DNA viruses: the really big ones (giruses)

Abstract

Viruses with genomes greater than 300 kb and up to 1200 kb are being discovered with increasing frequency. These large viruses (often called giruses) can encode up to 900 proteins and also many tRNAs. Consequently, these viruses have more protein-encoding genes than many bacteria, and the concept of small particle/small genome that once defined viruses is no longer valid. Giruses infect bacteria and animals although most of the recently discovered ones infect protists. Thus, genome gigantism is not restricted to a specific host or phylogenetic clade. To date, most of the giruses are associated with aqueous environments. Many of these large viruses (phycodnaviruses and Mimiviruses) probably have a common evolutionary ancestor with the poxviruses, iridoviruses, asfarviruses, ascoviruses, and a recently discovered Marseillevirus. One issue that is perhaps not appreciated by the microbiology community is that large viruses, even ones classified in the same family, can differ significantly in morphology, lifestyle, and genome structure. This review focuses on some of these differences than on extensive details about individual viruses.

Figure 1

(a) Transmission electron micrograph of Mimivirus. (b) Atomic force microscopy of defibered Mimivirus. The unique star-faced vertex is clearly visible. (c) Fivefold averaged cryo-electron micrographs of virus PBCV-1 reveal a long, thin, cylindrical spike structure at one vertex and protrusions (fibers) extending from one unique capsomer per trisymmetron. (d) Central cross section of panel c. Note the gap between the unique vertex and the membrane enclosing the DNA. Also the unique vertex contains a portal-like protein. (e) PBCV-1 attached to the cell wall as viewed by the quick-freeze, deep-etch procedure. Note fibers attach the virus to the wall. ( f ) Transmission electron micrograph of EhV. (g) Schematic of freshly isolated EhV (left) and stored EhV (right). Note the external membrane swells with age. (h, i) Morphology of the White spot shrimp virus (WSSV) virion. (h) Negative contrast electron micrograph of intact WSSV virion with its tail-like extension. (i ) Schematic based on panel h showing the layered structures of a WSSV virion, i.e., envelope, tegument, and nucleocapsid. ( j ) Electron micrograph of bacteriophage G. The insert shows coliphage lambda to the same scale. Panel a from Reference , b from Reference , c and d from Reference , e from Reference , f and g from Reference , h and i from Reference , and j from Reference —all published with permission.