> From: bap at med.pitt.edu (Bruce Phillips)
> Subject: Viruses versus Plasmids
> The responses to the viruses versus plasmids issue, I think first
> raised by some else, was quite interesting. I think we would mostly agree
> that the attribute best distinguishing viruses from plasmids is the ability
> to form particles, ie., the expression of capsid (and related) proteins.
> constraining the argument to wild-type "organisms," the ability to form
> precisely defined particles as a infectious vehicle would seem to be
> a unique attribute of viruses.
A good comment. I yesterday ended up writing this for a new edition
(the old one ate itself and is now inaccessible - ah, the joys of
multimedia!) of my Introductory Molecular Virology tutorial:
"There are a number of types of genomes which have some sort of
independence from cellular genomes: these include "retrons" or
retrotransposable elements (of which more later), bacterial and
fungal (and eukaryotic organelle) plasmids, satellite nucleic acids
and satellite viruses which depend on helper viruses for
replication, and viroids.
Plasmids may share a number of properties with viral genomes -
including modes of replication, as in ss circular DNA plasmids and
viruses - but are not pathogenic to their host organisms, and are
transferred by conjugation between cells rather than by free
Viroids are small naked circular ssRNA genomes which appear rodlike
under the EM, which are capable of causing diseases in plants. They
code for nothing but their own structure, and are presumed to
replicate by somehow interacting with host RNA polymerase, and to
cause pathogenic effects by interfering with host DNA/RNA metabolism
and/or transcription. A structurally similar disease agent in
humans is the hepatitis B virus-dependent hepatitis delta agent,
which additionally codes for a structural protein. "
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"