Typically, viruses are considered to be small particles that easily pass through 0.2 µm filters and have small genomes containing a few protein-encoding genes. However, large viruses with huge dsDNA genomes that encode hundreds of proteins are being discovered with increasing frequency. These large viruses have also been referred to as giruses in order to emphasize their unique properties. Examples of giruses include:
- Mimivirus and its close relative Mamavirus, which infect amoebae and have the largest genomes (~1.2 Mb). Mimivirus has 979 protein-encoding sequences (CDSs), six tRNA genes and 33 non-coding RNA genes.
- Viruses that infect algae (phycodnaviruses) and have genomes up to ~560 kb.
- Viruses, such as bacterophage G, that infect bacteria and have genomes up to ~670 kb (~498 kb is unique sequence).
A recent report describes the newest girus, a lytic virus (named CroV) that infects the marine microzooplankton Cafeteria roenbergensis. CroV has a ~730 kb genome and contains 544 CDSs and 22 tRNAs encoding genes in the 618 kb central region of its genome. Viruses with genomes ranging from 100 to 280 kb, such as herpesviruses and baculoviruses, are not discussed in this commentary, and poxviruses, asfarviruses, iridoviruses, and ascoviruses are only briefly mentioned because of their evolutionary connection to some giruses. Another group of viruses with dsDNA genomes >500 kb are the polydnaviruses.
To place the size of these large viruses into perspective, the smallest free-living bacterium, Mycoplasma genitalium, encodes ~470 CDSs. Although estimates of the minimum genome size required to support life are ~250 CDSs, some symbiotic bacteria such as Carsonella ruddii and Hodgkinia cicadicola have genomes of 160 kb and 144 kb, respectively. Thus, many large viruses have more CDSs than some single-celled organisms.
Another Really, Really Big Virus. (2011) Viruses 3(1): 32-46; doi:10.3390/v3010032
Viruses with genomes larger than 300 kb and up to 1.2 Mb, which encode hundreds of proteins, are being discovered and characterized with increasing frequency. Most, but not all, of these large viruses (often referred to as giruses) infect protists that live in aqueous environments. Bioinformatic analyses of metagenomes of aqueous samples indicate that large DNA viruses are quite common in nature and await discovery. One issue that is perhaps not appreciated by the virology community is that large viruses, even those classified in the same family, can differ significantly in morphology, lifestyle, and gene complement. This brief commentary, which will mention some of these unique properties, was stimulated by the characterization of the newest member of this club, virus CroV (Fischer, M.G.; Allen, M.J.; Wilson, W.H.; Suttle, C.A. Giant virus with a remarkable complement of genes infects marine zooplankton. PNAS USA 2010, 107, 19508-19513). CroV has a 730 kb genome (with ~544 protein-encoding genes) and infects the marine microzooplankton Cafeteria roenbergensis producing a lytic infection.