Enhanced production of polyhydroxyalkanoates by mutation in exopolysaccharide biosynthetic genes

The formation of exopolysaccharides (EPS) by microorganisms is considered a disadvantage for commercial production of polyhydroxyalkanoates (PHA), not only for the partial loss of the carbon source, but also for the high viscosity of the resulting culture. In Azotobacter vinelandii it was shown that the synthesis of alginate and poly(3-hydroxybutyrate) (PHB) are connected, since they both play a role in the cell response to fluctuant growth conditions. Sinorhizobium meliloti is a nitrogen-fixing soil bacterium that can accumulate poly-3-hydroxybutyrate and poly-3(hydroxybutyrate-co-hydroxyvalerate), exopolysaccharides and glycogen. Mutants in phaC gene of S. meliloti showed a non-mucoid phenotype, resulting apparently in a reduction of succinoglycan production. Experiments were developed to verify the possibility to improve PHA accumulation in S. meliloti by reducing or hampering the synthesis of EPS. S. meliloti can grow on lactose, glucose and galactose with the same kinetics, and can also grow in lactose-containing waste material. It produces poly(3-hydroxybutyrate) during imbalanced growth on lactose, glucose and galactose, but also EPS under the same conditions. Analysis of polymer production in different mutants of EPS biosynthetic genes showed that PHA is enhanced in these strains. The reduced exopolysaccharide synthesis in PHA-negative mutants and the enhanced PHA accumulation in EPS mutants suggest that both biosynthetic pathways are interconnected and share common regulatory systems.