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Journal of Bacteriology, June 2000, p. 3437-3445, Vol. 182, No. 12
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

The N- and C-Terminal Portions of the Agrobacterium VirB1 Protein Independently Enhance Tumorigenesis

Matxalen Llosa,dagger John Zupan, Christian Baron,Dagger and Patricia Zambryski*

Department of Plant and Microbial Biology, University of California, Berkeley, Berkeley, California 94720-3102

Received 12 November 1999/Accepted 23 March 2000

Genetic transformation of plants by Agrobacterium tumefaciens is mediated by a virulence (vir)-specific type IV secretion apparatus assembled from 11 VirB proteins and VirD4. VirB1, targeted to the periplasm by an N-terminal signal peptide, is processed to yield VirB1*, comprising the C-terminal 73 amino acids. The N-terminal segment, which shares homology with chicken egg white lysozyme as well as lytic transglycosylases, may provide local lysis of the peptidoglycan cell wall to create channels for transporter assembly. Synthesis of VirB1* followed by its secretion to the exterior of the cell suggests that VirB1* may also have a role in virulence. In the present study, we provide evidence for the dual roles of VirB1 in tumorigenesis as well as the requirements for processing and secretion of VirB1*. Complementation of a virB1 deletion strain with constructs expressing either the N-terminal lysozyme-homologous region or VirB1* results in tumors intermediate in size between those induced by a wild-type strain and a virB1 deletion strain, suggesting that each domain has a unique role in tumorigenesis. The secretion of VirB1* translationally fused to the signal peptide indicates that processing and secretion are not coupled. When expressed independently of all other vir genes, VirB1 was processed and VirB1* was secreted. When restricted to the cytoplasm by deletion of the signal peptide, VirB1 was neither processed nor secreted and did not restore virulence to the virB1 deletion strain. Thus, factors that mediate processing of VirB1 and secretion of VirB1* are localized in the periplasm or outer membrane and are not subject to vir regulation.

* Corresponding author. Mailing address: Dept. of Plant and Microbial Biology, 111 Koshland Hall, University of California, Berkeley, CA 94720-3102. Phone: (510) 643-9204. Fax: (510) 642-4995. E-mail: zambrysk{at}nature.berkeley.edu.

dagger Present address: Departamento de Biologia Molecular (Unidad asociada al C.I.B.), Facultad de Medicina, C. Herrera Oria s/n, 39011 Santander, Spain.

Dagger Present address: Lehrstuhl für Mikrobiologie der Universität München, D-80638 Munich, Germany.

Journal of Bacteriology, June 2000, p. 3437-3445, Vol. 182, No. 12
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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