The effects of NaCl, KCl and the organic osmotica sucrose or glycine betaine on glutamine synthetase (GS) transferase activity in crude extracts of the marine and the halotolerant sp. DUN52, respectively, were examined. NaCl at low concentrations (≤ 0.75 ) stimulated the enzyme (up to 30%) in the halotolerant strain but was inhibitory above 0.2 in the marine strain. In both strains KCl (0.1-1.3 ) stimulated GS activity and was inhibitory only above 1.4 . Sucrose, the major intracellular organic osmoticum in , had only a slight inhibitory effect until concentrations above 0.3 ; 2.0 sucrose caused a 60% inhibition. The quaternary ammonium compound glycine betaine, the primary organic osmoticum in sp. DUN52, was not inhibitory at physiologically relevant concentrations up to 1.8-2.0 and reduced NaCl inhibition by 10-30% at NaCl concentrations from 0.8 to 2.0 in the halotolerant strain when used at 1.0 . At NaCl concentrations above 0.2 , 0.2 KCl reduced the activity of GS by 6-15%. Sucrose and KCl showed similar degrees of protection against NaCl inhibition in when used at concentrations similar to those measured in seawater-grown cells of Increasing concentrations of glycine betaine increased protection of GS activity against NaCl inhibition at NaCl concentrations above 0.5 . These results suggest that the extent of salt tolerance of a particular strain of cyanobacterium may depend, in part at least, on the metabolic effects of compounds that are accumulated as internal osmotica in that strain.


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