|MadSci Network: Science History|
1) First, I'll try to get our definitions straight. I presume that by micro-viruses you are referring to subviral organisms. I have enclosed a very nice description of these organisms. "There are basically two types of subviral agents: self replicating and self non-replicating. Self replicating agents are plant viroids. The non- self replicating agents are usually parasites of viruses. They depend on a host virus for repl ication and sometimes encapsidation. This type of subviral agents include satellite RNA, satellite virus, virusoid, defective interfering (DI) RNA, and hepatits delta virus. Viroids: Viroids are autonomously replicating pathogens that consist solely of unencapsidated, single-stranded, circular RNAs of 246-375 nucleotides. Viroids do not code for any protein products including a capsid protein. The native structure of viroids is an extremely stable and highly basepaired rod. Some viroid RNAs have ribozyme activity (self- cleavage). Satellite RNA: Satellite RNA are small RNA molecules that require a host virus for replication. They may or may not reduce the titer of the host virus. Satellite RNAs ranges from 200 to 1700 nt. Larger satellite RNAs may encode a protein. Virusoid: viroid-like satellite RNA. They are small satellite RNA with a circular, highly basepaired structured like a viroid. Virusoids depend on a host virus for replication and encapsidation. Like a viroid, it does not encode any protein. All virusoid RNAs studied so far have ribozyme activity. Satellite Virus: When a satellite RNA encodes its own capsid protein and encapsidate it own RNA, it is called satellite virus. Satellite viruses still depend on host virus for replication. Defective Interfering (DI) RNA. Small RNA molecules that derived from viral RNA by extensive deletion. They are mosaic of viral genomic RNA. DI depends on the original virus for replication and usually reduce the replication of the host v irus. Hepatitis delta virus: a covalently closed, circular ssRNA of approximately 1700 nt. It can fold into a unbranched rod-like structure and replicate through a rolling circle mechanism. Both genomic RNA and complementary RNA are capable of self cleavage. Hepatitis delta virus depends on hepatitis B virus for replication. Hepatitis delta virus is the only viroid-like infectious agent infecting animals. Another class of pathogens, yet to be reported infecting plants, are worth mentioning. They are Prions (Proteinaceous infectious particles). Prions have been defined traditionally as " small proteinaceous infectious particles which resist inactivation by procedures that modify nucleic acids." Prion are a form of a normal cellular protein (PrP, prion protein) existing in a special tertiary structure (conformation). Upon transmission to healthy individuals, prion can lead a change in conformation of the normal cellular prion protein into the diseased prion protein. " (web site quoted from http://ag.arizona.edu/~zxiong/plp611/lect18.html) 2) Your question is about the discovery of prions and "micro-viruses". Here again there could be a confusion of intent: do you mean what were the actions of people who discovered prions and micro-viruses, or do you mean what made them suspect inheritance through other forms than DNA replication or with naked RNA? I will presume that you are interested in how scientists thought, and since in biology one method of investigation tends to be repeated many times over (and a similar mechanism of action pops up in differing organisms, environments etc.), I will remind you of how DNA was established to have a hereditary function. Basically, in science (and for that matter in any thorough and logical investigation), you must list all possible options and methodically eliminate the impossible ones. Any theory left standing after the eliminatory process must be the correct one. (Sherlock Holmes speech in one of Arthur Conan Doyle's books, but very pertinent to science). Of course, any theory must be proven, so after guessing the correct mechanism, another round of experiments must be devised to go through the postulated process and obtained the predicted results. So, in finding the role of DNA, scientists took proteins and DNA (both suspected of being the replicating material) and tagged them with irradiated labels and looked at which material showed up in the newly replicated cells. DNA showed up, won and was declared the blueprint of all living things. (DNA function: http://biology.ux.com/Saddleback/Discovery.htm; http://library.thinkquest.org/18258/griffith.htm; http://library.thinkquest.org/18258/hersheychase.htm) However, later on there were some diseases investigated that were transmitted by, it seems, other ways than simple DNA replication of bacteria or viruses. So scientists got back to square 1, confirmed that it was not DNA that was ruining cows' brains (in the case of prions) and reinvestigated the possibility of proteins. And while logically this is just how investigating mysteries in science works, both the discovery of the function of DNA and prions were important discoveries because they went against the traditional thought regarding inheritance at the time (at the beginning of the century protein was supposed to be the replicating material, and after DNA was declared the blueprint af life, protein infection by protein, rogue DNA becoming infectious and other possibilities were thought at best bizarre). 3) In the case you just wanted a histoy of prion discovery, Stanley B. Prusiner, a professor at the University of California at San Francisco, discovered prions . Here is a little more detail as to the investigative process: "Scrapie, Creutzfelt-Jakob disease, and kuru are all shown to be transmissible by injecting material from diseased brains into the brains of healthy mammals. But it was unknown for many years just how this phenomenon occurred. Ultraviolet or ionizing radiation can degrade nucleic acids in the form of DNA or RNA, which are present in all other known pathogens. But when the nucleic acids in scrapie-infected brains were destroyed by these methods, the brain material was still able to transmit scrapie. This observation suggested that nucleic acids such as those present in DNA and RNA are not required for the transmission of spongiform encephalopathies. Determined to solve this mystery, although other researchers had failed at their attempts, Prusiner began to purify the infectious material in his U.C.S.F. laboratory in 1974. By 1982, Prusiner's research confirmed previous evidence that DNA and RNA were absent in the infectious material of the diseases. He eventually came across a further discovery that the responsible agent was in fact a protein. This was supported by the fact that in experiments where proteins were denatured, or unfolded, the infectivity of the diseased material decreased. Prusiner later found a single protein that was responsible for scrapie, and named it PrP, short for "prion protein." " (web site quoted from: http://www.esb.utexas.edu/palmer/bio303/group23/How Were Prions Discovered.html) For the latest news in prion science try this web site: http://www.uchospitals.edu/news/madcowbrew.html, or search using copernic search engine (at www.copernic.com). Regarding subviral organisms, you can also do a search (as above) and read about Barbara McClintock research on "jumping genes" (mobile genetic elements) in any biology book . Hope I have been of some help (as opposed to confusing you further). Ana
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