Identification of the Product of an Agrobacterium tumefaciens Chromosomal Virulence Gene. Angeles Zorreguieta. Instituto de Investigationes Bioquímicas “Fundación Campomar,” Facultad de Ciencias Exactas y Naturales and CONICET, Antonio Machado 151, 1405 Buenos Aires, Argentina. Roberto A. Geremia(1), Sonia Cavaignac(1), Gerard A. Cangelosi(2), Eugene W. Nester(2), and Rodolfo A. Ugalde(1). (1) Instituto de Investigaciones Bioquímicas “Fundación Campomar,” Facultad de Ciencias Exactas y Naturales and CONICET, Antonio Machado 151, 1405 Buenos Aires, Argentina, and (2) Department of Microbiology SC-42, University of Washington, Seattle, WA 98195, U.S.A.. MPMI 1:121-127. Accepted 15 February 1988. Copyright 1988 The American Phytopathological Society.

The chvB operon of Agrobacterium tumefaciens is required for bacterial attachment to plant cells and for efficient crown gall tumor formation. As defined by the virulence phenotypes of mutants with transposon insertions mapping in the region, the operon was previously mapped to a 5-kilobase (kb) stretch of chromosomal DNA. We report here that the operon is actually about 8.5 kb long and that it contains a 7-kb gene coding for a large membrane protein involved in the synthesis of cyclic β-1,2-glucan. Mutants with transpospon insertions within the 5-kb phenotypically defined operon do not synthesize this functional protein, do not synthesize ß-1,2-glucan, and do not form tumors. However, mutants with insertions that map up to 3.5 kb downstream of the phenotypically defined operon synthesize truncated proteins that are active in ß-1,2-glucan synthesis. These mutants form tumors. The truncated proteins correspond closely in size with the map positions of the insertions, suggesting that the insertions truncate the proteins by translational termination. A plasmid that contains only the phenotypically defined chvB operon also codes for a truncated protein. A fusion product between the protein and β-galactosidase carried on a Tn3-HoHo1 insertion was observed in one mutant. Partial trypsin digestion of wild-type inner membranes generated truncated proteins that were active in ß-1,2-glucan synthesis, demonstrating that a large portion of the protein is not required for ß-1,2-glucan synthesis. The correlation between ß-1,2-glucan synthesis by the truncated proteins and tumorigenesis strongly implicates the polysaccharide product of this protein in tumor formation.