Isolation and Physiological Characteristics of a Methylamine-resistant Mutant of the N2-fixing Cyanobacterium Nostoc muscorum

Abstract

Summary Cells of wild type N. muscorum (het+ nif+ met5) were highly sensitive to methylamine (1mM) and most of the metabolic activities like growth, nitrogenase, photosynthetic O2 evolution and activity of glutamine synthetase (GS) were inhibited in N-free and NH+4 supplemented conditions. Mutants were isolated following nitrosoguanidine treatment which were resistant to 10 mM methylamine under nitrogen-fixing conditions. In methylamine-resistant (metr) strain both heterocyst and nitrogenase activity were not repressed with ammonium supplementation. The level of GS activity in the strain was low but not affected by glutamate analogue L-methionine-DL-sulfoximine (MSX) and ammonical compounds. The photosynthetic O2 evolution in the mutant was not affected by methylamine toxicity which was instantaneously inhibited in the wild type with methylamine (1mM). The ammonium uptake in the wild-type N. muscorum exhibited biphasic pattern with two transport systems; the high-affinity (Km = 7.04μM) and “low-affinity” (Km = 285.7μM). However, as compared to wild type value for the “high affinity” transport system in the mutant was increased more than two fold (Km = 18.18μM). Methylamine supplementation to the cultures showed competitive inhibition of NH+4 transport in both the strains, although the rate of inhibition was faster in the wild type as compared to mef strain. These results suggest that methylamine-resistant phenotype in N. muscorum is perhaps due to change in the catalytic function of GS alongwith methylamine insensitive function of photosystem II.