If you know anything about me, you know that I’m mad about viruses. Although this madness extends to everything viral, I have a peculiar fondness for giant viruses. A new giant virus has been found that not only looks different from all the others, but has an amazing set of genes.
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.
On episode #380 of the science show This Week in Virology, the TWiVeroos deliver the weekly Zika Report, then talk about a cryoEM structure of a plant virus that reveals how the RNA genome is packaged in the capsid, and MIMIVIRE, a CRISPR-like defense system in giant eukaryotic viruses.
You can find TWiV at microbe.tv/twiv, or you can listen below.
The discovery of the giant Mimivirus and Megavirus amazed virologists (and also many others). Their virions (750 nanometers) and DNA genomes (1,259,000 base pairs) were the biggest ever discovered, shattering the notions that viruses could not be seen with a light microscope, and that viral genomes were smaller than bacterial genomes. Now two even bigger viruses have been discovered, which are physically and genetically unlike any previously known viruses. They have been called Pandoraviruses.
Both new viruses were isolated by culturing environmental samples in the amoeba Acanthamoeba castellani. Pandoravirus salinus was isolated from shallow marine sediment in a river at the coast of central Chile, and Pandoravirus dulcis was obtained from mud at the bottom of a freshwater pond near Melbourne, Australia. The P. salinus genome is at least 2.77 megabases in length (there is some uncertainty in the actual length due to the presence of repeated sequences at the ends of the DNA), while the P. dulcis genome is 2.47 megabases in length. The smaller P. dulcis genome is a subset of the P. salinus genome.
These new genomes are twice as large as those of previously described viruses, and bigger than the genomes of intracellular bacteria such as Tremblaya (138,927 base pairs) and Rickettsia (1,111,523 bp), some free living bacteria, and many free living Archaea.
While the huge sizes of the Pandoravirus virion and genomes are amazing, I find three other features of these viruses even more remarkable. The first is their atypical replication cycle. The virions are taken into amoebae by phagocytic vacuoles, and upon fusing with the vacuole membrane, the virion contents are released into the cytoplasm via a pore on the virion apex. Within 2-4 hours the cell nucleus is reorganized, and by 8-10 hours new particles appear where the nucleus once was. Pandoravirus DNA and virions are synthesized and assembled simultaneously, in contrast to eukaryotic DNA viruses and phages which fill pre-formed capsids with DNA. Virions are released by 10-15 hours as the cells lyse.
A second amazing feature is that most of the P. salinus open reading frames encode brand-new proteins. Of the 2,556 putative protein coding sequences in the P. salinus genome, 93% have no recognizable counterparts among known proteins. Some of the genes found in large DNA viruses are present, such as those encoding DNA polymerase and DNA-dependent RNA polymerase, and several amino acyl-tRNA synthetases, like members of the Megaviridae. Curiously, many of the Pandoravirus coding regions contain intervening sequences, which must be removed by RNA splicing. This process is known to occur only in the cell nucleus, suggesting that some Pandoravirus transcription occurs in that organelle. The lack of gene homology leads to authors to conclude that ‘no microorganism closely related to P. salinus has ever been sequenced’.
I am also impressed by what the authors describe as the ‘alien morphological features’ of the virions. The oval-shaped particles are 1 micron in length and 0.5 microns in diameter, easily visible by light microscopy. They are wrapped in a three-layered envelope with a pore at one end of the particle, and resemble nothing that has ever been seen before (see photograph).
How much bigger can viruses get? I don’t know the answer but I would guess even bigger than Pandoraviruses. The membranous Pandoravirus particle could easily accommodate even larger genomes. How big can a virus get and still be a virus? The answer to that question is easy: it is a virus as long as it requires a cell for replication.
These remarkable findings further emphasize the need for scientists to pursue their curiosity, and not only work on problems of obvious medical relevance. As the authors write,
This work is a reminder that our census of the microbial diversity is far from comprehensive and that some important clues about the fundamental nature of the relationship between the viral and the cellular world might still lie within unexplored environments.
Continuing their playful naming of giant viruses, the authors note that the name Pandoravirus reflects their ‘lack of similarity with previously described microorganisms and the surprises expected from their future study’.
Hosts: Vincent Racaniello, Alan Dove, and Dickson Despommier
Vincent, Alan, and Dickson discuss the reduction in genome size of Mimivirus upon passage in amoeba, and analysis of the microbiome of honeybees.
Click the arrow above to play, or right-click to download TWiV #139 (96 MB .mp3, 80 minutes).
Links for this episode:
- Mimivirus genome reduction after amoebal culture (PNAS)
- Carbohydrate-Active enZYmes database
- Analysis of the honey bee microbiome (PLoS One)
- The colony-collapse blues (TWiV 104)
- Blessed are the beekeepers (Wall Street Journal)
- TWiV on Facebook
- Letters read on TWiV 139
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Hosts: Vincent Racaniello and Dickson Despommier
On episode 17 of the podcast This Week in Parasitism, Vincent and Dickson continue their discussion of protozoan parasites that cause diarrhea with a review of amebic dysentery caused by Entamoeba histolytica.
TWiP is brought to you by the American Society for Microbiology at Microbeworld.org.
Links for this episode:
- Entamoeba histolytica cyst
- Entamoeba histolytica trophozoite
- Entamoeba histolytica life cycle
- Galactose prevents attachment of amoebae to cells
- Entamoeba histolytica life cycle
- Charcot-Leyden crystal
- Letters read on TWiP 17
Download TWiP #17 (55 MB .mp3, 76 minutes)
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