MULTICOPY PLASMID INSTABILITY - THE DIMER CATASTROPHE HYPOTHESIS

Multimer formation reduces plasmid copy number and is an established c ause of segregational instability. Nevertheless, it is difficult to ra tionalize observations that low levels of dimers can cause severe inst ability, if we assume they are distributed evenly in cell populations. We report here that dimer distribution is in fact heterogeneous in re combination-proficient strains. Most cells in the population contain o nly monomers; dimers are confined to a small subpopulation from which plasmid-free daughters arise at high frequency. In a rec+ culture wher e 4% of pBR322 molecules are dimers, more than half are in dimer-only cells. We show that this situation is inevitable because dimers replic ate at twice the rate of monomers. Runaway multimerization is avoided because dimer-containing cells grow more slowly than their monomer-con taining counterparts. A computer simulation is used to show how dimers proliferate after formation by homologous recombination. The equilibr ium concentration of dimers is proportional to the inter-plasmid recom bination rate and is essentially independent of the rate at which homo logous recombination converts dimers to monomers.