Partition of plasmid R1: a computer simulation.

A computer-simulated population of individual Escherichia coli cells harboring plasmid R1 parA+/parB- has been used to analyze three possible modes of plasmid segregation: equi-partition, in which plasmids are partitioned equally to daughter cells at cell division; single-site inheritance, in which the products of the most recent replication event are partitioned equally and the remaining plasmids are distributed randomly; and pair-site partition, in which a single, randomly chosen plasmid is actively partitioned to each daughter cell at division and the rest are distributed randomly. Comparison between predicted and experimental plasmid loss-frequency enabled us to rule out the first of these models as a likely mode of action in R1 but was inconclusive regarding the other two. The parA region would therefore seem to partition actively only one pair of plasmids to each daughter cell, the precise selection rule involved remaining unresolved. This question is not easily decided with current technology, as we show, but our simulation results also predict that the isolation of rep(Ts) mutants will provide an experimental system in which a clear distinction is possible between two plasmids that are the products of the most recent replication event and two that are chosen strictly at random.