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Journal of Bacteriology, May 2000, p. 2709-2715, Vol. 182, No. 10
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Conjugative Junctions in RP4-Mediated Mating of Escherichia coli

A. Lacey Samuels,1 Erich Lanka,2 and Julian E. Davies1,*

Department of Microbiology and Immunology, University of British Columbia, Vancouver, British Columbia, Canada,1 and Max-Planck-Institut für Molekulare Genetik, Dahlem, D-14195 Berlin, Germany2

Received 1 December 1999/Accepted 24 February 2000

The physical association of bacteria during conjugation mediated by the IncPalpha plasmid RP4 was investigated. Escherichia coli mating aggregates prepared on semisolid medium were ultrarapidly frozen using copper block freezing, followed by freeze substitution, thin sectioning, and transmission electron microscopy. In matings where the donor bacteria contained conjugative plasmids, distinctive junctions were observed between the outer membranes of the aggregates of mating cells. An electron-dense layer linked the stiffly parallel outer membranes in the junction zone, but there were no cytoplasmic bridges nor apparent breaks in the cell walls or membranes. In control experiments where the donors lacked conjugative plasmids, junctions were not observed. Previous studies have shown that plasmid RP4 carries operons for both plasmid DNA processing (Tra1) and mating pair formation (Tra2). In matings where donor strains carried Tra2 only or Tra2 plus the pilin-processing protease TraF, junctions were found but they were shorter and more interrupted than the wild type. If the donor strain had the pilin gene knocked out (trbC), junctions were still found. Thus, it appears that the electron-dense layer between the outer membranes of the conjugating cells is not composed of pilin.

* Corresponding author. Mailing address: Department of Microbiology and Immunology, University of British Columbia, 6174 University Blvd., Vancouver, BC V6T 1Z3, Canada. Phone: (604) 221-8896. Fax: (604) 822-6041. E-mail: jed{at}interchange.ubc.ca.

Journal of Bacteriology, May 2000, p. 2709-2715, Vol. 182, No. 10
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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