Submitted to: International Congress on Molecular Plant-Microbe Interactions
Publication Type: Abstract Only
Publication Acceptance Date: August 26, 1999
Publication Date: N/A
We have isolated and characterized the cyclic beta-(1,3)(1,6)-glucan synthesis locus from Bradyrhizobium japonicum and identified two genes (ndvB,C) which are required for their synthesis and for symbiotic N-fixation. NdvC, supposedly a beta-(1,6)-glucosyl transferase, is a 556 aa polypeptide with several membrane spanning regions. A single amino acid alteration (115 lysine to 115alanine, K115A) was introduced at the putative UDP-glucose binding site and (248 glutamate to 248glutamine, E248Q) at the glucan-binding site. The expression of mutated NdvC (K115A or E248Q) in a ndvC::Tn5 chromosomal background resulted in cyclic beta-glucans with altered ratios of (1,3)- and (1,6)-linkages. The synthesis of glucans with modified glycosyl linkage patterns allowed B. japonicum strains to infect and nodulate soybean roots, but the symbiosis was ineffective (fix -). This is contrasted with B. japonicum strain AB-1 (ndvC::Tn5) which synthesized glucans lacking beta -(1,6)-linkages, (cyclodecakis-(1,3)-beta-D-glucosyl), and formed only pseudonodules devoid of bacteroids. At 28 days after inoculation, more than 96% of the bacteroids from strain AB-1 carrying the altered ndvC plasmid (pK115A or pE248Q) had lost the plasmid. More than 85% of the bacteroids with a similar chromosomal background (i.e., ndvC::Tn5), carrying a wild-type beta-(1,6)-glucosyl transferase on plasmid, retained it. These results indicate that the nodule environment during infection and nodule development selects strains synthesizing wild-type beta-glucan.