Taxonomic Structure of the Family
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Phycodnaviridae |
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Virions are polyhedral with a multilaminate shell surrounding an electron dense core. Virions do not have an external membrane and are 130-190 nm in diameter. Some electron micrographs indicate the virions have flexible hair like appendages with swollen structures at the end; these appendages extend from at least some of the vertices. One virion vertex may contain a 20-25 nm spike structure (Fig. 1).
Physicochemical and Physical Properties
Virion Mr of the Chlorovirus type species PBCV-1 is about 1 109; S20w is more than 2,000S. The virions of the members of the genus Chlorovirus are disrupted in CsCl. The infectivity of chloroviruses is not affected by non-ionic detergents but are inactivated by organic solvents.
Virions contain large dsDNA genomes, ranging from 160 to 380 kbp. The chloroviruses have linear, nonpermuted dsDNA genomes with crosslinked hairpin ends. The DNA termini, contain identical inverted 1-2.2 kbp repeats. The remainder of the genome is primarily single copy DNA. The phaeoviruses have circular dsDNA genomes and some of them contain stretches of single-stranded regions. Phaeovirus genomes can become integrated into the genomes of their hosts. The genome structures of the prasinoviruses and the prymnesioviruses are unknown.
The G + C content of the viral genomes range from 40-52%. The DNA of many, if not all, of the viruses contains methylated bases, both 5-methyl-cytosine (5mC) and N6-methyl-adenine (6mA). Proportions of methylated bases in the chloroviruses range from no 6mA and 0.1% 5mC to 37% 6mA and 47% 5mC.
Purified virions contain as many as 50 or more proteins ranging in size from Mr <10 to >200 103. The chlorovirus PBCV-1 has three glycoproteins, three myristillated proteins (the major capsid protein (CP), Vp54, is both glycosylated and myristillated), and several phosphoproteins. Four proteins, including Vp54, are located on the virus surface.
Five to 10% of the chlorovirus PBCV-1 virion is composed of lipid. The lipid component is located inside the glycoprotein shell and is required for virus infectivity.
At least three of the chlorovirus PBCV-1 proteins are glycosylated including the major CP Vp54. The glycan portion of Vp54 is on the external surface of the virion. Unlike any other known viruses, PBCV-1 appears to code for at least some of the enzymes involved in its glycosylation.
Genome Organization and Replication
The 330,747 bp chlorovirus PBCV-1 genome has been sequenced. The virus encodes 701 ORFs, 65 codons or longer, of which 376 are predicted to encode proteins. About 40% of these ORFs match proteins in the databases. Two of the PBCV-1 genes are interrupted by introns: a transcription factor-like gene has a self-splicing type I intron and the DNA polymerase gene has a spliceosomal processed type of intron. The PBCV-1 genome also encodes ten tRNA genes, of which one is predicted to contain a small intron.
The intracellular site of virion DNA replication is unknown. DNA packaging occurs in localized regions in the cytoplasm. Transcription probably occurs in the nucleus because PBCV-1 does not encode an identifiable DNA-directed RNA polymerase.
Four distinct antigenic variants of the chlorovirus PBCV-1 can be isolated which are resistant to polyclonal antibody prepared against wildtype PBCV-1. These variants occur at a frequency of about 1 10-6-1 10-7. The antibodies react primarily with the glycan portion of the major CP. Additional variants of these viruses can easily be isolated from natural sources.
The phycodnaviruses, depending on whether they infect freshwater algae or marine algae, are ubiquitous in freshwater or seawater collected throughout the world. Typically, the viruses are host specific and only infect single isolates of species of algae. For example, the chloroviruses only attach to cell walls of certain unicellular, eukaryotic, chlorella-like green algae. Virus attachment is followed by dissolution of the host wall at the point of attachment and entry of the viral DNA and associated proteins into the cell, leaving an empty capsid on the host surface. Beginning about 2-4 hours post-infection, progeny virions are assembled in the cytoplasm of the host. Infectious virions can be detected inside the cell about 30-40 minutes prior to virus release; virus release occurs by cell lysis.
Infection of the prasinoviruses occurs when virions adhere to the wall-less host cell surface, followed by fusion of adjacent host and particle surfaces. Empty particles remain on the cell surface following the release of core contents. An eclipse period of approximately 3 hours follows the attachment stage. The virus growth cycle is complete after approximately 14 hours. During the replication cycle, particles appear in the cytoplasm and are associated with the production of cytoplasmic fibrils (ca. 5-8 nm in diameter) and clusters of membrane bound vesicles absent in healthy cells. Particles are released into the medium via localized ruptures in the cell membrane; ruptures often appear at several locations on the same cell.
Less is known about the replication of prymnesioviruses. Virus formation is observed in the cytoplasm and the nucleus remains intact and separate from the viroplasm that consists of a fibrillar matrix. Ultimately, viral production results in the disruption of organelles, lysis of the cell and release of the virus particles.
The phaeoviruses infect the wall-less spore stage of filamentous brown algae. These viruses appear as virus particles in sporangial cells of the host. Depending on the virus, some infect both unilocular and plurilocular sporangia (Ectocarpus siliculosus virus 1, EsV-1) and others only form in unilocular sporangia (Feldmania species virus, FsV). A few of these viruses can infect more than one species of brown algae.
The hosts for some of the chloroviruses can easily be grown in the laboratory and the viruses can be plaque-assayed. The hosts for some of the other viruses, e.g. the prasinoviruses, have not been cultured aseptically. The brown algal viruses, which only appear in mature gametangia or sporangia cells, can also be grown in the laboratory.
The chloroviruses, prasinoviruses, and prymnesioviruses are transmitted horizontally. The phaeoviruses are transmitted both horizontally and vertically.
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