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Genetically engineered tomato

Patrick O'Neil patrick at corona
Sun Feb 12 23:56:42 EST 1995

On 12 Feb 1995 klier at cobra.uni.edu wrote:

> One of the concerns that have been voiced about genetically engineered
> organisms deals with the method used to select cells which have
> incorporated the target gene.  Usually the target gene is transferred
> with a gene for antibiotic resistance.  The cells are then treated
> with the antibiotic, and those which survive have incorporated the
> resistance gene and (we hope) the target gene.
> The concern is that the antibiotic resistance gene may later transfer
> to other organisms under field conditions, potentially giving rise
> to antibiotic resistant pathogens.

A reasonable concern, though something else, such as luciferase or lacZ 
could be used as a screen - and neither are in any way harmful.  In any 
case, an antibiotic resistance gene is not completely stable.  As with 
any gene that offers a selective advantage to an organism under a given 
set of circumstances (in this case a resistence to, say, chloramphenicol) 
is quickly lost when that selection is removed.  If you introduce and 
ampicillin resistence gene into E.coli, then as long as ampicillin is 
maintained in the media within which the bacteria is growing, then the 
gene/plasmid will be selected for and retained.  If, on the other hand, 
you grow that transformed bacteria in non-ampicillin media for a week or 
so - let's say, to be safe, three weeks - at the end of that time, you 
will most likely find no bacteria resistent to the antibiotic.  The gene 
will have mutated, and why not?  There is nothing selecting for a 
functional copy of the gene.  Moreover, the entire plasmid that harbored 
the gene in the first place is liable to be lost entirely unless it 
carries another gene that gives selective advantage in the circumstances 
within which the bacteria finds itself.  My long-winded reply is meant to 
state that the threat is still minimal since the genes for antibiotic 
resistence are not "coded in stone" and they mutate into nothingness when 
no antibiotic is present within relatively few turnovers.

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