plasmid n-mers

harvey at harvey at
Sat Jul 24 04:59:00 EST 1993

> From:	CBS%000001002100.FTP.MAIL::PO.CWRU.EDU::RAM3 23-JUL-1993 22:52:51.60
> To:	bao,harvey
> CC:	
> Subj:	RE:Re: 2x vector plasmid

> From: ram3 at po.CWRU.Edu (Russell A. Maurer)
> Reply-To: ram3 at po.CWRU.Edu (Russell A. Maurer)

> In a previous article, GIETZ at () says:
> >I know for a fact that I have run across (years ago) a copy of pBR322 
> >that is a dimer!
> >A direct repeat not an inverted repeat.  It seemed stable and
> >cannot be identified as such until you run supercoiled DNA on agarose
> >to compare to a standard.  I have run across many other things in my cloning
> >adventures that look like direct repeats and seem to be stable.  I have 
> >produced
> >a number of plasmids with large direct repeats that seem stable in coli.  I 
> >never
> >recover inverted repeats of any type!  This is my $0.02 worth.
> >Dan Gietz

> >I was also under the impression that a vector dimer would be unstable once
> >it starts replicating. That's why my confusion, and that's why my original
> >question. If someone really knows that such dimers can indeed exist I would
> >like to know (reference?).
> >
> >Johan
> >
> Such dimers and higher multimers have been known for a long time.  For a
> recent (and provocative) reference see Summers et al., "Multicopy plasmid
> instability: the dimer catastrophe hypothesis" in Molecular Microbiol.
> 8:1031-1038 (1993).
> Russ Maurer

Plasmid n-mers are a consequence of relication in a RecA+ host. All n-mers, inc
the odd number series are generated. If you take such a population and transform
a RecA- host you can isolate a fixed n-mer population. A trimer for example 
remains as such - dimers and monomers are never produced. You do however get
the odd hexamer as a consequence of a replicating trimer mobius circle 
and the few catenanes that are also a product are resolved by topoisomerase.

So if you do a ligation and transform into a RecA- host vector dimers would 
be one of the expected products



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