In the first episode for 2018, the TWiV team reviews the amazing virology stories of 2017.
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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).
No problem not being nice to Dickson in this episode, because he’s absent for a discussion of a new giant virus that replicates in the cytoplasm yet transiently accesses the nucleus to bootstrap infection.
You can find TWiV #440 at microbe.tv/twiv, or listen below.
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Among the multitudes of eukaryotic viruses with DNA genomes, some replicate in the cell nucleus, while others avoid the nuclear bureaucracy and remain in the cytoplasm. But biology is not always so rigid: a new giant virus has been found that replicates in the cytoplasm, where it seems to recruit components of the nuclear transcription machinery (link to paper).
Noumeavirus was isolated from a pond near – where else? – Noumea airport in New Caledonia. The 200 nanometer icosahedral particles infect the amoeba Acanthamoeba castellani and have a double-stranded DNA genome of 376,207 base pairs encoding 452 proteins. Sequence comparisons revealed Noumeavirus to be a new member of the family Marseilleviridae, which includes other previously discovered giant viruses.
Other members of the Marseilleviridae replicate in the cytoplasm of the host cell, so it was assumed that the related Noumeavirus would do the same. However an analysis of the proteins in purified virus particles revealed an absence of components of the transcriptional machinery – which is needed for ths synthesis of mRNA. RNA polymerase, for example, is readily detected in other cytoplasmic viruses such as Mimivirus and poxviruses.
If proteins involved in transcription are not present in the Noumeavirus particle, and the virus does not enter the nucleus, how are viral mRNAs produced? It appears that early in infection, the required proteins are moved from the nucleus to sites of viral replication in the cytoplasm. When nuclear proteins were labled with green fluorescent protein, within one hour after infection they can be seen moving out of the nucleus into the cytoplasm to sites of viral replication. The nuclear integrity remains intact, as host DNA does not leave the organelle. This recruitment of nuclear proteins is transient: after 2-4 hours proteins are no longer leaving the nucleus.
This series of events suggests that nuclear proteins needed to initiate viral mRNA synthesis are recruited from the nucleus to sites of viral replication in the cytoplasm. Once viral mRNAs are made, the viral transcriptional machinery can be assembled and the nuclear proteins are no longer needed. The authors call this ‘remote control of the host nucleus.’
Confirmation of this hypothesis will require the demonstration that nuclear proteins involved in viral mRNA synthesis are recruited to early sites of viral replication in the cytoplasm. It will also be essential to identify the mechanism by which these nuclear proteins are extracted. Perhaps one or more virion proteins, such as an abundant 150 amino acid protein of unknown function, is involved.
Other giant viruses, such as Mimivirus, package the viral transcriptional machinery in the virus particle and are independent of the cell nucleus. At the other extreme are viruses that undergo transcription and DNA synthesis entirely in the nucleus (e.g., herpesviruses). Perhaps Noumeavirus is a relic of an evolutionary transition between the two replication strategies.
The TWiVsters reveal new giant viruses that argue against a fourth domain of life, and discovery of viruses in the oceanic basement.
You can find TWiV #437 at microbe.tv/twiv, or listen below.
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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 molecules involved in translation. Analysis 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. 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 of 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?
The TWiVrific gang reveal how integration of a virophage into the nuclear genome of a marine protozoan enhances host survival after infection with a giant virus.
You can find TWiV #419 at microbe.tv/twiv, or listen below.
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Virophages (the name means virus eater) were first discovered to replicate only in amoeba infected with the giant mimiviruses or mamaviruses. They reduce yields of the giant viruses, and also decrease killing of the host cell. Another virophage called mavirus has been found to integrate into the genome of its host and behaves like an inducible antiviral defense system (link to paper).
The host cell of the virophage mavirus is Cafeteria roenbergensis, Cro (pictured), a marine phagotropic flagellate, that is infected with the giant virus CroV (Cafeteria roenbergensis virus). When Cro cells are infected with a mixture of mavirus and CroV, the virophage integrates into the host cell genome. There it remains silent; the cells survive, and no virophage particles are produced. Such cells can be called lysogens, a name applied to bacteria containing integrated bacteriophage genomes, or prophages.
How does the mavirus genome integrate into the Cro cell? The viral genome encodes an integrase, an enzyme that cuts host DNA and inserts a copy of the viral genome. Retroviruses achieve the same feat via an integrase.
When Cro-mavirus lysogens are infected with CroV, the integrated mavirus genome is transcribed to RNA, the viral DNA replicates, and new virus particles are formed. These virophages inhibit the replication of CroV by 100-1000 fold. As a consequence, the host cell population survives.
These findings suggest that the virophage mavirus is altruistic: induction of the integrated genome leads to killing of the host cell, but other members of the cell population are protected. Altruism is not unknown in Nature, but how it evolved is an intriguing question.
All this work was done in a laboratory. It will be necessary to determine if integration of mavirus into Cro cells in the wild has any influence on the ecology of these organisms.
On episode #354 of the science show This Week in Virology, the esteemed doctors of TWiV review a new giant virus recovered from the Siberian permafrost, why influenza virus gain of function experiments are valuable, and feline immunodeficiency virus.
You can find TWiV #354 at www.microbe.tv/twiv.