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Multicopy plasmid instability: the dimer catastrophe hypothesis (1993)

Summers, David K. ; Beton, Christopher W. H. ; Withers, Helen L.

Oxford, UK : Blackwell Publishing Ltd

Molecular microbiology 8 (1993), S. 0

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ISSN: 1365-2958

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Biology , Medicine

Notes: Multimer formation reduces plasmid copy number and is an established cause of segregational instability. Nevertheless, it is difficult to rationalize observations that low levels of dimers can cause severe instability, if we assume they are distributed evenly in cell populations. We report here that dimer distribution is in fact heterogeneous in recombination-proficient strains. Most cells in the population contain only monomers; dimers are confined to a small sub-population from which plasmid-free daughters arise at high frequency. In a rec+ culture where 4% of pBR322 molecules are dimers, more than half are in dimer-only cells. We show that this situation is inevitable because dimers replicate at twice the rate of monomers. Runaway multimerization is avoided because dimer-containing cells grow more slowly than their monomer-containing counterparts. A computer simulation is used to show how dimers proliferate after formation by homologous recombination. The equilibrium concentration of dimers is proportional to the inter-plasmid recombination rate and is essentially independent of the rate at which homologous recombination converts dimers to monomers.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1365-2958.1993.tb01648.x

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ColE1 multimer formation triggers inhibition of Escherichia coli cell division (1993)

Patient, Michaela E. ; Summers, David K.

Oxford, UK : Blackwell Publishing Ltd

Molecular microbiology 9 (1993), S. 0

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ISSN: 1365-2958

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Biology , Medicine

Notes: Multimer formation and consequent copy number depression are acknowledged causes of multicopy plasmid instability. Multimer resolution sites (among which ColE1 cer is best-characterized) have been identified in a variety of plasmids. They participate in the conversion of multimers to monomers, maximizing the number of independently segregating molecules and minimizing the frequency of plasmid loss. We show that multimer resolution alone is insufficient to ensure stable maintenance of ColE1-like plasmids in a recombination-proficient host. The expression of Red, a transcript encoded within cer and expressed in multimer-containing cells, is also required. The appearance of Red correlates with the inhibition of division of multimer-containing cells, presumably allowing time for the conversion of multimers to monomers by site-specific recombination.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1365-2958.1993.tb01238.x

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