Random diffusion can account for topA-dependent suppression of partition defects in low-copy-number plasmids.

The maintenance of partition-defective (Par-) mini-P1 and mini-F plasmids was studied in topA strains of Escherichia coli, which are defective in topoisomerase I activity. The partition defects were substantially but not completely suppressed in broth-grown cultures. This suppression was not due to a large increase in copy number. However, the absolute number of copies of Par- mini-P1 plasmids per average dividing cell is sufficiently high to account for the modest stability observed if a random distribution of the copies to daughter cells is assumed. The similar number of Par- plasmid copies in wild-type cells are distributed in a considerably worse-than-random fashion. Thus, it is unnecessary to propose, as was suggested previously, that an active, par-independent pathway operates in topA strains to ensure proper segregation of the plasmids to daughter cells. Rather, it seems likely that the lack of topoisomerase I activity aids the random distribution of the partition-defective plasmids, perhaps by facilitating their separation after replication. The results of studies carried out at reduced growth rates were consistent with this view; when topA cells containing Par- mini-P1 plasmids were cultured in minimal medium, in which the copy number of the plasmids per average cell is sharply reduced, very little suppression of the partition defect was observed.