Until October 2011 it had largest capsid diameter of all known viruses, as well as a large and complex genome compared with other viruses. Distant Mimivirus relative with a larger genome highlights the fundamental features of Megaviridae
Billionaire adventurer Richard Branson may have large-scale plans for deep sea exploration, but a new ocean discovery makes big waves on a microscopic level. A team of researchers trawling the ocean floor have just published their findings of the world’s new largest virus, found lurking off the coast of Las Cruces, Chile. It’s so big, it’s actual scientific name is Megavirus chilensis — and you can even view it with a basic light microscope. The previous virus record holder was Mimivirus, which boasted the largest diameter of any virus to date until Megavirus came along.
The virus’ DNA features 1,259,197 base pairs, which encode some parasitic bacteria-like features. Unlike a virus, bacteria is a cellular organism — a virus can only infect and replicate itself within the cells of other organisms. It even has some built-in DNA repairing enzymes which allow the virus to repair damage from ultraviolet light, chemicals, and radiation.
Mimivirus was found in 1992, lurking in an amoeba in Bradford, England. Its capsid, the protein shell housing the virus’ genetic material, was 400 nanometres wide. If that still sounds pretty tiny, most viruses fall in the range between 20 and 300 nanometres, making both Mimivirus and the new Megavirus titans of the microscopic world. Mimivirus and Megavirus are believed to have diverged from a shared viral ancestor somewhere along the evolutionary road, both developing into giant viruses in their own right.
Mimivirus has been placed into a viral family by the International Committee on Taxonomy of Viruses as a member of the Mimiviridae and has been placed into Group I of the Baltimore classification system. Although not strictly a method of classification, Mimivirus joins a group of large viruses known as nucleocytoplasmic large DNA viruses (NCLDV). They are all large viruses which share both molecular characteristics and large genomes. The mimivirus genome also possesses 21 genes encoding homologs to proteins which are seen to be highly conserved in the majority of NCLDVs, and further work suggests that mimivirus is an early divergent of the general NCLDV group.
Mimivirus possesses many characteristics which place it at the boundary of living and non-living. It is as large as several bacterial species, such as Rickettsia conorii and Tropheryma whipplei, possesses a genome of comparable size to several bacteria, including those above, and codes for products previously not thought to be encoded by viruses. In addition, mimivirus possesses genes coding for nucleotide and amino acid synthesis, which even some small obligate intracellular bacteria lack. This means that unlike these bacteria, mimivirus is not dependent on the host cell genome for coding the metabolic pathways for these products. They do however, lack genes for ribosomal proteins, making mimivirus dependent on a host cell for protein translation and energy metabolism. These factors combined have thrown scientists into debate over whether mimivirus is a distinct form of life, comparable on a domain scale to Eukarya, Archaea and Bacteria. Nevertheless, mimivirus does not exhibit the following characteristics, all of which are part of many conventional definitions of life: homeostasis, response to stimuli, growth in the normal sense of the term (instead replicating via self-assembly of individual components) or undergoing cellular division.
Because its lineage is very old and could have emerged prior to cellular organisms, mimivirus has added to the debate over the origins of life. Some genes unique to mimivirus, including those coding for the capsid, have been conserved in a variety of viruses which infect organisms from all domains – Eukarya, Archaea and Bacteria. This has been used to suggest that mimivirus is related to a type of DNA virus that emerged before cellular organisms and played a key role in the development of all life on Earth. An alternative hypothesis is that there were three distinct types of DNA viruses that were involved in generating the three known domains of life