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The ndvA locus of Rhizobium meliloti is homologous to and can substitute for the chvA locus of Agrobacterium tumefaciens. We have previously shown that an ndvA mutant exhibited reduced motility and formed small, white, empty nodules on alfalfa roots. Here we show that this ndvA mutant is defective in the production of the cyclic extracellular polysaccharide beta-(1----2)glucan, even though a 235,000-dalton protein intermediate, known to be involved in the synthesis of this molecule, is present and active in vitro. The DNA sequence of the ndvA locus revealed a single large open reading frame encoding a 67,100-dalton protein that was homologous to a number of bacterial ATP-binding transport proteins. The greatest degree of relatedness was seen with Escherichia coli HlyB, a protein involved in the export of hemolysin, and with the mdr gene product of mammalian cells, which is also homologous to HlyB and thought to be involved in export. Based on the overall symbiotic phenotype of ndvA mutants, the extensive homology between NdvA and HlyB, the fact that ndvA mutants retained an active 235,000-dalton membrane intermediate, and the absence of extracellular beta-(1----2)glucan, we propose that NdvA is involved in export of beta-(1----2)glucan from the cell and that this process is fundamentally important for normal alfalfa nodule development.

The ndvA gene product of Rhizobium meliloti is required for beta-(1----2)glucan production and has homology to the ATP-binding export protein HlyB