Factors That Control the Rate of Exopolysaccharide Production by Agrobacterium radiobacter NCIB 11883

SUMMARY: Under carbon limitation the efficiency of aerobic energy conservation of Agrobacterium radiobacter was independent of the carbon source. Under nitrogen limitation the chemical composition of the exopolysaccharide produced was independent of carbon source; however, the rate of production was highest on glucose and decreased on other substrates in the order glucose, succinate, gluconate, xylose, sorbitol, glycerol and ethanol. When the observed yields of exopolysaccharide from glucose, gluconate and xylose were corrected for cell production they came very close to the theoretical values; however, the yields of exopolysaccharide decreased sharply when the carbon source was either more reduced or oxidized than glucose. Although the rate of exopolysaccharide production on glucose was greater than that on gluconate or xylose, the rate of ATP utilization for polymer synthesis was similar and accounted for most of the respiratory activity occurring in excess of that required for cell biosynthesis. The proportion of respiratory activity that is dissociated from polymer synthesis increased sharply with carbon sources that are significantly more reduced or oxidized than glucose. A mutant of A. radiobacter that was unable to synthesize exopolysaccharide had the same efficiency of growth as the parent strain. Under nitrogen-limited growth the respiratory activity in excess of growth requirements was similar to the polymer-producing strain, supporting the view that the rate of ATP turnover was similar in both strains. This suggests that the rate of ATP turnover rather than exopolysaccharide synthesis is physiologically important. It is proposed that the rate of ATP turnover in excess of that required for growth under nitrogen limitation (N-limited qo 2 × ATP/O quotient) is controlled to a level that is governed by the oxidation/reduction state and the Y ATP of the given substrate.