ISOLATION AND CHARACTERIZATION OF AN OXYGEN SENSITIVE MUTANT OF AZORHIZOBIUM-CAULINODANS

An oxygen sensitive mutant of Azorhizobium caulinodans strain IRBG 46 was isolated by NTG mutagenesis. It was defective in NP fixation under 3% O-2 level, while under 1% O-2 if was almost as active as the paren t strain IRBG 46, The mutant was also found to be a slow grower with r educed respiratory activity, low azide tolerance and no catalase activ ity. However, it did not differ from its parent strain with respect to nitrate respiration. Under symbiotic condition the mutant formed smal ler, light green nodules as compared to bigger, dark green nodules for med by the wild type strain. The mutant was also defective in Np fixat ion under symbiotic condition, Complementation analysis showed that th e mutation might be in either fixL or fixJ gene which are involved in O-2 regulation of nif/fix gene expression. A possible role of all thes e factors in conferring a highly O-2 tolerant nitrogen fixing system i n the organism, has been discussed.