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Effect of the CopB Auxiliary Replication Control System on Stability of Maintenance of Par⁺ Plasmid R1

Jan A. Olsson, Johan Paulsson, and Kurt Nordström^*

Department of Cell and Molecular Biology, Biomedical Center, Uppsala University, S-751 24 Uppsala, Sweden

Received 31 March 2003/ Accepted 1 October 2003

Plasmid R1 is a low-copy-number plasmid that is present at a level of about four or five copies per average cell. The copy number is controlled posttranscriptionally at the level of synthesis of the rate-limiting initiator protein RepA. In addition to this, R1 has an auxiliary system that derepresses a second promoter at low copy numbers, leading to increased repA mRNA synthesis. This promoter is normally switched off by a constitutively synthesized plasmid-encoded repressor protein, CopB; in cells with low copy numbers, the concentration of CopB is low and the promoter is derepressed. Here we show that the rate of loss of a Par⁺ derivative of the basic replicon of R1 increased about sevenfold when the cells contained a high concentration of the CopB protein formed from a compatible plasmid.

Present address: Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge CB3 0WA, United Kingdom.