Enhanced symbiotic performance by Rhizobium tropici glycogen synthase mutants

We isolated a Tn5-induced Rhizobium tropici mutant that has enhanced capaci ty to oxidize N,N-dimethyl-p-phenylendiamine (DMPD) and therefore has enhan ced respiration via cytochrome oxidase, The mutant had increased levels of the cytochromes c(1) and CycM and a small increase in the amount of cytochr ome aa(3). In plant tests, the mutant increased the dry weight of Phaseolus vulgaris plants by 20 to 38% compared with the control strain, thus showin g significantly enhanced symbiotic performance. The predicted product of th e mutated gene is homologous to glycogen synthases from several bacteria, a nd the mutant larked glycogen, The DNA sequence of the adjacent gene region revealed six genes predicted to encode products homologous to the followin g gene products from Escherichia coli: glycogen phosphorylase (glgP), glyco gen branching enzyme (glgB), ADP glucose pyrophosphorylase (glgC), glycogen synthase (glgA), phosphoglucomutase pgm), and glycogen debranching enzyme (glgX). All six genes are transcribed in the same direction, and analysis w ith lacZ gene fusions suggests that the first fire genes are organized in o ne operon, although pgm appears to have an additional promoter; glgX is tra nscribed independently. Surprisingly, the glgA mutant had decreased levels of high-molecular-weight exopolysaccharide after growth on glucose, but lev els were normal after growth on galactose, A deletion mutant was constructe d in order to generate a nonpolar mutation in glgA. This mutant had a pheno type similar to that of the Tn5 mutant, indicating that the enhanced respir ation and symbiotic nitrogen fixation and decreased exopolysaccharide were due to mutation of glgA and not to a polar effect on a downstream gene.