Elucidation of the role of osmoprotective compounds and osmoregulatory genes: The key role of bacteria

A series of compounds, including proline and several betaines (glycine betaine, proline betaine), known to accumulate in plants during osmotic stress, have been found to function as osmoprotective compounds for bacteria such as enteric bacteria and Rhizobium meliloti. These molecules can be accumulated in massive amounts in the bacterial cytoplasm without precluding normal cellular functions. Accumulation of glycine betaine, one of the most efficient osmoprotec-tants, contributes to maintaining the proper osmotic strength of the cytoplasm, and thus prevents osmotic dehydration. For the accumulation of glycine betaine, the bacteria need an external supply of this compound or its precursor choline. Uptake systems for both molecules have been characterized. Escherichia coli has both a high-affinity and a low-affinity transport system for choline, and glycine betaine uptake is also mediated by two transport systems (low-affinity and high-affinity). Within the cells, choline never accumulates but is converted into glycine betaine by two dehydrogenases. In E. coli glycine betaine serves only in osmoregulation and cannot be catabolized, whereas in R. meliloti glycine betaine can function as a carbon, nitrogen and energy source, as well as an osmoprotectant. When choline and glycine betaine are not available, the bacteria can also achieve a moderate level of osmotic tolerance by endogenous synthesis of other compatible solutes, such as glutamate and the non-reducing disaccharide trehalose.