Once thought to be a type of bacteria, the mimivirus is a fascinating organism, appearing to be half-way between a virus and a living cell.
One of the largest known viruses, the mimivirus is a double-stranded DNA virus that infects amoeba. It was discovered rather inadvertently in 1992 in the Acanthamoeba polyphaga amoeba. The researchers were actually searching for the Legionella bacteria that causes Legionellosis.
Given the large size of the virus - it is ten times larger than the common cold virus and is visible under a light microscope - the researchers mistook it for a bacteria. They called it "Bradfordcoccus" after Bradford in England; the amoeba in which it had been found was from this area. It wasn't until 2003 that French researchers from Marseille's Université de la Méditerranée identified it as a virus.
The mimivirus has since been examined under an electron microscope and although much about it still remains to be explained, we do now have a clearer picture of its viral structure.
Structure of the Mimivirus:
- The mimivirus has a capsid diameter of around 400 nm. This is larger than that of all other viruses.
- The capsid has outer protein filaments that extend total virus length to around 600 nm or 800 nm.
- The mimivirus capsid has an icosahedral symmetry.
- The mimivirus does not have an outer viral envelope.
- It may, however, have an internal lipid layer.
The DNA of the mimivirus contains around 911 genes.
The exact details of mimivirus replication are not known as yet. However, by studying infected cells under the electron microscope, researchers think the viral replication might take the following steps -
- The mimivirus attaches itself to the outer surface of an amoeba.
- It is absorbed into the cell and appears to be inactive for a while.
- However the virus is replicating and this becomes evident a few hours later.
- The mimivirus continues to replicate and fill the amoeba cell with mimivirus virions.
- The cell eventually bursts and the virions are released.
Importance of the Mimivirus
The mimivirus is unusual not only for its size, but also for certain features that seem to straddle it between viruses and living cells. It does, for example, have several morphological characteristics similar to those found in many nucleocytoplasmic large DNA viruses (NCLDV). It does not contain ribosomal protein genes. The absence of these genes means that the mimivirus is incapable of carrying out any kind of protein translation without its amoebic host. It also cannot carry out any energy metabolism on its own. However, the mimivirus does contain certain genes for sugar, lipid, and amino acid metabolism that are not seen in other viruses. On the other hand, unlike living cells, the mimivirus does not respond to stimuli, does not undergo cell division, and does not grow.
There is ongoing debate amongst scientists regarding two possibilities about the mimivirus -
- It could be a distinct life-form
- It is an ancient virus that may have originated before cellular organisms, and in fact may have participated in the development of cellular organisms.