Chlorovirus: a genus of Phycodnaviridae that infects certain chlorella‐like green algae

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Abstract
SUMMARY: Taxonomy: Chlorella viruses are assigned to the family Phycodnaviridae, genusChlorovirus, and are divided into three species:ChlorellaNC64A viruses,ChlorellaPbi viruses andHydra viridis Chlorellaviruses. Chlorella viruses are large, icosahedral, plaque‐forming, dsDNA viruses that infect certain unicellular, chlorella‐like green algae. The type member isParamecium bursariachlorella virus 1 (PBCV‐1).Physical properties: Chlorella virus particles are large (molecular weight ∼1 × 109 Da) and complex. The virion of PBCV‐1 contains more than 100 different proteins; the major capsid protein, Vp54, comprises ∼40% of the virus protein. Cryoelectron microscopy and three‐dimensional image reconstruction of PBCV‐1 virions indicate that the outer glycoprotein‐containing capsid shell is icosahedral and surrounds a lipid bilayered membrane. The diameter of the viral capsid ranges from 1650 Å along the two‐ and three‐fold axes to 1900 Å along the five‐fold axis. The virus contains 5040 copies of Vp54, and the triangulation number is 169. The PBCV‐1 genome is a linear, 330 744‐bp, non‐permuted dsDNA with covalently closed hairpin ends. The PBCV‐1 genome contains ∼375 protein‐encoding genes and 11 tRNA genes. About 50% of the protein‐encoding genes match proteins in the databases.Hosts: ChlorellaNC64A andChlorellaPbi, the hosts for NC64A viruses and Pbi viruses, respectively, are endosymbionts of the protozoanParamecium bursaria. However, they can be grown in the laboratory free of both the paramecium and the virus. These two chlorella species are hosts to viruses that have been isolated from fresh water collected around the world. The host for hydra chlorella virus, a symbiotic chlorella fromHydra viridis, has not been grown independently of its host; thus the virus can only be obtained from chlorella cells freshly released from hydra.