THE MATING PAIR FORMATION SYSTEM OF PLASMID RP4 DEFINED BY RSF1010 MOBILIZATION AND DONOR-SPECIFIC PHAGE PROPAGATION

Transfer functions of the conjugative plasmid RP4 (IncPalpha) are dist ributed among distinct regions of the genome, designated Tral and Tra2 . By deletion analyses, we determined the limits of the Tra1 region, e ssential for intraspecific Escherichia coli matings. The Tral core reg ion encompasses approximately 5.8 kb, including the genes traF, -G, -H , -I, -J, and -K as well as the origin of transfer. The traM gene prod uct, however, is not absolutely required for conjugation but significa ntly increases transfer efficiency. To determine the transfer phenotyp e of genes encoded by the Tra2 core region, we generated a series of d efined Tra2 mutants. This revealed that at least trbB, -C, -E, -G, and -L are essential for RP4 conjugation. To classify these transfer func tions as components of the DNA transfer and replication (Dtr) or of th e mating pair formation (Mpf) system, we analyzed the corresponding de rivatives with respect to mobilization of IncQ plasmids and donor-spec ific phage propagation. We found that all of the Tra2 genes listed abo ve and the traG and traF genes of Tra1 are required for RSF1010 mobili zation. Expression of traF from Tra1 in conjunction with the Tra2 core was sufficient for phage propagation. This implies that the TraG prot ein is not directly involved in pilus formation and potentially connec ts the relaxosome with proteins enabling the membrane passage of the D NA. The proposed roles of the RP4 transfer gene products are discussed in the context of virulence functions encoded by the evolutionarily r elated Ti T-DNA transfer system of agrobacteria.