TUFTS UNIVERSITY 
°‘ Molec,,Ur ®‘ olo 6y »nd Schoo | of Medicine 
Microbiology > 
- 2 - 
Dr. Fredrickson - cont. 
I would therefore suggest that an additional safety measure for 
investigators using plasmid recombinant DNA vectors would be to use a 
poorly mobilizable plasmid. By that I mean one that is mobilizable at 
frequencies of ^10“^ by a derepressed conjugative plasmid. Such vectors 
are available. It would also be advisable to assess the frequency of 
mobilization of nonconjugative vectors when these plasmid vectors are 
proposed for use. 
It is possible that evidence will be provided in the near future 
that the mobilization of nonconjugative (mobilizable) plasmids is so 
low in the environmental situation that the values will be far below 
any potential risk of spread. While I am aware that nonconjugative 
plasmids offer a major containment advantage over conjugative plasmids, 
I still believe that the use of poorly mobilizable plasmids should be 
advised until more data have been collected to speak to this point. 
and of Molecular Biology and 
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