Temperature-sensitive photosynthesis-deficient mutants of the cyanobacterium Anabœna variabilis impaired in nitrogen assimilation.

Temperature-sensitive (Ts) mutants of Anabœna variabilis exhibited differences in the rates of oxygen evolution at 28 and 40°C. They were unable to perform photosynthesis and nitrogen fixation at 40°C beyond a period of 3 d in a nitrogen-deficient medium. However, the addition of combined nitrogen sources enhanced the growth of all the Ts mutants at both temperatures. Studies on nitrate uptake ty Ts mutants revealed the existence of a low affinity system, where K values were found to be lower than 1 mmol/L. The activity of nitrate reductase gradually increased with time at 28°C with the exception of Ts-161 where the activity decreased with time at 40°C. The rate of ammonia uptake by A. variabilis and its Ts mutants greatly differed and the results suggest the existence of a single phase of uptake. The activity of glutamate-ammonia ligase (GAL) of the parent and Ts mutants was slightly higher in cells from nitrogen-deficient medium when compared to nitrate grown cells at 28°C. At 40°C, the GAL activity decreased after 3 d. The inability of the Ts mutants to grow at 40°C appears to be due to an impairment in nitrogen asimilation. [ABSTRACT FROM AUTHOR]