Mimivirus - portal view
Giruses - Mimivirus
Discovered in 1992, Mimivirus greatly expanded the scientific view about viruses due to its unusually large
size. With a capsid diameter of over 400 nm (excluding the fibres) it was almost twice the diameter of the
largest virus known at the time and became the first member of the Mimiviridae (Megaviridae). It has a very
large genome for a virus, larger than that of some bacteria, at just under 1.2 Mbp (1.181404 Mbp) and
consisting of double-stranded DNA (dsDNA, the genetic material most commonly found in large viruses). This
contains some 911 genes and produces some 979 proteins. Such a giant virus is also called a girus.
Above: a computer model of Mimivirus. The icosahedral capsid is coated in long
filaments or fibres of 125 nm length(only shown on one face for clarity).
The large capsid has a triangulation number, T somewhere between 972 and 1141 (1200 ?). Projecting from
the capsid are close-packed 125 nm fibres (giving the virion a total diameter of 750 nm). One apex of 5-fold
symmetry has a starfish-shaped structure which seems to be a portal through the capsid and so has been
called the 'stargate'. evidence suggests that the fibres may be cross-linked by peptidoglycan, which is a
compound normally found in bacterial cell walls. Since bacteria is a natural food of the amoeba, this possibly
acts as a decoy, tricking the amoeba into ingesting the virus which can then infect the cell.

Unusual for a virus, the genome of Mimivirus contains many 'cellular genes' including the first four aminoacyl
tRNA synthetases (ArgRS, CysRS, MetRS, TyrRS: see
protein synthesis). It is very unusual for viruses to
encode enzymes which are part of cellular synthetic pathways. The virus encodes some genes for sugar,
lipid, nucelotide and amino acid metabolism. There is apparently much less economy on genetic material,
which is so important to small viruses, indeed 10% of its genome appears to be junk DNA (still not very much
but in some viruses just about every base codes for a protein).

Mimivirus parasitises amoeba, single-celled protoctistans (protozoa). Since the discovery of Mimivirus, other
megaviruses (giruses) have been discovered. Megavirus, which also infects amoebae, has a 1.259197 Mbp
which possibly codes for as many as 1120 proteins (including 7 aminoacyl tRNA synthetase enzymes). Other
forms apparently infect protozoa. Mamavirus is a larger strain of Mimivirus.

Beneath the capid is a gap of 30 to 50 nm and then a lipid envelope which encloses the genetic material,
which is folded into an asymmetric shape, as revealed by X-ray laser scanning and illustrated below. This is
possibly a portal for the dsDNA genome to exit the capsid, once in the host cell, to initiate replication.
Above: the pentagonal portal at one 5-fold vertex of Mimivirus.
Mimivirus- packaged genome
Satellite Viruses

Mimivirus is itself parasitised by a smaller virus called Sputnik virus. Sputnik is a subvirus or satellite virus,
meaning that it can only reproduce inside a host cell which is also infected by Mimivirus. Mimivirus is
described as a helper virus for Sputnik. Another example of a satellite virus is HDV (hepatitis D virus) which
paraistises the HBV (heptatitis B virus) helper virus. Unusually, however, due to the large size of Mimivirus,
several satellite virus particles have been seen enclosed inside a single Mimivirus particle, waiting to co-infect
a new host amoeba. Sputnik virus has a genome of 18.343 kb (circular dsDNA, cdsDNA) and about 21
genes. It is 50 nm in diameter, icosahedral, and its reproduction disrupts or reduces Mimivirus reproduction.

Girus Evolution

A key question in virology is: how did viruses first evolve? Are they ancient relics of early (possibly
precellular) life or derived from cells by dgeneration. Certainly some virus genes are ancient, but others are
acquired more recently in evolution from host cells. Are giruses viruses enlarging and acquiring more genetic
material or descended from cellular life-forms by compaction (and generation of the protective protein capsid
around the dormant 'spore').
Article created: 24th Oct 2015
Left: fibres illustrated on two faces to roughly illustrate their length.