Transfer-defective mutants of the Tral region of RP1 were isolated. Complementation studies involving stable heterozygotes combined with the mapping of TnS insertion mutations revealed two pilus cistrons, pi4and pilB, at positions 46.9 to 48.2 kb and 46.0 to 46.4 kb, respectively. All pilB mutants were Dps-(i.e., resistant to donor-specific phages PR4 and PRR1), whereas piL4 mutants were Dps-(promoter-proximal mutations), Dps+'-(sensitive only to PR4 [more centrally located mutations]), or Dps' (sensitive to both phages [promoter-distal mutations]). The correlation between the site mutated and the Dps phenotype, together with the finding that certain Dps+ piL4 mutants continued to mobilize nonconjugative plasmids, suggested that piL4 is bifunctional, contributing both to pilus function (at the promoter-proximal end) and to RP1 mobilization. It was also shown that the 43.5-to 49.5-kb region that includes pilA and pilB encodes all of the Tral pilus functions required for propagation of donor-specific phages and hence, probably, for pili that are active in conjugation. Finally, three cistrons that specifically affect RP1 mobilization were identified. Two of these, mobA and mobB, occur immediately anticlockwise to oriT and probably correspond to the traJ and traI genes characterized by other workers. The third cistron, mobC, occurs clockwise to oriT and may be a new mobilization gene, since its function can be substituted by IncPp plasmids, a feature different from that of the traK mobilization gene which occurs in the same region but is RP1 specific. None of the mob cistrons was required for mobilization of nonconjugative plasmids, except for mobB, which was required by pVS99.The broad-host-range IncPa plasmid RP1 (60 kb; synonymous with R18, RP4, and RK2) carries two regions (Tral and Tra2) that encode conjugal functions (22,37,45
