M13 Filamentous Bacteriophage
M13 and related bacteriophages (36 species, e.g. Ff, f1, fd, Pf1) are types of Inovirus. M13 infects the bacterium
Escherichia coli. Each virus particle consists of a filament about 7 nm in diameter by 880 nm in length. M13 infects
Escherichia coli cells which carry the F episome (a group of genes, often carried on a plasmid) which encodes the F
pilus (see conjugation). The virus binds to the F pilus by the 5 or so copies of the g3p glycoprotein at the head end of
the filament (stalked cyan spheres in the model above). This triggers retraction of the pilus. Upon reaching the target
cell surface, the g3p proteins bind to the bacterial envelope protein TolA, which, with the help of certain other
proteins, causes the head of the virus to breach the outer membrane of the target cell and then insert into the target's
inner membrane. at this point the cap of the viral head opens and the viruses genetic material is injected into the host
F1 filament
Above: the Inovirus particle consists of 2750 copies of the g8p glycoprotein (magenta) which
binds to the genetic material, which is single-stranded DNA (ssDNA), forming a helix with a
pitch of about 16 degrees (it is at 16 degrees to the axis of the filament). The g3p are shown in
cyan. The proteins in yellow are units of g6p, to which the g3p are anchored. This protein.
along with g3p, is also essential for entry of the viral genome: the two proteins form a g3p-g6p
adsorption complex for adhesion to the envelope of the target cell.
Above: the tail-end of the virion. In orange are copies of the g7p and in green the g9p

Once the viral ssDNA is in the host cell (this is designated the plus-strand or (+)ssDNA since if
directly converted into RNA this strand contains the actual mRNA code - it is the sense strand)
host cell polymerase enzyme synthesises the complementary (-)ssDNA, resulting in a
double-stranded DNA (dsDNA) intermediate which is covalently closed into a circle: cc-dsDNA
or the replicative form, RF. Host cell RNA polymerase then synthesises viral RNAs by reading
the (-) strand (which is complimentary to the actual mRNA code). There is apparently a specific
promoter of each of the ten genes.

The viral protein g2p is a nuclease and it nicks the RF at the ori (origin of replication) which
allows replication of the positive strand by
rolling circle replication. These copies of the (+)
strand are then converted into more dsDNA RFs to amplify viral replication and viral protein

When enough copies of the g5p protein have been synthesised, these inhibit convertion of
(+)ssDNA molecules into RF dsDNA, so that the (+)ssDNA can be used as genomic DNA and
packaged into new virus particles. The g5p coats the (+)ssDNA, but is later replaced by g8p,
the capsid protein, which triggers assembly of the virus particles.

New virus particles are extruded or secreted from the bacterial host. This does not kill the host,
which continues to divide and can produce viral particles indefinitely. Viral proteins g1p and
g4p are necessary for this extrusion process as they increase the number of contact sites
between the outer and inner membranes which is where the new virus particles are extruded.
They are extruded tail-end (g7p end) first. Viral proteins g3p and g6p also play a role in
termination of filamentous phage assembly, by forming the tail-end of completed phages.

Viral proteins g9p (green in our model) and g7p (orange) possibly initiate extrusion in response
to the packaging signal (given by the genome when it is packaged). This is the first end to
emerge from the host cell.
Article written: 24th Oct 2015