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.
However, aren't there certain virus mutants, eg of lambda, in whcih
lysogeny results but there is no longer the capacity to express structural
proteins from the proviral state. Since many plasmids have the capability
to integrate into host cell chromosomes and replicate in concert with the
cell genome, this adds to the blur between viruses and plasmids. Still,m
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.