Physiological Alterations and Regulation of Heterocyst and Nitrogenase Formation in Het super(-) Fix super(-) Mutant Strain of Anabaena variabilis

Physiological alterations and regulation of heterocyst and nitrogenase formation have been studied in Het super(-) Fix super(-) mutant strain of diazotrophic cyanobacterium Anabaena variabilis. Het super(-) Fix super(-) mutant strain of A. variabilis has been isolated by N-methyl-N'-nitro-N double prime -nitrosoguanidine (NTG) mutagenesis and was screened with the penicillin enrichment (500 mu g ml super(-1)). Growth, heterocyst differentiation, nitrogenase and glutamine synthetase (biosynthetic and transferase), super(14)CO sub(2)-fixation, nitrate reductase (NR), nitrite reductase (NiR), glucose-6-phosphate dehydrogenase (G6PDH), and isocitrate dehydrogenase (IDH) activities, and NO sub(3) super(-), NO sub(2) super(-), and NH sub(4) super(+) uptake and whole cell protein profile in different metabolic conditions were studied in the Het super(-) Fix super(-) mutant strain taking wild-type A. variabilis as reference. Het super(-) Fix super(-) mutant strain was incapable of assimilating elemental nitrogen (N sub(2)) due to its inability to form heterocysts and nitrogenase and this was the reason for its inability to grow in BG-11 sub(0) medium (free from combined nitrogen). In contrast, wild-type strain grew reasonably well in the absence of combined nitrogen sources and also showed heterocyst differentiation (8.5%) and nitrogenase activity (10.8 eta mol C sub(2)H sub(4) formed mu g super(-1) Chl a h super(-1)) in N sub(2)-medium. Wild-type strain also exhibited higher NR, NiR, and GS activities compared to its Het super(-) Fix super(-) mutant strain, which may presumably be due to acquisition of high uptake of NO sub(3) super(-), NO sub(2) super(-), and NH sub(2) super(+). Wild-type strain in contrast to its Het super(-) Fix super(-) mutant strain also exhibited high level of G6PDH, IDH, and super(14)CO sub(2) fixation activities. Low levels of G6PDH and IDH activities in Het super(-) Fix super(-) mutant strain further confirmed the lack of heterocyst differentiation and nitrogenase activity in the Het super(-) Fix super(-) mutant strain. NR, NiR, and GS activities in both the strains were energy-dependent and the energy required is mainly derived from photophosphorylation. Furthermore, it was found that de novo protein synthesis is necessarily required for the activities of NR, NiR, and GS in both wild-type and its Het super(-) Fix super(-) mutant strain.