Transport and catabolism of proline betaine in salt-stressed Rhizobium meliloti

Exogenous proline betaine (N,N-dimethylproline or stachydrine) highly stimulated the growth rate of Rhizobium meliloti, in media of inhibitory concentration of NaCl whereas proline was ineffective. High levels of proline betaine uptake occurred in cells grown in media of elevated osmotic strength; on the contrary, only low activity was found in cells grown in minimal medium. The apparent K_m was 10 M with a maximal transport rate of 25 nmol min^-1 mg^-1 of protein in 0.3 M NaCl-grown cells. The concentrative transport was totally abolished by KCN (2 mM), 2,4-dinitrophenol (2 mM), and carbonyl cyanide-m-chlorophenyl hydrazone (CCCP 10 M) but was insensitive to arsenate (5 mM). Glycine betaine was a very potent inhibitor of proline betaine uptake while proline was not. Proline betaine transport was not reduced in osmotically shocked cells and no proline betaine binding activity was detected in the crude periplasmic shock fluid. In the absence of salt stress, Rhizobium meliloti actively catabolized proline betaine but this catabolism was blocked by increasing the osmotic strength of the medium. The osmolarity in the growth medium regulates the use of proline betaine either as a carbon and nitrogen source or as an osmoprotectant.Abbreviations LAS lactate-aspartate-salts - MSY mannitol-salts-yeast - CCCP carbonyl cyanide-m-chlorophenyl hydrazone - DCCD dicyclohexylcarbodiimide - KCN potassium cyanide - Hepes 4-(2-hydroxyethyl)-1-piperzine-ethanesulphonic acid