OSMOTICALLY INDUCED OLIGOSACCHARIDE AND POLYSACCHARIDE SYNTHESIS BY RHIZOBIUM-MELILOTI SU-47

In standard liquid medium containing 5 g mannitol l-1 and 1 g glutamic acid l-1, Rhizobiium meliloti SU-47 cells accumulated 350 mg cyclic, 1,2-beta-glucans (g protein)-1. The cyclic glucans were 36% glycerol-1-phosphate-substituted and 64% were uncharged. In the same medium with 10 g mannitol l-1, repeating units of succinoglycan (1110 mg l-1) were found as extracellular carbohydrates, and only low amounts of the succinoglycan polymer (up to 300 mg l-1) were excreted. By raising the osmotic pressure of the medium by the addition of NaCl or other ionic and non-ionic osmolytes, succinoglycan production could be stimulated: up to 2.4 g l-1 at 0.2 M-NaCl was produced at the expense of the repeating units. Above 0.2 M-NaCl growth was slowed down, and succinoglycan excretion diminished. At 1 M-NaCl growth stopped completely. In standard medium containing 0.6 M-NaCl the amount of cellular cyclic 1,2-beta-glucans was lowered to 150 mg (g protein)-1 out of which the glycerol-1-phosphate-substituted glucan fraction was reduced to 15%. Instead, high amount of oligosaccharides were synthesized as osmoprotectants, with trehalose as the major component [up to 200 mg (g protein)-1]. Glycogen synthesis was completely suppressed at this salt concentration, while poly beta-hydroxybutyric acid synthesis was unaffected.