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Journal of Bacteriology, August 1999, p. 5033-5041, Vol. 181, No. 16
Departments of Crop
Sciences1 and
Microbiology,2 University of Illinois
at Urbana-Champaign, Urbana, Illinois 61801
Received 2 April 1999/Accepted 11 June 1999
The trb operon from pTiC58 is one of three loci that
are required for conjugal transfer of this Ti plasmid. The operon,
which probably codes for the mating bridge responsible for pair
formation and DNA transfer, contains 12 genes, 11 of which are related
to genes from other members of the type IV secretion system family. The
12th gene, traI, codes for production of
Agrobacterium autoinducer (AAI). Insertion mutations were
constructed in each of the 12 genes, contained on a full-length clone
of the trb region, using antibiotic resistance cassettes or
a newly constructed transposon. This transposon, called
mini-Tn5Ptrb, was designed to express genes
downstream of the insertion site from a promoter regulated by TraR and
AAI. Each mutation could trans complement downstream Tn3HoHo1 insertions in the trb operon of
full-sized Ti plasmids. When marker-exchanged into the
transfer-constitutive Ti plasmid pTiC58
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Essential Components of the Ti Plasmid
trb System, a Type IV Macromolecular
Transporter

and
accR mutations in
trbB, -C, -D, -E,
-L, -F, -G, and -H abolished conjugal transfer from strain UIA5, which lacks the 450-kb
catabolic plasmid pAtC58. However, these mutants retained residual
conjugal transfer activity when tested in strain NT1, which contains
this large plasmid. The trbJ mutant failed to transfer at a
detectable frequency from either strain, while the trbI
mutant transferred at very low but detectable levels from both donors. Only the trbK mutant was unaffected in conjugal transfer
from either donor. Transfer of each of the marker-exchange mutants was
restored by a clone expressing only the wild-type allele of the
corresponding mutant trb gene. An insertion mutation in
traI abolished the production of AAI and also conjugal
transfer. This defect was restored by culturing the mutant donor in the
presence of AAI. We conclude that all of the trb genes
except trbI and trbK are essential for conjugal
transfer of pTiC58. We also conclude that mutations in any one of the
trb genes except traI and trbJ can
be complemented by functions coded for by pAtC58.
*
Corresponding author. Mailing address: Department of
Crop Sciences, University of Illinois at Urbana-Champaign, 240 Edward R. Madigan Laboratory, 1201 West Gregory Dr., Urbana, IL 61801. Phone:
(217) 333-1524. Fax: (217) 244-7830. E-mail:
stephenf{at}uiuc.edu.
Present address: Plant Protectants Research Unit, Korean Research
Institute of Bioscience and Biotechnology, Yusung, Taejon, South Korea
305-600.
Present address: Howard Hughes Medical Institute, Department of
Molecular Genetics and Cell Biology, The University of Chicago, Chicago, IL 60637.
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