SHORT COMMUNICATION - PHENOTYPIC-EXPRESSION OF AMMONIA-EXCRETING MUTANTS OF ANABAENA-7120 UNDER NITROGEN LIMITATION

Mutants of Anabaena 7120 defective in glutamine synthetase (GS) activi ty were isolated following transposon mutagenesis. Mutants M11, M55 an d M73 showed about 60% less GS activity in N-2-grown aerobic cultures than the wild-type strain and were resistant to the glutamate analogue L-methionine-DL-sulphoximine (MSX). These mutants had the capacity to excrete Na-fixed ammonia continuously into the culture medium and sho wed an enhanced level of aerobic nitrogenase activity. The intracellul ar ammonium pool generated in N-2-grown cells of mutants was found to be less than that of the wild-type strain. Similarly, ammonium uptake by these mutants was 50% less in mutants compared to the wild-type, su ggesting a possible role of GS in controlling this function.