Abstract
The chvA gene product of Agrobacterium tumefaciens is required for virulence and attachment of bacteria to plant cells. Three chvA mutants were studied. In vivo, they were defective in the synthesis, accumulation, and secretion of beta-(1-2)glucan; however, the 235-kilodalton (kDa) protein known to be involved in the synthesis of beta-(1-2)glucan (A. Zorreguieta and R. Ugalde, J. Bacteriol. 167:947-951, 1986) was present and active in vitro. was present and active in vitro. Two molecular forms of cyclic beta-(1-2)glucan, designated types I and II, were resolved by gel chromatography. Type I beta-(1-2)glucan was substituted with nonglycosidic residues, and type II beta-(1-2)glucan was nonsubstituted. Wild-type cells accumulated type I beta-(1-2)glucan, and chvA mutant cells accumulated mainly type II beta-(1-2)glucan and a small amount of type I beta-(1-2)glucan. Inner membranes of wild-type and chvA mutants formed in vitro type II nonsubstituted beta-(1-2)glucan. A 75-kDa inner membrane protein is proposed to be the chvA gene product. chvA mutant inner membranes had increased levels of 235-kDa protein; partial trypsin digestion patterns suggested that the 235-kDa protein (the gene product of the chvB region) and the gene product of the chvA region form a complex in the inner membrane that is involved in the synthesis, secretion, and modification of beta-(1-2)glucan. All of the defects assigned to the chvA mutation were restored after complementation with plasmid pCD522 containing the entire chvA region.
Highlights
The chvA gene product of Agrobacterium tumefaciens is required for virulence and attachment of bacteria to plant cells
It was recently reported that one R. meliloti ndvA mutant, equivalent to A. tumefaciens chi'A mutants (7, 23), neither secreted nor accumulated cellular P-(1-2)glucan; it was previously reported that an A. tiumefaciens chvA mutant produced,B-(1-2)glucan (20)
trichloroacetic acid (TCA) extracts of A. tumefaciens A348, ME42, or NIl (ME42 complemented with plasmid pCD522 [Table 1]) contained polysaccharides eluting in the void volume and in the partial included volume of the column (Fig. 1)
Summary
The chvA gene product of Agrobacterium tumefaciens is required for virulence and attachment of bacteria to plant cells. Mutants with insertions in this 3.5-kilobase DNA region are virulent and form truncated proteins that are active in the 3-(1-2)glucan synthesis (28), which demonstrates that only the part of the chvB locus necessary for ,B-(1-2)glucan synthesis is required for effective plant infection. This glucan has been observed only in Agrobacterium and Rhizobium species (24) and is believed to play a central role in the interaction of these bacteria with plants, since in both genera, mutants affected in synthesis of the glucan are avirulent or form ineffective empty nodules
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