Agrobacterium tumefaciens causes tumors on plants by transferring part
of its Ti (tumor inducing) plasmid, the T-DNA or transferred DNA, to
plant cells. The virB operon, the largest operon of the virulence (Vir
) region located on the Ti plasmid, is necessary for tumor induction o
n all plant species. Previously, the complete nucleotide sequences of
the virB operons of several Ti plasmids of A. tumefaciens were determi
ned. The 11 predicted proteins mostly have signal sequences and/or hyd
rophobic domains. Hence, these proteins are thought to be located in o
r transported over the agrobacterial inner membrane. The VirB proteins
are suggested to form a pore structure in the membrane through which
the T-DNA-protein complex is transported. To obtain direct evidence fo
r transport of these proteins over the inner membrane, we made fusions
between genes of the virB operon and the gene for the enzyme alkaline
phosphatase. Here we show the localization of several VirB proteins i
n the bacterial membrane and predict the topology of the membrane-loca
lized VirB proteins. The finding of a fusion between the VirB7 protein
and the enzyme alkaline phosphatase provides the first evidence for t
he expression of the small virB7 gene. The VirB2 protein shows similar
ity with the TraA propilin of the Escherichia coli F plasmid. Here we
show that the predicted topology of the VirB2 protein in the inner mem
brane is identical to that of the TraA protein. Therefore, we hypothes
ize that the VirB2 protein is part of an agrobacterial pilus-like stru
cture. (C) 1994 Academic Press, Inc.