Paramecium bursaria chlorella virus (PBCV-1) is the prototype of a group (family Phycodnaviridae, genus Chlorovirus) of large, icosahedral, plaque-forming, double-stranded DNA viruses that replicate in certain unicellular, eukaryotic chlorella-like green algae. In nature, the chlorella host is a hereditary endosymbiont of the cilliated protozoan Paramecium bursaria and while inside its protozoan host, the alga is protected from the viruses. The alga host can be grown in the laboratory in liquid and on solid media and since the viruses are lytic to the host, this lead to the development of a plaque assay, the only plaque assay for a virus infecting a photosynthetic eukaryote. Using the plaque assay, chloroviruses have been found in freshwater sources throughout the world and many genetically distinct isolates usually can be found within the same sample. Typically titers of 1-100 plaque forming unit per milliliter (pfu/ml) of water are observed for most native water samples; however, the titer of the viruses within a single source can reach as high as 100,000 pfu/ml.
Analysis of the 330,611-bp PBCV-1 genome leads to the prediction that this chlorovirus has 416 protein-encoding genes and 11 transfer RNA genes. The predicted products of ~50% of these genes resemble proteins of known function. Limited sequence analysis of additional chlorovirus isolates indicates that the viral genomes are probably co-linear with PBCV-1 and that there are genes encoded by PBCV-1 not found in other isolates as well as genes encoded by other isolates not present in PBCV-1. This suggests that the entire "gene pool" of this genus exceeds the 375 PBCV-1-encoded genes. Besides their large genome, the chloroviruses have other unusual features including : (a) The viruses encode multiple DNA methyltransferases and DNA site-specific endonucleases; (b) PBCV-1 encodes at least part, if not the entire machinery to glycosylate its glycoproteins; (c) PBCV-1 has at least two types of introns : a self splicing intron in a transcription factor-like gene and a splicesomal processed type of intron in its DNA polymerase gene.
The Chlorovirus genus consists of four known species.
1. NC64A viruses (type = PBCV-1): These viruses infect Chlorella variabilis strain NC64A isolated from Paramecium bursaria originally collected in the southeastern United States.
2. Pbi viruses (type = CVA-1): These viruses infect Micractinium conductrix (aka M. reisseri) Pbi isolated from Paramecium bursaria originally collected in Germany.
3. Hydra viruses (type = HVCV-1): These viruses infect Chlorella originally isolated from Hydra viridis. This chlorovirus host has not been cultured free of the virus and consequently, the virus can be isolated only from chlorella cells freshly released from hydra.
4. SAG 3.83 viruses (type = ATCV-1): These viruses infect Chlorella heliozoae strain SAG 3.83 isolated from the the heliozoon Acanthocystis turfacea.
These viruses are each specific to their own host isolate; they do not infect nor attach to the other hosts.
These viruses are each very specific to their own host isolate; they do not infect nor even attach to the other hosts.