Isolation and characterization of temperature-sensitive mutants ofAnabaena variabilis impaired in nitrogen fixation

Temperature-sensitive (ts) mutants of the cyanobacteriumAnabaena variabilis ATCC 29413 were isolated following mutagenesis with N-methyl-N′-nitro-N-nitrosoguanidine (MNNG) and post-treatment with metronidazole at 40°C. Of the 8000 clones isolated and tested, six mutants were conditionally lethal at the restrictive temperature (40°C). All the ts mutants exhibited differences in their rates of growth, chlorophyll content, pigment (phycocyanin and/or chlorophyll) ratios, heterocyst frequency, oxygen evolution and nitrogenase activity at the permissive temperature (28°C). A gradual loss of all the above features occurred after a period of 3 d at 40°C, followed by lysis of the cultures. Cessation of nitrogenase activity was found to be different in the different ts mutants. The temperature-sensitive nature of the mutants is suggested to be due to an impairment in iron metabolism since addition of ferric citrate to cultures at 40°C restored the ability to grow, produce heterocysts and fix nitrogen.