Division of Biochemistry, Indian Agricultural Research Institute, New Delhi 110 012, India
* Corresponding author.
1Present address: Division of Biochemistry, Central Rice Research Institute, Cuttack 753 006, India
An oxygen sensitive mutant of Azorhizobium caulinodans strain IRBG 46 was isolated by NTG mutagenesis. It was defective in N2 fixation under 3% O2 level, while under 1% O2 it was almost as active as the parent strain IRBG 46. The mutant was also found to be a slow grower with reduced respiratory activity, low azide tolerance and no catalase activity. However, it did not differ from its parent strain with respect to nitrate respiration. Under symbiotic condition the mutant formed smaller, light green nodules as compared to bigger, dark green nodules formed by the wild type strain. The mutant was also defective in N2 fixation under symbiotic condition. Complementation analysis showed that the mutation might be in either fixL or f/xJ gene which are involved in O2 regulation of nif/fix gene expression. A possible role of all these factors in conferring a highly O2 tolerant nitrogen fixing system in the organism, has been discussed.
oxygen sensitive, Azorhizobium caulinodans, azide tolerance, respiration rate, oxygen regulation, nitrogen fixation
