Isolation and partial characterization of NaCl-tolerant mutant strain of Anabaena variabilis with impaired glutamine synthetase activity

NaCl-tolerant mutant of diazotrophic cyanobacterium Anabaena variabilis has been isolated by N-methyl-N-nitro-N-nitrosoguanidine (NTG)(1)) mutagenesis and selection for NaCl resistance. The effect of NaCl (300 mM) on growth, heterocyst differentiation, nitrogenase, GS, NR, NO3- and NO2- uptake was s tudied by maintaining wild-type A. variabilis as reference. The NaCl-tolera nt mutant grew reasonably well both in presence and absence of NaCl. It als o produced more heterocysts and exhibited greater nitrogenase than its wild -type counterpart in N-2-medium without NaCl. Exposure of wild-type to NaCI (300 mM) for 7 days resulted a significant inhibition of growth and hetero cyst differentiation with reduced nitrogenase activity. These activities we re, however, significantly stimulated by NaCl in NaCl-tolerant mutant. GS a ctivity in NaCl-tolerant mutant was reduced to about 50% causing liberation of ammonia in the external medium. No ammonia was however, liberated by wi ld-type having normal GS activity, suggesting a metabolic linkage between G S (ammonia assimilation or release) and nitrogenase in NaCl-tolerant mutant . GS activity in wild-type was inhibited by NaCl, however, no such inhibiti on by NaCl was recorded in GS activity of NaCl-tolerant mutant. Untreated c ells of NaCl-tolerant mutant showed greater levels of NO3- and NO2- uptake as compared to its wild-type, which were further increased after NaCl-treat ment. In contrast, NaCl-treatment to wildtype cells resulted in stimulation of NO3- and NO2- uptake systems, however, it also led to almost complete i nhibition of NR activity. This suggests that although wild-type take up NO3 - and NO2- more rapidly in response to NaCl stress, it could not metabolize these due to NaCl-caused inhibition in NR and GS systems. Thus, NaCl-cause d impairment in nitrogen metabolism may be one of the reasons for reduced g rowth of wild-type under NaCl stress.