Plasmid R1 is present as clusters in the cells of Escherichia coli

Fluorescence microscopy was used to determine the location(s) of the replic ation origin of plasmid R1 in exponentially growing cells of Escherichia co li. The number of oriR1 foci per cell was smaller than the number of R1 cop ies per cell and was found to be the same for a copA mutant of R1 and for t he wild-type plasmid. The intensities of individual foci were stronger for the cop mutant than for the wild type. We interpreted these results to impl y that the plasmid DNA molecules were localized in small groups/clusters, a result that seems contrary to the earlier observations that plasmid R1 rep licates randomly and segregates as a single-copy unit. The implications for the quantitative behavior of plasmid R1 in stability, incompatibility test s, replication, and partition experiments are discussed. (C) 2000 Academic Press.