THE CONJUGAL TRANSFER SYSTEM OF AGROBACTERIUM-TUMEFACIENS OCTOPINE-TYPE TI PLASMIDS IS CLOSELY-RELATED TO THE TRANSFER SYSTEM OF AN INCP PLASMID AND DISTANTLY RELATED TO TI PLASMID VIR GENES

We have determined the DNA sequences of two unlinked regions of octopi ne-type Ti plasmids that contain genes required for conjugal transfer. Both regions previously were shown to contain sequences that hybridiz e with tra genes of the nopaline-type Ti plasmid pTiC58. One gene clus ter (designated tra) contains a functional oriT site and is probably r equired for conjugal DNA processing, while the other gene cluster (des ignated trb) probably directs the synthesis of a conjugal pilus and ma ting pore. Most predicted Tra and Trb proteins show relatively strong sequence similarity (30 to 50% identity) to the Tra and Trb proteins o f the broad-host-range IncP plasmid RP4 and show significantly weaker sequence similarity to Vir proteins found elsewhere on the Ti plasmid. An exception is found in the Ti plasmid TraA protein, which is predic ted to be a bifunctional nickase-helicase that has no counterpart in I ncP plasmids or among Vir proteins but has homologs in at least six ot her self-transmissible and mobilizable plasmids. We conclude that this Ti plasmid tra system evolved by acquiring genes from two or three di fferent sources. A similar analysis of the Ti plasmid vir region indic ates that is also evolved by appropriating genes from at least two con jugal transfer systems. The widely studied plasmid pTiA6NC previously was found to be nonconjugal and to have a 12.65-kb deletion of DNA rel ative to other octopine-type Ti plasmids. We show that this deletion r emoves the promoter-distal gene of the trb region and probably account s for the inability of this plasmid to conjugate.