Genetical studies on the heterocyst and nitrogen fixation of the blue-green alga Nostoc muscorum

Summary The short-trichome-forming, non-heterocystous and non-nitrogen-fixing (het^– nif^–) mutant of the nitrogen-fixing blue-green alga Nostoc muscorum was isolated by N-methyl-N-nitro-N-nitrosoguanidine (NTG)-mutagenesis after penicillin enrichment technique and characterized. The mutant did not grow and fix nitrogen in combined-nitrogen-free medium while in nitrate-containing medium it grew well (K=0.112/day, G=64.27 h), although its growth was comparatively poor than the parent alga (K=0.128/day; G=56.14 h). The mutant was stable and both the het^– and nif^– characters reverted to wild type (het⁺ nif⁺) with the reversion frequency of 2.62×10^-7.The het^– nif^– mutant tolerated 0.5 g/ml of streptomycin sulphate on the agar medium and its streptomycin resistant mutant capable of growing in presence of 10g/ml of streptomycin was isolated spontaneously with a frequency of 1.45×10^-8. These streptomycin resistant isolates (het^– nif^– str^R) resisted 100 g/ml of streptomycin sulphate on the agar medium and 200 g/ml in liquid medium. Spontaneous virus-resistant mutant of het^– nif^– str^R was isolated with a mutation frequency of 4.02×10^-4.The data of genetic recombination experiments suggested that there is transfer of both het and nif genes to het^– nif^– strain with the frequency of 2×10^-6 to 2×10^-5 simultaneously. There was increase in recombination frequency with increasing the incubation period. The virus-resistance marker is also transferred to the sensitive recipient.Abbreviations CFU colony forming units - C–N Chu-10 medium without combined nitrogen - C+N Chu-10 medium with 0.232 g/l calcium nitrate - G generation time - het heterocyst differentiating genes - K specific growth rate constant - MOI multiplicity of infection - nif nitrogen-fixing genes - NTG N-methyl-N-nitro-N-nitrosoguanidine - PFU plaque forming units - str^R streptomycin resistance - str^R streptomycin sensitive