The ndvB locus of Rhizobium meliloti encodes a 319-kDa protein involved in the production of beta-(1----2)-glucan

The ndvB locus of Rhizobium meliloti was sequenced and found to encode a 319-kDa protein involved in the production of beta-(1----2)-glucan. Transposon Tn5 mutagenesis revealed that a large portion of the downstream half of this gene is not essential for symbiosis but is required for optimal production of beta-(1----2)-glucan. A high molecular weight inner membrane protein, believed to be the ndvB gene product, was absent from two different upstream ndvB::Tn5 mutants. This protein could be labeled in vitro with UDP-U-14C]glucose in the wild type but not in the symbiotically defective mutants. Inner membrane preparations from the symbiotically competent downstream mutants labeled less well than did those from wild type with UDP-U-14C] glucose and did not show distinct bands after polyacrylamide gel electrophoresis and fluorography, suggesting that C-terminal truncations of NdvB might affect the stability of this molecule. These downstream mutants had reduced amounts of periplasmic beta-(1----2)-glucan and exhibited several vegetative defects seen also in the upstream mutants. These included alterations in phage and antibiotic sensitivity, in motility, and in growth in low osmolarity media. Bacteroids produced by two of the downstream mutants were morphologically abnormal, indicating that ndvB is involved not only in invasion but also in bacteroid development.