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. Additionally, the titer of the viruses within a single source can reach as high as 40,000 plaque forming units per milliliter of water.

Analysis of the 330,744-bp PBCV-1 genome leads to the prediction that this chlorovirus has ~375 protein-encoding genes and 10 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 species.

1. NC64A viruses (type = PBCV-1): These viruses infect Chlorella-NC64A isolated from Paramecium bursaria originally collected in the southeastern United States.

2. Pbi viruses (type = CVA-1): These viruses infect Chlorella-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 alga host was never cultured preumably because the virus is either lysogenic or integrates into the host genome. Therefore, all or a portion of the algae are infected while in the Hydra and the virus only enters its lytic phase once the algae are released.

4. SAG 3.83 viruses (type = ATCV-1): These viruses infect Chlorella strain SAG 3.83 isolated from the the heliozoon Acanthocystis turfacea.

These viruses are each very specific to their own host isolate; they do not infect nor even attach to the other hosts.

Click on one of the icons below to view PBCV-1 plaques, Paramecium bursaria and the PBCV-1 Genomic Map

PlaquesProtozoan host of the chlorellaPBCV-1 Genomic Map

Infection of Chlorella strain NC64A by PBCV-1.

(A) Viral particle in close proximity to the alga.
(B and C) Attachment of PBCV-1 to the algal wall and digestion of the wall at the point of attechment.
(D) Viral DNA beginning to enter the host.
(E) An empty viral capsid remaining on the surface of the host.
(F) PBCV-1 attachment and dissolution of a Chlorella cell wall fragment.

Note that (i) viral attachment always occurs on the external side of the wall (i.e., the internal side of the wall curls inward) and (ii) DNA is not released from viral particles attached to wall fragments.

The size markers in panel E and F represent 100 nm and 200 nm, respectively.

View the Infection Cycle Slideshow

Since the virus also attaches to and digests wall fragments which have been boiled or extracted by harsh procedures, the virus carries the digestive enzyme(s).

How to isolate your own Chlorella Virus

© 1999-2000 James L. Van Etten, University of Nebraska, Lincoln, Department of Plant Pathology - World of Chlorella Viruses Home Page
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