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Viruses vs plasmids

Thu Feb 1 03:03:01 EST 1996

> From:          Ulrich Melcher <umelcher at bmb-fs1.biochem.okstate.edu>
> Subject:       Re: Viruses vs plasmids

> 1) Before the viral nature of the molecules responsible for the killer 
> phenomenon in fungi (brewer's yeast and smut fungus) was discovered, the 
> ds RNA molecules were, I think, called killer plasmids.  So, there is 
> precedence for using the term "plasmid" with RNA.

I think dsRNAs found in mitochondria have been caled plasmids too - 
but incorrectly, as it was found they were in fact products of 
opposing transcription that annealed.  So although there is 
precedent, it is not too germane if in fact the nature of the agent 
was incorrectly deduced.

> 2) Please also note that these were pathogenic RNAs.  So, I disagree 
> with Ed who believes, I think, that a distinction between plasmids and 
> viruses can be made on the basis of pathogenicity.

Yes, but as you have said...they are in fact viral.  Therefore the 
distinction can still be made, should one wish to do so.

> 3) Most plasmids we use or look at have genes because we use the genes 
> for selection or are interested in the properties carried by them.  If I 
> were to take all the protein-coding genes out of a plasmid and put it 
> back in a cell, it would probably still replicate extrachromosomally, 
> though I might have a hard time finding it.  Is it a plasmid?  I think 
> so.

Until it gets a gene or two which allows it to stop being innocuous 
and escape the cell in order to get into another, then it will be a 

As I have said previously, there is a continuum of nucleic acid forms 
which contain an internal self-image of an organism, from chromosomes 
to transposons to retrotransposons to plasmids to satellite RNAs to 
satellite virus genomes to viruses.  Deciding which is what is purely 
a matter of operational decision-making; there will inevitably be 
forms which transcend/crossover our best attempts at defining 

I have always favoured a "fuzzy" definition, sort of on the basis of 
fuzzy sets.  The problem is arriving at such a definition and 
teaching it to students in a way that they will remember.
                     Ed Rybicki, PhD  
      Dept Microbiology     |  ed at molbiol.uct.ac.za   
   University of Cape Town  | phone: x27-21-650-3265
   Private Bag, Rondebosch  |  fax: x27-21-650 4023
      7700, South Africa    |   
    WWW URL: http://www.uct.ac.za/microbiology/ed.html      
"And then one day you find, ten years have got behind you"

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