DIMERIZATION OF PLASMID DNA ACCELERATES SELECTION FOR ANTIBIOTIC-RESISTANCE

Dimerization of multicopy plasmids is widely assumed to be disadvantag eous both for plasmid maintenance and for the host cell. It is known t hat dimerization causes plasmid instability; dimer-containing cells gr ow slower than their monomer-containing counterparts. However, as we d emonstrate here, under conditions of selective stress, dimers provide an advantage for bacteria. Dimers facilitate segregation of mutants fr om numerous copies of the parental plasmid. Accelerated segregation gr eatly increases the rate of accumulation of plasmids carrying mutation s that are adaptive for bacteria. In contrast, resolution of dimers by site-specific recombination decreases, 10(3)-10(5)-fold, the efficien cy of selection of spontaneous reversions in the tet gene of pBR327.