Stability of pBR322-derived plasmids

Abstract

The stability of pBR322-derived plasmids was studied during growth of their Escherichia coli host in the absence of antibiotics. Plasmid pBR322, as well as its Δ rom and Δ bla derivatives, were lost from their host within 60 generations, but a number of Δ tet derivatives were quite stable under the same conditions. An evaluation of the data indicated that primary plasmid loss due to random partitioning corresponds to the generation of a plasmid-free cell about every 10 4 divisions (probability P 0; = “intrinsic” instability). Secondary loss of plasmid-carrying cells resulted from a growth advantage of the plasmid-free cells when bacteria die, perhaps due to unrepaired lethal damage in the DNA, under conditions of stationary incubation (= “apparent” instability). This cell death also occurred in the absence of plasmids but was accelerated by the presence of extra plasmid DNA in the cell and further accelerated by a functional tet gene. This was the reason for the differential apparent stabilities of Δ bla and Δ tet plasmids. There was no indication that an accumulation of plasmid multimers contributed to the plasmid instability, as has been suggested in the literature. The value of P 0 = 10 −4 is 14 orders of magnitude greater than expected under the assumption of a random (Poisson) distribution of plasmid copy numbers in a population of cells.