Question: Origin of plasmids

Monica Osterblad Unknown at
Tue Oct 17 07:43:50 EST 1995

In article <45jn6k$qcm at epervier.CC.UMontreal.CA>, szat at ERE.UMontreal.CA (Szatmari George) says:
>In article <45e13r$8rn at> Lesley Robertson <l.a.robertson at> writes:
>>Steven Sanyal <steven.sanyal at> wrote:
>>>I am a third year Molecular Biology student at the university of Toronto 
>>>and am also taking a microbiology course out of general interest.
>>>I am curious as to the proposed origin of plasmids.  I have heard that 
>>>bacteria can actually scavenge their surroundings for useful pieces of 
>>>Most plasmids I have come across thus far have dealt with antibiotic 
>>>resistance genes - used for selections.  Did these genes evolve only 
>>>since the advent of antibiotics this century, or is it believed that they 
>>>existed before?  ie: were their natural antibiotic agents that existed 
>>>before - a sort of interbacterial warfare.
>>>steven.sanyal at
>>Most, if not all, were probably already around in one form or another, 
>>but not necessarily dominant in the community except where selective 
>>pressures gave "owners" an advantage. Not only that, it doesn't have to 
>>be interbacterial "warfare" (although that does occur - look up 
>>bacteriocins) - penicillin is a natural product of fungi (although most 
>>precribed modern penicillins have been chemically modified - hence the 
>>term "semi-synthetic penicillin"). Indeed, the oldest known herbals 
>>describe the use of bread mould as a good way of treating infected 
>>Lesley Robertson
True, but also remember that plasmids have their own mechanisms for 
spreading laterally, that is, by conjugation to other bacteria. And
they don't like competition from related plasmids either: 
incompatibility. What it all adds up to, is that plasmids sort of
have a life of their own, too. They are not only just tools for
their "host" bacteria - although they are that too.
Could one say that they in a way are symbiontic "viruses"?

And about the scavenging of DNA - that would be transformation, and
it has been shown to occur naturally only in some bacteria (the classic
encapsulated Strep. pneumoniae experiment).


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