What is: the toxic fragment in pBR322?

Ashok Aiyar aiyar at ebv.oncology.wisc.edu
Mon Jan 20 17:18:08 EST 1997

On Mon, 20 Jan 1997 14:49:11 -0500, Richard Pelletier <rpelle1 at po-box.mcgill.ca> wrote:
>We have a modified pBR322 plasmid from which the toxic fragment has been
>removed. It is called pML2 and has the sequences 1120-2490 bp removed (out
>of 4.3 kb). I am not sure about the reason for the so called toxicity, 

I believe that the toxicity you refer to was originally reported in 
the following publication by Monika Lusky and Michael Botchan.
UI  - 81270767
AU  - Lusky M
AU  - Botchan M
TI  - Inhibition of SV40 replication in simian cells by specific pBR322
      DNA sequences.
SO  - Nature 1981 Sep 3;293(5827):79-81

>I heard that these sequences were somehow toxic to eukaryotic cells,
>therefore decreasing the transformation efficiency. 

Plasmids that contained the "toxic" sequences were reported to be
refractory to the replication of SV40 in simian cells.

>However, we have
>performed transfections with the whole pBR322 plasmid, and saw no
>toxicity. I think the toxicity issue have been disproved by now.

I think that the specific constructions used by Lusky and Botchan
may have been toxic to SV40 replication.  

There are other replicative SV40 vectors that contain these sequences.  
Additionally, these sequences are also not toxic to the replication of 
other episomal plasmids in simian and human cells.

Ashok Aiyar, Ph.D.
McArdle Laboratory for Cancer Research
aiyar at ebv.oncology.wisc.edu

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