Characterization ofndvD, the third gene involved in the synthesis of cyclic β-(13),(16)-D-glucans inBradyrhizobium japonicum

Abstract

Previously, we identified two genes in Bradyrhizobium japonicum (ndvB, ndvC) that are required for cyclic β-(1[Formula: see text]3),(1[Formula: see text]6)-D-glucan synthesis and successful symbiotic interaction with soybean (Glycine max). In this study, we report a new open reading frame (ORF1) located in the intergenic region between ndvB and ndvC, which is essential for β-glucan synthesis and effective nodulation of G. max. This new gene is designated ndvD (nodule development). The ndvD translation product has a predicted molecular mass of 26.4 kDa with one transmembrane domain. Genetic experiments involving gene deletion, Tn5 insertion, and gene complementation revealed that the mutation of ndvD generated pleiotropic phenotypes, including hypoosmotic sensitivity, reduced motility, and defects in conjugative gene transfer, in addition to symbiotic ineffectiveness. Although deficient in in vivo β-glucan synthesis, membrane preparations from the ndvD mutant synthesized neutral β-glucans in vitro. Therefore, ndvD does not appear to be a structural gene for β-glucan synthesis. Our hypothesis for the mechanism of β-(1[Formula: see text]3),(1[Formula: see text]6)-D-glucan synthesis is presented. Key Words: β-glucans,Bradyrhizobium, soybean, nitrogen fixation.