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J. Bacteriol., Jul 1996, 4233-4247, Vol 178, No. 14
SK Farrand, I Hwang and DM Cook
The Ti plasmids of Agrobacterium tumefaciens encode two transfer systems.
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 determinants
responsible for conjugal transfer map to two regions, tra and trb, of the
nopaline-type Ti plasmid pTiC58. By using transposon mutagenesis with
Tn3HoHo1, we localized the tra determinants to an 8.5- kb region that also
contains the oriT region. Fusions to lacZ formed by transposon insertions
indicated that this region is expressed as two divergently transcribed
units. We determined the complete nucleotide sequence of an 8,755-bp region
of the Ti plasmid encompassing the transposon insertions defining tra. The
region contains six identifiable genes organized as two units divergently
transcribable from a 258-bp inter-genic region that contains the oriT site.
One unit encodes traA, traF, and traB, while the second encodes traC, traD,
and traG. Reporter insertions located downstream of both sets of genes did
not affect conjugation but were expressed, suggesting that the two units
encode additional genes that are not involved in transfer under the
conditions tested. Proteins 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 RSF1O1O and to the
helicase domain of TraI of plasmid F. The translation product of traF is
related to TraF of RP4, and that of traG is related to TraG of RP4 and to
VirD4 of the Ti plasmid T-DNA transfer system. Genetic analysis indicated
that at least traG and traF are essential for conjugal transfer, while
sequence analysis predicts that traA also encodes an essential function.
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 share common ancestry with
genes of the RP4 conjugation system.
Copyright © 1996, American Society for Microbiology
The tra region of the nopaline-type Ti plasmid is a chimera with elements related to the transfer systems of RSF1010, RP4, and F
Department of Crop Sciences, University of Illinois at Urbana- Champaign, 61801, USA.
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