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Journal of Bacteriology, July 2005, p. 4767-4773, Vol. 187, No. 14
0021-9193/05/$08.00+0     doi:10.1128/JB.187.14.4767-4773.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Mutations in the C-Terminal Region of TraM Provide Evidence for In Vivo TraM-TraD Interactions during F-Plasmid Conjugation

Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E9

Received 31 January 2005/ Accepted 11 April 2005

Conjugation is a major mechanism for disseminating genetic information in bacterial populations, but the signal that triggers it is poorly understood in gram-negative bacteria. F-plasmid-mediated conjugation requires TraM, a homotetramer, which binds cooperatively to three binding sites within the origin of transfer. Using in vitro assays, TraM has previously been shown to interact with the coupling protein TraD. Here we present evidence that F conjugation also requires TraM-TraD interactions in vivo. A three-plasmid system was used to select mutations in TraM that are defective for F conjugation but competent for tetramerization and cooperative DNA binding to the traM promoter region. One mutation, K99E, was particularly defective in conjugation and was further characterized by affinity chromatography and coimmunoprecipitation assays that suggested it was defective in interacting with TraD. A C-terminal deletion (S79*, where the asterisk represents a stop codon) and a missense mutation (F121S), which affects tetramerization, also reduced the affinity of TraM for TraD. We propose that the C-terminal region of TraM interacts with TraD, whereas its N-terminal domain is involved in DNA binding. This arrangement of functional domains could in part allow TraM to receive the mating signal generated by donor-recipient contact and transfer it to the relaxosome, thereby triggering DNA transfer.

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