Summary: Addition of glycine betaine (1 m) stimulated aerobic growth of Sp 7 in the presence of 0·3 m-NaCl. The nitrogenase activity of whole cells was particularly sensitive to salt stress, being almost totally inhibited in the presence of the same concentration of salt. Added glycine betaine strongly enhanced nitrogen fixation activity under salt stress. Under such conditions, maximal nitrogenase activity was obtained at a pO value (1 kPa) that inhibits nitrogen fixation activity in the absence of salt. We demonstrated the presence of a highaffinity transport system for glycine betaine, with an apparent of 10 μ. The osmolarity of the medium regulated the activity of the transport system. The maximal transport rates were 4 and 20 nmol min (mg protein) in cells grown in low-salt and high-salt medium, respectively. A high intracellular concentration of glycine betaine (480 μ) was observed only at a high osmolarity (0·3 -NaCl). Glycine betaine uptake was significantly reduced in osmotically shocked cells and a glycine betaine binding activity was detected in the crude periplasmic shock fluid. This suggests a transport mechanism involving a periplasmic glycine betaine binding protein. was unable to use the transported glycine betaine as a carbon- or nitrogen-source, in low- or high-salt medium. Intracellular glycine betaine was not catabolized.


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