Trends in Plant Science
ReviewChloroviruses: not your everyday plant virus
Section snippets
Algal viruses
Viruses infecting higher plants are typically small RNA viruses that encode only a few genes [1]. Although small viruses have recently been discovered that infect algae (e.g. [2]), many viruses infecting eukaryotic algae are huge dsDNA viruses with genomes ranging from 160 to 560 kb with up to 600 protein-encoding genes (CDSs) and are the subject of this review. These large viruses (family Phycodnaviridae), are found in aqueous environments throughout the world and play dynamic, albeit largely
Chloroviruses
Chloroviruses exist in freshwater throughout the world with titers as high as 100 000 plaque-forming units (PFU) per ml of indigenous water, although titers are typically 1–100 PFU/ml. Titers fluctuate during the year with the highest titers occurring in the spring 7, 9. Chlorovirus hosts, which are normally symbionts and are often referred to as zoochlorellae, are associated with the protozoan Paramecium bursaria (Figure 1b), the coelenterate Hydra viridis or the heliozoon Acanthocystis turfacea
PBCV-1 structure
Cryo-electron microscopy and fivefold symmetry averaging three-dimensional reconstruction of PBCV-1 indicate the outer capsid is an icosahedron and covers a single lipid bilayered membrane, which is required for infection (Figure 1e and f) 12, 13, 14. One of the PBCV-1 vertices has a 560 Å long spike-structure; 340 Å protrudes from the surface of the virus. The part of the spike structure that is outside the capsid has an external diameter of 35 Å at the tip, expanding to 70 Å at the base (Figure 1
PBCV-1 life cycle
Three-dimensional reconstruction of PBCV-1 in the presence of C. variabilis cell walls established that the spike first contacts the cell wall (Figure 1j) and that the fibers then aid in holding the virus to the wall (Figure 1i) [14]. PBCV-1 attachment to its host receptor is specific and attachment is a major factor in limiting its host range. The spike is too narrow to deliver DNA and likely serves to puncture the wall and is then jettisoned. Following host cell wall degradation by a
Chlorovirus genomes
The PBCV-1 genome is a linear ∼331-kb, nonpermuted dsDNA molecule with covalently closed hairpin termini. Identical ∼2.2-kb inverted repeats flank each hairpin end. The remainder of the PBCV-1 genome contains primarily single-copy DNA [7]. The GC content of the PBCV-1 genome is ∼40%; by contrast, its host nuclear genome is ∼67% GC. The PBCV-1 genome was recently re-sequenced to correct mistakes in the original sequence. Using 40 codons as the minimum CDS size, PBCV-1 contains 416 predicted CDSs
Chlorovirus genes
Many PBCV-1 encoded enzymes are either the smallest or among the smallest proteins in their family including the smallest eukaryotic ATP-dependent DNA ligase [33], the smallest type II DNA topoisomerase [34], the smallest prolyl-4-hydroxylase [35], the smallest histone methyltransferase [36] and the smallest protein to form a functional K+ channel named Kcv [37]. Whereas K+ channels from prokaryotes and eukaryotes often consist of several hundred amino acids, Kcv from PBCV-1 consists of 94
Evolutionary history
Phycodnaviruses are believed to have an ancient, common evolutionary ancestry with some other large dsDNA viruses such as the poxviruses (e.g. smallpox virus), asfarviruses, iridoviruses, ascoviruses and mimiviruses 68, 69, 70. Collectively these viruses are referred to as nucleocytoplasmic large dsDNA viruses (NCLDVs). Comparative analysis of 45 NCLDVs identified five genes common to all the viruses and 177 additional genes exist in at least two of the virus families [71].
Although a common
Concluding remarks
With the increasing interest in using algae for producing biofuels, it is obvious that pathogens, including viruses, will affect yields [80]. Although this review focuses on certain large dsDNA viruses that infect algae, ssRNA, dsRNA and ssDNA viruses have recently been discovered that infect algae, e.g. [2]. Algal viruses are potentially a greater problem than plant viruses are to higher plants because plant viruses are usually transmitted by biological vectors. Thus two components are
Disclosure statement
The authors are not aware of any affiliations, memberships, funding or financial holdings that might be perceived as affecting the objectivity of this review.
Acknowledgments
We thank Dr. Leslie Lane for critical reading of the manuscript, and Dr. Alexander Tchourbanov for composing the gene map, and Wim van Egmond for the micrograph of zoochlorella–paramecium. Research in the Van Etten laboratory was supported in part by NSF-EPSCoR EPS-1004094, DOE DE-FG36-08GO88055, DOE DE-EE0003142 and NIH/NCRR 1P20RR15635 from the COBRE program of the National Center for Research Resources.
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