Coupling between the Basic Replicon and the Kis-Kid Maintenance System of Plasmid R1: Modulation by Kis Antitoxin Levels and Involvement in Control of Plasmid Replication

Abstract

<i>kis-kid</i>, the auxiliary maintenance system of plasmid R1 and <i>copB</i>, the auxiliary copy number control gene of this plasmid, contribute to increase plasmid replication efficiency in cells with lower than average copy number. It is thought that Kis antitoxin levels decrease in these cells and that this acts as the switch that activates the Kid toxin; activated Kid toxin reduces <i>copB</i>-mRNA levels and this increases RepA levels that increases plasmid copy number. In support of this model we now report that: (i) the Kis antitoxin levels do decrease in cells containing a mini-R1 plasmid carrying a <i>repA</i> mutation that reduces plasmid copy number; (ii) <i>kid</i>-dependent replication rescue is abolished in cells in which the Kis antitoxin levels or the CopB levels are increased. Unexpectedly we found that this coordination significantly increases both the copy number of the <i>repA</i> mutant and of the wt mini-R1 plasmid. This indicates that the coordination between plasmid replication functions and <i>kis-kid</i> system contributes significantly to control plasmid R1 replication.