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Journal of Bacteriology, January 1999, p. 186-196, Vol. 181, No. 1
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

pSa Causes Oncogenic Suppression of Agrobacterium by Inhibiting VirE2 Protein Export

Lan-Ying Lee,1,2 Stanton B. Gelvin,1,* and Clarence I. Kado2

Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907-1392,1 and Davis Crown Gall Group, University of California, Davis, California 956162

Received 27 March 1998/Accepted 9 October 1998

When coresident with the Ti (tumor-inducing) plasmid, the 21-kDa product of the osa gene of the plasmid pSa can suppress crown gall tumorigenesis incited by Agrobacterium tumefaciens. Neither T-DNA processing nor vir (virulence) gene induction is affected by the presence of osa in the bacterium. We used Arabidopsis thaliana root segments and tobacco leaf discs to demonstrate that Osa inhibits A. tumefaciens from transforming these plants to the stable phenotypes of tumorigenesis, kanamycin resistance, and stable beta -glucuronidase (GUS) expression. When A. tumefaciens contained osa, the lack of expression of transient GUS activity in infected plant tissues, as well as the lack of systemic viral symptoms following agroinfection of Nicotiana benthamiana by tomato mottle virus, suggested that oncogenic suppression by Osa occurs before T-DNA enters the plant nucleus. The extracellular complementation of an A. tumefaciens virE2 mutant (the T-DNA donor strain) by an A. tumefaciens strain lacking T-DNA but containing a wild-type virE2 gene (the VirE2 donor strain) was blocked when osa was present in the VirE2 donor strain, but not when osa was present in the T-DNA donor strain. These data indicate that osa inhibits VirE2 protein, but not T-DNA export from A. tumefaciens. These data further suggest that VirE2 protein and T-DNA are separately exported from the bacterium. The successful infection of Datura stramonium plants and leaf discs of transgenic tobacco plants expressing VirE2 protein by an A. tumefaciens virE2 mutant carrying osa confirmed that oncogenic suppression by osa does not occur by blocking T-DNA transfer. Overexpression of virB9, virB10, and virB11 in A. tumefaciens did not overcome oncogenic suppression by osa. The finding that the expression of the osa gene by itself, rather than the formation of a conjugal intermediate with pSa, blocks transformation suggests that the mechanism of oncogenic suppression by osa may differ from that of the IncQ plasmid RSF1010.

* Corresponding author. Mailing address: Department of Biological Sciences, Purdue University, West Lafayette, IN 47907-1392. Phone: (765) 494-4939. Fax: (765) 496-1496. E-mail: gelvin{at}bilbo.bio.purdue.edu.

Journal of Bacteriology, January 1999, p. 186-196, Vol. 181, No. 1
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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