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J. Bacteriol., 02 1997, 1291-1297, Vol 179, No. 4
DM Cook, PL Li, F Ruchaud, S Padden and SK Farrand
Two regions of the nopaline-type Ti plasmid pTiC58 are important for
conjugal transfer of this element to recipient bacteria. These two regions
were cloned into two independent replicons to produce a binary transfer
system. For one region, oriT/tra, we constructed two derivatives, pFRtra
and pDCtra-5. Each contains the oriT site and the two flanking, divergently
transcribed tra operons that encode the DNA processing functions associated
with the relaxosome. These two plasmids also carry traR, which encodes the
transcriptional activator necessary for expression of transfer genes. The
two plasmids differ by the amounts of traB sequence or sequence downstream
of traG present in the construct. The second replicon, pPLE2, carries the
traI/trb region. The traI gene confers production of the Agrobacterium
tumefaciens N-acyl homoserine lactone autoinducer, while the remaining
genes in the trb operon encode components of the mating bridge. Donors
harboring the two plasmids mobilized the transfer of the plasmid carrying
the oriT/tra region to an A. tumefaciens recipient at frequencies similar
to that at which the intact Ti plasmid transferred. Plasmid pFRtra, which
encodes most of traB, was mobilized at a frequency almost 10-fold higher
than was pDCtra-5, which lacks most of the gene. A. tumefaciens donors also
mobilized pFRtra to Escherichia coli and Pseudomonas fluorescens recipients
at frequencies similar to those observed with A. tumefaciens recipients.
Rhizobium meliloti harboring the binary system also transferred the
oriT/tra component to these recipients. However, E. coli or P. fluorescens
donors harboring the binary system did not transfer pFRtra to any of the
recipients. Furthermore, while the A. tumefaciens and R. meliloti donors
produced high levels of the autoinducer, the P. fluorescens and E. coli
donors produced only trace amounts of this signal molecule. These results
indicate that the tra system of pTiC58 is fully contained within the
characterized tra and trb regions of the Ti plasmid, that conjugation does
not require functions encoded by the vir system for maximal activity, and
that while the Ti plasmid tra system recognizes diverse gram-negative
bacteria as recipients, of the hosts tested, it functions only in members
of the family Rhizobiaceae.
Copyright © 1997, American Society for Microbiology
Ti plasmid conjugation is independent of vir: reconstitution of the tra functions from pTiC58 as a binary system
Department of Crop Sciences, University of Illinois at Urbana- Champaign, 61801, USA.
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