Plasmid conjugation systems are composed of two components, the DNA transfer and replication system, or Dtr, and the mating pair formation system, or Mpf. During conjugal transfer an essential factor, called the coupling protein, is thought to interface the Dtr, in the form of the relaxosome, with the Mpf, in the form of the mating bridge. These proteins, such as TraG from the IncP1 plasmid RP4 (TraG RP4 ) and TraG and VirD4 from the conjugal transfer and T-DNA transfer systems of Ti plasmids, are believed to dictate specificity of the interactions that can occur between different Dtr and Mpf components. The Ti plasmids of Agrobacterium tumefaciens do not mobilize vectors containing the oriT of RP4, but these IncP1 plasmid derivatives lack the trans-acting Dtr functions and TraG RP4 . A. tumefaciens donors transferred a chimeric plasmid that contains the oriT and Dtr genes of RP4 and the Mpf genes of pTiC58, indicating that the Ti plasmid mating bridge can interact with the RP4 relaxosome. However, the Ti plasmid did not mobilize transfer from an IncQ relaxosome. The Ti plasmid did mobilize such plasmids if TraG RP4 was expressed in the donors. Mutations in traG RP4 with defined effects on the RP4 transfer system exhibited similar phenotypes for Ti plasmid-mediated mobilization of the IncQ vector. When provided with VirD4, the tra system of pTiC58 mobilized plasmids from the IncQ relaxosome. However, neither TraG RP4 nor VirD4 restored transfer to a traG mutant of the Ti plasmid. VirD4 also failed to complement a traG RP4 mutant for transfer from the RP4 relaxosome or for RP4-mediated mobilization from the IncQ relaxosome. TraG RP4 -mediated mobilization of the IncQ plasmid by pTiC58 did not inhibit Ti plasmid transfer, suggesting that the relaxosomes of the two plasmids do not compete for the same mating bridge. We conclude that TraG RP4 and VirD4 couples the IncQ but not the Ti plasmid relaxosome to the Ti plasmid mating bridge. However, VirD4 cannot couple the IncP1 or the IncQ relaxosome to the RP4 mating bridge. These results support a model in which the coupling proteins specify the interactions between Dtr and Mpf components of mating systems.Plasmid conjugation conceptually can be divided into two functions. In the first, the DNA is processed by a complex of proteins, one of which introduces a single-strand nick at the nic site within the oriT recognition sequence. Called the relaxosome, the proteins of this complex are coded for by genes of the Dtr (DNA transfer and replication) component of the transfer system. In the second, the nucleoprotein transfer intermediate comprised of the nicked strand covalently linked at the 5Ј end to the relaxase is secreted from the donor directly into the recipient via a bridge that forms between the mating pair. This translocation apparatus is a complex membraneassociated structure coded for by the Mpf (mating pair formation) genes.The relaxosome of one conjugal plasmid may or may not be transferrable by the Mpf system of another. Specificity is conferred, in part, by a...

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We examined the role of exposure duration and scene complexity on the weapon focus effect (WFE). Memory for the a mock crime was affected more by a weapon than an unusual but nonthreatening object. Threat reduced correct identifications when the event was short but not long; duration of the event did not interact with unusualness. Additionally, we found a WFE for target-absent lineup decisions, but only for the accomplice lineup, not the object-wielding perpetrator's lineup. We discuss the implications of these results for illuminating the mechanisms that elicit the WFE.