Cafeteria roenbergensis virus

The virus was isolated from seawater samples collected from the Gulf of Mexico during 1989 to 1991, on a flagellate host that was misidentified as belonging to the genus Bodo; hence the original designation of the virus as BV-PW1. The virus was shown to be about 300 nm in diameter and have a complex internal structure, as well as evidence of a putative tail-like structure. CroV has one of the largest genomes of all marine viruses known, consisting of ~730,000 base pairs of double-stranded DNA. CroV is itself parasitized by a virophage named "Mavirus". ==Viral protein composition and structure==

Viral protein composition includes 141 encoded proteins that have been identified in CroV, a number believed to be in close proximity to the entirety of the virion proteome. The virus packages several distinct groups of proteins, including a presumably complete base excision repair (BER) pathway. This is the most extensive DNA repair machinery that has yet been observed in a virus. It is also the first virus to be found with a mechanosensitive ion channel protein, which may protect the genome from osmotic damage. Mature CroV consists of a 300 nm diameter outer protein shell with icosahedral symmetry, an underlying lipid membrane, and an inner core that contains the genome. Resolution of the virus structure by cryo-electron microscopy yielded an icosahedral virus capsid with a T number of 499 and a new model for capsid assembly for giant viruses. ==Viral genome==

CroV is the sole member of the genus Rheavirus in the family Mimiviridae within the proposed order Imitervirales. Phylogenetic analysis indicates that the virus is a nucleocytoplasmic large DNA virus (NCLD virus). Acanthamoeba polyphaga mimivirus is its closest known relative, although the two viruses share less than one-third of homologous genes. ==Viral replication==

Viral reproduction occurs in large constructs known as large cytoplasmic factories or viral factories. This is the site where DNA replication, transcription, and particle assembly are thought to take place. These factories are also the primary targets of the virophage Mavirus, which utilizes CroV machinery to replicate. Mavirus is a 19,000 kb circular double stranded DNA virus. Maviral infection reduces host cell death by interfering with CroV infection and replication. Mavirus integrates into the genome of cells of Cafeteria roenbergensis, and thereby confers immunity to the population. CroV enters cells via phagocytosis. Once inside the cell, the CroV capsid disassembles and the viral proteins and genome are released. CroV does not use the transcription or translation machinery of the host cell. It remains in the cytoplasm, where a “virus factory” forms and replicates independent of the host cell nucleus. The CroV genome is not integrated into the host cell genome. CroV encodes eight subunits of DNA-dependent RNA polymerase and it also encodes at least six transcription factors, which allows the DNA genome to be transcribed into mRNA without the use of the cell’s proteins. CroV can then translate the mRNAs into proteins with help of the cell's translation machine and by using its own tRNA synthetase, tRNA, and translation initiation factors to fine-tune the translation to its own advantage. ==Host interaction==

CroV infects Cafeteria roenbergensis, which is a marine zooflagellate. CroV is fatal to the host cell. This impacts coastal ecology because Cafeteria roenbergensis feeds on bacteria found in the water. When there are low numbers of Cafeteria roenbergensis due to extensive CroV infections, the bacterial populations rise exponentially. ==References==