THE CONJUGAL INTERMEDIATE OF PLASMID RSF1010 INHIBITS AGROBACTERIUM-TUMEFACIENS VIRULENCE AND VIRB-DEPENDENT EXPORT OF VIRE2

Agrobacterium tumefaciens causes crown gall disease by transferring on cogenic, single-stranded DNA (T strand), covalently attached to the Vi rD2 protein, across the bacterial envelope into plant cells where its expression results in tumor formation. The single-stranded DNA binding protein VirE2 is also transferred into the plant cell, though the loc ation at which VirE2 interacts with the T strand is still under invest igation. The movement of the transferred DNA and VirE2 from A. tumefac iens to the plant cell depends on the membrane-localized VirB and VirD 4 proteins. Further, the movement of the IncQ broad-host-range plasmid RSF1010 between Agrobacterium strains or from Agrobacterium to plants also requires the virB-encoded transfer system. Our earlier studies s howed that the presence of the RSF1010 plasmid in wild-type strains of Agrobacterium inhibits both their virulence and their capacity to tra nsport VirE2, as assayed by coinfection with virE mutants. Here we dem onstrate that the capacity to form a conjugal intermediate of RSF1010 is necessary for this inhibition, suggesting that the transferred form of the plasmid competes with the VirD2-T strand and/or VirE2 for a co mmon export site.