Question: Origin of plasmids

Monica Osterblad Unknown at utu.fi
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 mo6.rc.tudelft.nl> Lesley Robertson <l.a.robertson at stm.tudelft.nl> writes:
>>Steven Sanyal <steven.sanyal at utoronto.ca> 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 
>>>DNA.
>>>
>>>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.
>>>
>>>Steve
>>>
>>>steven.sanyal at utoronto.ca
>>>
>>
>>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 
>>wounds.
>>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).

Monica



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