The TWiVanguardians take on Bodo saltans virus, a leviathan which infects an abundant flagellated eukaryote in Earth’s waters.
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Show notes at microbe.tv/twiv
The discovery of Mimivirus in a French cooling tower amazed virologists and changed our view of the biology and evolution of giant viruses. Since then, many other giant viruses have been identified, and with three exceptions, they all appear to infect species of Acanthamoeba. Now a new member of the Mimivirus family has been discovered that infects the flagellated eukaryote Bodo saltans (pictured: image credit).
When giant viruses were discovered – with genomes much larger than any previously seen – some suggested that they had descended from a fourth domain of life (the current three are bacteria, archaea, and eukaryotes). Part of the reason for such a claim was the finding of homologs of bacterial and eukaryotic genes, including moleculesÂ involved in translation. AnalysisÂ of new giant viruses encoding even more components of the translation machinery has thrown cold water on the fourth domain hypothesis.
Klosneuvirus, with a 1.57 million base pair DNA genome, was discovered in a wastewater treatment plant in Austria, and three related viruses – Indivirus, Hokovirus, and Catovirus – were found in environmental samples. Â Sequence analyses suggests that these viruses should be classified in a subfamily of the Mimiviridae.
The Klosneuviruses encode far more components of the translational machinery than do mimiviruses – 25 tRNAs, 19 aminoacyl tRNA synthetases, 11 initiation and elongation proteins, a chain release factor, and tRNA modifying enzymes.
Phylogenomic analyses demonstrate that the aminoacyl tRNA synthetase and translation factor genes are likely derived from protists. This finding is not compatible with the hypothesis that these viruses are derived from a fourth domain of life. It is more likely that smaller ancestors ofÂ giant viruses acquired these genes from known eukaryotes.
Why these components of the translational system have been maintained in these giant virus genomes is an excellent question. They might confer some advantage to the viruses, for example when host translation is shut off as a viral defense. Having components of the translational apparatus might allow viral protein synthesis to proceed.
Note that genes encoding ribosomal RNAs or proteins have not been found in any virus. In fact no virus encodes a complete protein synthesis machinery. Maybe they have yet to be discovered? Or perhaps these energetically costly activities are best left to the cell?
On episode #379 of the science show This Week in Virology,Â Scott TibbettsÂ joins the TWiVirateÂ to describe his work on the role of a herpesviral nocoding RNA in establishment of peripheral latency, and then we visit two last minute additions to theÂ Zika virus literature.
You can find TWiV #379 at microbe.tv/twiv, or listen below.