presence of termination signals are twice the normal length. Even larger inserts are naturally selected against. The non-terminated phage will remain anchored to the host, and attachment of another V-DNA complex will continue the elongation process. The PS site is only needed to initiate elongation, not for the attachment of additional genomes.
Phage can be assembled and released at a very high rate, with up to 1000 progeny released within an hour of infection of the host.