Effect of poly-3-hydroxybutyrate synthase mutation on the metabolism of Ensifer (formerly Sinorhizobium) meliloti.

In order to investigate the effect of poly-3-hydroxybutyrate synthase mutation (phbC) on the synthesis of exopolysaccharides (EPS) and glycogen, on the symbiotic properties and on the survival under specific conditions of Ensifer meliloti (formerly Sinorhizobium), a new stable phbC mutant of Ensifer meliloti 41 was isolated and characterized.Under poly-3-hydroxybutyrate accumulation conditions, the phbC -minus mutant (strain 41003) accumulates more glycogen and less exopolysaccharides as compared to the wild-type strain, and grows poorly in pyruvate as carbon source. The inactivation of aniA, encoding for a global carbon flux regulator, restores in E. meliloti 41003 the ability to grow on pyruvate, indicating a new role for this gene. Survival studies of E. meliloti 41 and 41003 under carbon free medium in both liquid and soil microcosms showed prolonged survival of E. meliloti 41 under these adverse conditions as compared to the mutant strain unable to accumulate the polyester. On the other hand, the accumulation of P(3HB) gave no significant advantage in survival under oxygen-limiting conditions. In both strains, E. meliloti 41 and 41003, nodule-inducing ability on alfalfa plants and acetylene reduction activity did not significantly differ from each other, although the mutant strain was less competitive in terms of root colonization.