TWiV 512: Flexuous SUMO wrestlers

Anne Simon joins the TWiV team to talk about plant viruses, including plum pox virus that devastates nut and stone fruit trees, and a geminivirus protein that regulates viral DNA synthesis.

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TWiV 469: Mycovirus stuffed potatoes

The TWiV hosts discuss a plant virus that infects a fungus, and whether you need to work insane hours to succeed in science.

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Have a methyl with your viral RNA

N6-MethyladenosineChemical modification of RNA by the addition of methyl groups is known to alter gene expression without changing the nucleotide sequence. The addition of a methyl group to adenosine has been found to regulate gene expression of animal viruses, and most recently of plant viruses.

The illustration shows a methyl (CH3-) group added to the nitrogen  that is attached to the #6 carbon of the purine base adenine. The entire molecule, with the ribose, is called N6-methyladenosine (m6A). Methylation of adenosine is carried out by enzymes that bind the RNA in the cell cytoplasm.

The m6A modification is found in multiple RNAs of most eukaryotes. It has also been found in the genome of RNA animal viruses. The modification is added to RNAs by a multi-protein enzyme complex, and is removed by demethylases. Silencing of the methylases decreases HIV-1 replication, while depletion of demethylases has the opposite effect. The replication of other viruses, including hepatitis C virus and Zika virus, is also regulated by m6A modification, but the details differ. For example, m6A negatively affects the replication of the flaviviruses hepatitis C virus and Zika virus.

Methylation of adenosine has been recently shown to modulate the replication of plant viruses. The RNA genomes of alfalfa mosaic virus (AMV) and cucumber mosaic virus (CMV) were found to contain m6A. An m6A demethylase was identified in Arabidopsis thaliana, a small flowering plant commonly used in research. This demethylase protein bound the capsid protein of AMV but not of CMV. Elimination of the demethylase from Arabidopsis reduced the replication of AMV but not CMV. These results show that m6A methylation negatively regulates the replication of AMV. Binding of the AMV capsid protein to the m6A demethylase might be a mechanism for ensuring that the enzyme demethylates viral RNA, allowing for efficient viral replication.

While it is clear that m6A regulates the replication of RNA viruses, the mechanisms involved are not well understood. Methylation of adenosine is likely to affect multiple functions, including the structure, celllular localization, splicing, stability, and translation of viral RNA (link to review). As m6A is also found in cellular RNAs, studies of its effect on viral processes is likely to provide insight into its role in cellular biology.

Behind the scenes: TWiV 202 at the University of Nebraska

We recorded This Week in Virology #202 at the University of Nebraska in Lincoln, Nebraska on 5 October 2012, as part of the 12th Annual Symposium in Virology. Terence Dermody, Shou-Wei Ding, Grant McFadden and I spoke about our research, and then we recorded TWiV with University of Nebraska virologists James Van Etten, T. Jack Morris, and Charles Wood.

I was impressed by the fine virology being done at the Nebraska Center for Virology, as well as the collegiality of the virology community in Nebraska, Iowa, South Dakota, and Kansas – Symposium attendees were from all of those states! I met many young virologists at the poster session and I was touched by how many of them wanted to say hello and thank us for doing TWiV.

Many thanks to all the virologists who came to the Symposium and stayed to watch TWiV. Special thanks to Charles Wood, the Director of the Nebraska Center for Virology, who participated in TWiV #202 along with T. Jack Morris and James Van Etten.

Here are some behind the scenes photographs of this short but very informative visit.

TWiV 202: Huskers go viral

In episode #202 of the science show This Week in Virology, Vincent travels to the University of Nebraska to meet with members of the Nebraska Center for Virology and discuss their work on algal viruses, plant viruses, HIV and Kaposi’s sarcoma herpesvirus.

You can find TWiV #202 at

The viruses in your food

cole slawAt the recent Brazilian Virology Society meeting George Rohrmann gave me a copy of his book, Baculovirus Molecular Biology. Baculoviruses are double-stranded DNA containing viruses that infect insects and other arthropods. Early in the first chapter of George’s book I learned that baculoviruses may be present on uncooked cabbage leaves:

…in one study it was found that cabbage purchased from 5 different supermarkets in the Washington D.C. area were all contaminated with baculoviruses to such an extent that each serving (about 100 cm2 of leaf material) would contain up to 108 polyhedra of an NPV pathogenic for the cabbage looper, Trichoplusia ni!

We ingest many other non-animal viruses regularly with foods. In TWiV #79, Red hot chili viruses, we discussed the finding of pepper mild mottle virus, one of the major pathogens of chili peppers, in human feces as well as in pepper or spice-containing food products. Metagenomic analysis of the RNA viruses present in human feces revealed that most viral sequences are similar to plant viruses.  Of 36,769 sequences obtained, 25,040 (91%) resembled plant viruses. In this study, the most abundant human fecal virus was pepper mild mottle virus, present in concentrations of up to 109 virions per gram of dry fecal matter.

The plant (and perhaps insect) viruses that we ingest on a regular basis do not appear to replicate or cause disease in humans. Might they play important roles in development of the immune system, as do the commensal bacteria in our gut?

