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Microbiology 155 (2009), 2676-2682; DOI  10.1099/mic.0.029777-0
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Microbiology 155 (2009), 2676-2682; DOI  10.1099/mic.0.029777-0
© 2009 Society for General Microbiology

The role of FIS in the Rcd checkpoint and stable maintenance of plasmid ColE1

I. K. Blaby{dagger} and D. K. Summers

Department of Genetics, University of Cambridge, Downing Street, Cambridge CB2 3EH, UK

Escherichia coli plasmid ColE1 lacks active partitioning, and copies are distributed randomly to daughter cells at division. The plasmid is maintained stably in the bacterial population as long as its copy number remains high. The accumulation of plasmid dimers and higher multimers depresses copy number, and is an important cause of multicopy plasmid instability. ColE1 dimers are restored to the monomeric state by site-specific recombination, which requires the host-encoded proteins XerCD, ArgR and PepA acting at the plasmid cer site. In addition, a 70 nt RNA expressed from the cer site of plasmid dimers delays the division of dimer-containing cells. Here, we report that the global regulator FIS binds to cer in a sequence-specific manner, close to the Rcd promoter (Pcer). FIS is not required for plasmid dimer resolution, but is essential for repression of Pcer in plasmid monomers. Repression also requires the XerCD recombinase, but not ArgR or PepA. We propose a model for monomer–dimer control of Pcer in which the promoter is repressed in plasmid monomers by the concerted action of FIS and XerCD. Rcd transcription is triggered in plasmid dimers by the lifting of XerCD-mediated repression in the synaptic complex.

Correspondence
D. K. Summers
dks11{at}cam.ac.uk

Abbreviations: EMSA, electrophoretic mobility shift assay

{dagger}Present address: Department of Microbiology and Cell Science, PO Box 110700, University of Florida, Gainesville, FL 32611-0700, USA.






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