Eclipse period during replication of plasmid R1:: contributions from structural events and from the copy-number control system

The eclipse period ( the time period during which a newly replicated plasmid copy is not available for a new replication) of plasmid R1 in Escherichia coli was determined with the classic Meselson-Stahl density-shift experiment. A mini-plasmid with the wild-type R1 replicon and a mutant with a thermo-inducible runaway-replication phenotype were used in this work. The eclipses of the chromosome and of the wild-type plasmid were 0.6 and 0.2 generation times, respectively, at temperatures ranging from 30 degreesC to 42 degreesC. The mutant plasmid had a similar eclipse at temperatures up to 38 degreesC. At 42 degreesC, the plasmid copy number increased rapidly because of the absence of replication control and replication reached a rate of 350 - 400 plasmid replications per cell and cell generation. During uncontrolled replication, the eclipse was about 3 min compared with 10 min at controlled replication ( the wild-type plasmid at 42 degreesC). Hence, the copy-number control system contributed significantly to the eclipse. The eclipse in the absence of copy-number control ( 3 min) presumably is caused by structural requirements: the covalently closed circular plasmid DNA has to regain the right degree of superhelicity needed for initiation of replication and it takes time to assemble the initiation factors.