THE TRA REGION OF THE NOPALINE-TYPE TI PLASMID IS A CHIMERA WITH ELEMENTS RELATED TO THE TRANSFER SYSTEMS OF RSF1010, RP4, AND F

The Ti plasmids of Agrobacterium tumefaciens encode two transfer syste ms, One mediates,the translocation of the T-DNA from the bacterium to a plant cell, while the other is responsible for the conjugal transfer of the entire Ti plasmid from one bacterium to another. The determina nts responsible for conjugal transfer map to two regions, tra and trb, of the nopaline-type Ti plasmid pTiC58, By using transposon mutagenes is with Tn3HoHo1, we localized the tva determinants to an 8.5-kb regio n that also contains the oriT region, Fusions to lacZ formed by transp oson insertions indicated that this region is expressed as two diverge ntly transcribed units, We determined the complete nucleotide sequence of an 8,755-bp region of the Ti plasmid encompassing the transposon i nsertions defining tra. The region contains six identifiable genes org anized as two units divergently transcribable from a 258-bp intergenic region that contains the oriT site. One unit encodes traA, traF, and traB, while the second encodes traC, traD, and traG. Reporter insertio ns located downstream of both sets of genes did not affect conjugation but were expressed, suggesting that the two units encode additional g enes that are not involved in transfer under the conditions tested, Pr oteins of the predicted sizes were expressible from traA, traC, traD, and traG, The products of several Ti plasmid tra genes are related to those of other conjugation systems, The 127-kDa protein expressed from traA contains domains related to MobA of RSF1010 and to the helicase domain of TraI of plasmid F. The translation product of traF is relate d to TraF of RP4, and that of traG is related to TraG of RP4 and to Vi rD4 of the Ti plasmid T-DNA transfer system, Genetic analysis indicate d that at least traG and traF are essential for conjugal transfer, whi le sequence analysis predicts that traA also encodes an essential func tion, traB, while not essential, is required for maximum frequency of transfer. Patterns of sequence relatedness indicate that the oriT and the predicted cognate site-specific endonuclease encoded by traA share lineage with those of the transfer systems of RSF1010 and plasmid F, while genes of the Ti plasmid encoding other essential tra functions s hare common ancestry with genes of the RP4 conjugation system.