Mechanism of Osmotic Stress Adaptation in Rhizobium meliloti

Rhizobium meliloti is able to survive osmotically stressful environments by the intracellular accumulation of compatible solutes, which restores the osmotic balance between the cell and the environment. R meliloti was found to accumulate the highly effective compatible solute glycine betaine. This osmolyte cannot be synthesized by R. meliloti. Betaine, or its precursor, choline, must be transported from the environment. (Smith et al. 1989). When betaine or choline was excluded from the growth medium, R. meliloti accumulated via de novo synthesis glutamate, trehalose and an unusual dipeptide N-acetylglutaminylglutamine amide (NAGGN) (Smith and Smith, 1989). While this dipeptide was not observed in other Rhizobia, it was found in several Pseudomonas species including P. aeruginosa, P. fluorescens, P. putida, P. mendocina, P. pseudoalcaligenes, and P. cepacia.