Zhang, T., Breitbart, M., Lee, W., Run, J., Wei, C., Soh, S., Hibberd, M., Liu, E., Rohwer, F., & Ruan, Y. (2006). RNA Viral Community in Human Feces: Prevalence of Plant Pathogenic Viruses PLoS Biology, 4 (1) DOI: 10.1371/journal.pbio.0040003

TWiV 92: Live at ASV in Bozeman

Hosts: Vincent Racaniello, Rich Condit, Karla Kirkegaard, and Marilyn Roosinck

On episode #92 of the podcast This Week in Virology, Vincent, Rich, Karla, and Marilyn recorded TWiV at the 29th Annual Meeting of the American Society for Virology in Bozeman, where they discussed plant viruses and how they make plants resistant to adverse conditions, and identification of dominant negative drug targets.

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Weekly Science Picks

Marilyn – Viruses in the faecal microbiota of monozygotic twins and their mothers (Nature)
Rich –
The Known Universe by the American Museum of Natural History
Vincent – The Red Queen by Matt Ridley (thanks, Jesper!)

Send your virology questions and comments (email or mp3 file) to or leave voicemail at Skype: twivpodcast. You can also post articles that you would like us to discuss at and tag them with twiv.

Can a plant virus make you sick?

Tabasco virusIt has been estimated that approximately one hundred trillion bacteria colonize the human intestine. That’s about ten times the number of cells that constitute the entire human body. These bacteria are believed to have a mutualistic symbiotic relationship with their hosts. What is known about the viral communities that inhabit our alimentary tract?

The vast majority of DNA viruses in the human gut are bacteriophages, which infect the resident bacteria. The most plentiful RNA viruses in our feces are plant viruses, and the most abundant is pepper mild mottle virus (PMMV). This plant virus is present at high levels, up to  109 virions per gram. The virus has been identified in 12 of 18 fecal specimens taken from healthy humans in the USA and Singapore.

Pepper mild mottle virus is present worldwide in field-grown peppers. It is composed of an RNA genome wrapped with many copies of a viral protein that forms a rod-like particle with helical symmetry (pictured). The origin of PMMV in human feces is the food that we eat. The virus can be readily detected in fresh peppers, and in food products produced from peppers, such as chili powders, sauces, and spices. High levels of PMMV are found in Tabasco sauce, which contains virions that are not only visible in the electron microscope, but which are infectious for plants.

A case-control study was done to test whether presence of PMMV virus in feces is associated with clinical symptoms. Eighteen adults with PPMV positive feces were compared with 31 control, PPMV-negative individuals. Abdominal pain, diverticulosis or diverticulitis, and fever were significantly associated with the presence of PPMV RNA. The authors conclude:  ‘We provide the first evidence that plant viruses may cause disease in humans’. However they admit that ‘clinical symptoms may be imputable to another cofactor, including spicy food’. It’s interesting that 22 of 304 adults, but only 1 of 137 children, were positive for PPMV. The positive child was a 5 year old admitted to hospital because of abdominal pain, fever, asthenia, and hyperventilation. He probably ate too much spicy food, an activity most children don’t engage in.

Could PMMV replicate in human cells? I believe it is highly unlikely. Viruses such as PMMV are transmitted among plants by mechanical means (such as by contaminated farm equipment) and do not enter cells via specific receptors, as do animal viruses. Therefore I see no efficient way for PMMV to enter a human cell. If tomato spotted wilt virus is artificially introduced into HeLa cells, it does not reproduce. However, if the gene encoding the viral RNA polymerase is introduced into HeLa cells along with tomato spotted wilt virus, the viral RNA does undergo replication, although new particles are not produced. These are highly artificial conditions which are unlikely to occur in the human intestine.

The next time you have abdominal cramps after eating spicy food, it’s probably not caused by PMMV, although you will be shedding that virus in your feces. It’s because the chemical constituents of spicy foods affect your digestive tract – in ways that we don’t understand.

Breitbart, M., Hewson, I., Felts, B., Mahaffy, J., Nulton, J., Salamon, P., & Rohwer, F. (2003). Metagenomic Analyses of an Uncultured Viral Community from Human Feces Journal of Bacteriology, 185 (20), 6220-6223 DOI: 10.1128/JB.185.20.6220-6223.2003

Colson P, Richet H, Desnues C, Balique F, Moal V, Grob JJ, Berbis P, Lecoq H, Harlé JR, Berland Y, & Raoult D (2010). Pepper mild mottle virus, a plant virus associated with specific immune responses, Fever, abdominal pains, and pruritus in humans. PloS one, 5 (4) PMID: 20386604

de Medeiros, R. (2005). Expression of a viral polymerase-bound host factor turns human cell lines permissive to a plant- and insect-infecting virus Proceedings of the National Academy of Sciences, 102 (4), 1175-1180 DOI: 10.1073/pnas.0406668102

TWiV #70: Hacking aphid behavior

Hosts: Vincent Racaniello, Dickson Despommier, and Alan Dove

On episode #70 of the podcast ‘This Week in Virology’, Vincent, Dickson, and Alan consider a broad spectrum antiviral against enveloped viruses, how a plant virus induces chemical signals in the host to maximize its spread, a new way to preserve viral vaccines at tropical temperatures, and the continuing story of XMRV and chronic fatigue syndrome.

This episode is sponsored by Data Robotics Inc. Use the promotion code VINCENT to receive $50 off a Drobo or $100 off a Drobo S.

Win a free Drobo S! Contest rules here.

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Weekly Science Picks

Dickson Chemical Ecology – edited by Thomas Eisner and Jerrold Meinwald
Alan Scripps Center for Mass Spectrometry
Vincent Folding@home (thanks Jesper!)

Send your virology questions and comments (email or mp3 file) to or leave voicemail at Skype: twivpodcast. You can also post articles that you would like us to discuss at and tag them with twiv.