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Journal of Bacteriology, March 2000, p. 1564-1574, Vol. 182, No. 6
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Components of the RP4 Conjugative Transfer Apparatus Form an Envelope Structure Bridging Inner and Outer Membranes of Donor Cells: Implications for Related Macromolecule Transport Systems

A. Marika Grahn,1 Jana Haase,2 Dennis H. Bamford,1 and Erich Lanka2,*

Department of Biosciences and Institute of Biotechnology, FIN-00014 University of Helsinki, Finland,1 and Max-Planck-Institut für Molekulare Genetik, Dahlem, D-14195 Berlin, Germany2

Received 21 October 1999/Accepted 23 December 1999

During bacterial conjugation, the single-stranded DNA molecule is transferred through the cell envelopes of the donor and the recipient cell. A membrane-spanning transfer apparatus encoded by conjugative plasmids has been proposed to facilitate protein and DNA transport. For the IncPalpha plasmid RP4, a thorough sequence analysis of the gene products of the transfer regions Tra1 and Tra2 revealed typical features of mainly inner membrane proteins. We localized essential RP4 transfer functions to Escherichia coli cell fractions by immunological detection with specific polyclonal antisera. Each of the gene products of the RP4 mating pair formation (Mpf) system, specified by the Tra2 core region and by traF of the Tra1 region, was found in the outer membrane fraction with one exception, the TrbB protein, which behaved like a soluble protein. The membrane preparation from Mpf-containing cells had an additional membrane fraction whose density was intermediate between those of the cytoplasmic and outer membranes, suggesting the presence of attachment zones between the two E. coli membranes. The Tra1 region is known to encode the components of the RP4 relaxosome. Several gene products of this transfer region, including the relaxase TraI, were detected in the soluble fraction, but also in the inner membrane fraction. This indicates that the nucleoprotein complex is associated with and/or assembled facing the cytoplasmic site of the E. coli cell envelope. The Tra1 protein TraG was predominantly localized to the cytoplasmic membrane, supporting its potential role as an interface between the RP4 Mpf system and the relaxosome.

* Corresponding author. Mailing address: Max-Planck-Institut für Molekulare Genetik, Abteilung Lehrach, Ihnestrasse 73, Dahlem, D-14195 Berlin, Germany. Phone: 49 30 8413 1696. Fax: 49 30 8413 1130. E-mail: lanka{at}molgen.mpg.de.

Journal of Bacteriology, March 2000, p. 1564-1574, Vol. 182, No. 6
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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