Control of Plasmid R1 Replication: Kinetics of Replication in Shifts Between Different Copy Number Levels

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Abstract
Plasmid R1 replication was studied in shifts between 2 steady states of copy number. The copy number was varied in 2 ways. First, the fact that it decreases with increasing growth rate was utilized. To minimize the metabolic effects of changes in the growth rate, the downshifts were obtained by adding .alpha.-methylglucoside to [Escherichia coli] cultures growing in glucose-minimal medium, and the upshifts were obtained by adding glucose to cultures growing in the presence of glucose plus .alpha.-methylglucoside. Second, a temperature-dependent copy mutant of plasmid R1 (pKN301) was used. Plasmid pPK301 shows a 3-fold higher copy number at 40 than at 30.degree. C. In both types of shift, plasmid replication immediately adjusted to the postshift differential rate. The copy number asymptotically adjusted to the new steady state. Hence, the system that controls plasmid R1 replication sets the frequency of replication without measuring the actual copy number. Plasmid R1 replication is probably under negative control by an R1-mediated repressor protein. Among the replication control models that involve negative control, the Pritchard inhibitor dilution model, the Sompayrac-Maale autorepressor model and the plasmid .lambda.dv system all predict gene dose-independent copy number control.