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Entry exclusion in F-like plasmids requires intact TraG in the donor that recognizes its cognate TraS in the recipient

Gerald F. Audette

William A. Klimke

Department of Biological Sciences, CW-405 Biological Sciences Building, University of Alberta, Edmonton, AB T6G 2E9, Canada

Correspondence
Laura S. Frost
laura.frost{at}ualberta.ca

The mating pair stabilization (Mps) protein of the F plasmid, TraG, is unique to F-like type IV secretion systems. TraG is a polytopic inner-membrane protein with a large C-terminal periplasmic domain that is required for piliation and Mps, whereas the N-terminal region is sufficient for pilus synthesis. The C-terminal region of TraG is thought to be cleaved by the host signal peptidase I to give a fragment called TraG* that is responsible for Mps. Using mutational analysis and cell localization studies, it was shown that TraG* is most probably an artifact caused by non-specific degradation. TraS (173 aa in F), which is involved in entry exclusion (Eex), blocks redundant conjugative DNA synthesis and transport between donor cells, suggesting that it interferes with a signalling pathway required to trigger DNA transfer. Using the F and R100 plasmids, TraG in the donor cell was found to recognize TraS in the recipient cell inner membrane, in a plasmid-specific manner. This activity mapped to aa 610–673 in F TraG, the only region that differs significantly from R100 TraG. Expression of traG or traG* in a recipient cell did not affect mating ability or Eex. These results suggest that TraG may be translocated to the recipient cell, where it contacts the inner membrane, initiating transfer, a process that is blocked by TraS.

Present address: Department of Chemistry, 456 Chemistry Building, York University, 4700 Keele St, Toronto, ON M3J 1P3, Canada.

Present address: National Center for Biotechnology Information/National Institutes of Health, 6th Floor, 45 Center Drive, Bethesda, MD 20892, USA.

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