Genomics Revolution Podcast

Episode 38: The Viral Age Begins

Host: Brad Goodner, Professor of Biology & Biomedical Humanities at Hiram College

 

Transcript:

Welcome back.  Now that we have a basic definition of a virus – a RNA or DNA genome surrounded by a protein shell called a capsid, let us put that definition into some historical context.  When did we humans come to realize that viruses existed?  When did we realize that they were different from cellular life forms?  The Viral Age began, in terms of human recognition, around 120 years ago.  Yet, it was not a human disease that brought us to this realization or even a disease of another animal.  It was a disease of tobacco.  You heard me right, tobacco.

Tobacco was a major cash crop in the U.S. and around the world in the late 19th century. As you know, it it the leaves that matter.  They get dried and cured, then rolled and chopped to make smoking products, chewing tobacco and snuff.  Tobacco mosaic disease ruins leaves.  It is called mosaic because the leaves of infected tobacco plants have a mottled color.  This pattern reflects the path of infection as the virus moves throughout a given leaf.  The disease causes leaves to be much less efficient at photosynthesis.  Slower growth means fewer leaves to use.

By the late 19th century, the germ theory of disease had gained a lot of support from European researchers such as Robert Koch in Germany and Louis Pasteur in France.  Pasteur had put the last nail in the coffin of spontaneous generation, the very old idea that some diseases, food spoilage and rotting meat was due to new life forms arising out of non life.  In fact, these issues were due to the presence of specific microbes.  Scientists started finding microbial causes for many diseases and other issues such as wine spoiling into vinegar.  Koch came up with a set of steps for proving that a particular microbe caused a particular disease – what we now call Koch’s postulates.  In 1886, Mayer showed that he could take leaves from a tobacco plant suffering from mosaic disease, crush the leaves and squeeze out some juice, paint that juice onto the leaves of a healthy plant and see the disease occur.  Ivanowski went further in 1892 to show that he could pass the juice from infected leaves through a filter that normally catches bacteria and the filtered juice could still cause disease.  Mayer and Ivanowski both thought that tobacco mosaic disease was caused by some type of bacterium, maybe just a really small one (Bos, 1999).  Martinus Beijerinck thought differently.  He repeated Ivanowski’s filtration experiment and went on to show that the causal agent of tobacco mosaic disease could diffuse through a block of agar like a small chemical.  Even as small as they are, most bacteria cannot do that and so in 1898 Beijerinck put forward a strikingly different proposal – tobacco mosaic disease is caused by something different, in his words “a contagium vivum fluidum” (Bos, 1999).  He could not define it further, but it was clear to him that the pathogen was not a bacterium.  Beijerinck’ s “contagium vivum fluidum” turned out to be the first virus ever discovered which now goes by the name Tobacco Mosaic Virus or TMV.

Next, TMV was the first virus to be crystallized by the American Wendell Stanley in 1935 for which he won a partial share of a Nobel Prize in 1946 (Norby, 2008).  It is important to note that Stanley thought the particles he crystallized were pure protein.  It was actually not pure protein as shown by Bawden and Pirie in England in 1936 – there was RNA in the crystallized sample as well.  Many scientists at the time and since then believed that Bawden and Pirie’s work was equal to or more important than that of Stanley, but it was Stanley whose name is on the prize (Norby, 2008).

In 1956, Frankel-Conrat showed that TMV RNA alone could produce full blown disease when applied to the surface of wounded tobacco leaves (Creager, 1999).  This data proved that the viral genome codes for all the necessary proteins needed for disease symptoms and to produce infectious viral particles (Scholthof et al., 2011).  Just a year later, Andre Lwoff, who would go on to share a Nobel Prize for his work on gene regulation, wrote out a description of a virus that still works today – “…infectious, potentially pathogenic, nucleoproteinic entities possessing only one type of nucleic acid, which are reproduced from their genetic material, are unable to grow and to undergo binary fission ...’’ (Lwoff, 1957).

TMV has continued to be the subject of much basic and applied research (Scholthof, 2004; Zaitlin, 1999).  Yet for us, it will always be the start of the Viral Age!

See you next time on Genomics Revolution as we begin to hear from guest hosts of the 2020 Hiram College Genetics course on some other important viruses and their genomes.

 

For More Information on TMV:

Bos, 1999. Philosophical Transactions of the Royal Society of London, part B 354:675-85. Beijerinck’ s work on tobacco mosaic virus: historical context and legacy.

Creager et al., 1999. The Plant Cell 11:301-8. Tobacco mosaic virus: pioneering research for a century.

Norrby, 2008. Archives of Virology 153:1109-23. Nobel prizes and the emerging virus concept.

Lwoff, 1957. Journal of General Microbiology 17:239–253. The concept of virus. 

Scholthof, 2004. Annual Review of Phytopathology 42:13-34. Tobacco mosaic virus: a model system for plant biology.

Scholthof et al., 2011. Molecular Plant Pathology 12:938-54. Top 10 plant viruses in molecular plant pathology.

Zaitlin, 1999. Philosophical Transactions of the Royal Society of London, part B 354:587-91. Elucidation of the genome organization of tobacco mosaic virus.