Contaminating plasmid

John Brunstein brunstei at
Wed Jan 12 16:29:59 EST 1994

	Hi there, I was really interested in your message as I am
currently having the same pboblem (although my plasmid seems to be about
1.3 kb and I only see it if the clone is also carying by
antibiotic-resistance plasmid (ususally a pUC derivitive). I note that if
I gel isolate fragments from a 'contaminated' pUC digest, the mystery band
reappears upon subsequent transformation (CaCl2 prepped SURE cells).  My
only conclusion can thus be that the competent cells already carry this
mystery plasmid, as I am sure it was not transferred during the gel
isolation.  Although to date its presence has not caused me any problems,
I am planning to attempt to cure the SURE cells of this by growth in
non-selective media for a few days.  You might wish to try the same thing.
 In any event, I would be interested if you would forward any direct
replies you get to your question;  I am glad to hear I'm not crazy and
someone else has a similar situation.

On 12 Jan 1994, Dr R. Dalgleish wrote:

> A number of us have had problems with a plasmid which seems to
> contaminate our plasmid stocks. It's ampicillin resistant and
> has a linear size of around 2.2 kb. The strangest aspect is the
> lack of sites for common restriction enzymes. It has an SstI (SacI)
> site but lacks sites for BamHI, HindIII, KpnI, PstI, EcoRI, SalI
> and EcoRV.
> It often appears as a faint band in preps of plasmid DNA from
> constructs in pUC and Bluescript type vectors.
> Can anybody suggest what it might be? I'd be very grateful for
> suggestions.
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