title: Isolation and characterization of nitrogenase-derepressed mutant strains of cyanobacterium Anabaena variabilis.
abstract: A positive selection method for isolation of nitrogenase-derepressed mutant strains of a filamentous cyanobacterium, Anabaena variabilis, is described. Mutant strains that are resistant to a glutamate analog, L-methionine-D,L-sulfoximine, were screened for their ability to produce and excrete NH4+ into medium. Mutant strains capable of producing nitrogenase in the presence of NH4+ were selected from a population of NH4+-excreting mutants. One of the mutant strains (SA-1) studied in detail was found to be a conditional glutamine auxotroph requiring glutamine for growth in media containing N2, NO3-, or low concentrations of NH4+ (less than 0.5 mM). This glutamine requirement is a consequence of a block in the assimilation of NH4+ produced by an enzyme system like nitrogenase. Glutamate and aspartate failed to substitute for glutamine because of a defect in the transport and utilization of these amino acids. Strain SA-1 assimilated NH4+ when the concentration in the medium reached about 0.5 mM, and under these conditions the growth rate was similar to that of the parent. Mutant strain SA-1 produced L-methionine-D,L-sulfoximine-resistant glutamine synthetase activity. Kinetic properties of the enzyme from the parent and mutant were similar. Mutant strain SA-1 can potentially serve as a source of fertilizer nitrogen to support growth of crop plants, since the NH4+ produced by nitrogenase, utilizing sunlight and water as sources of energy and reductant, respectively, is excreted into the environment.
authors.forename: H
authors.initials: H
authors.lastname: Spiller
authors.forename: C
authors.initials: C
authors.lastname: Latorre
authors.forename: M E
authors.initials: ME
authors.lastname: Hassan
authors.forename: K T
authors.initials: KT
authors.lastname: Shanmugam
mesh_terms: D000641:Ammonia / Q000494:pharmacology; D000458:Cyanobacteria / Q000235:genetics* / Q000254:growth & development; D005786:Gene Expression Regulation; D005974:Glutamate-Ammonia Ligase / Q000378:metabolism; D007700:Kinetics; D009154:Mutation; D009566:Nitrates / Q000378:metabolism; D009584:Nitrogen / Q000378:metabolism; D009591:Nitrogenase / Q000235:genetics*
chemical_list: D009566:Nitrates; D000641:Ammonia; D009591:Nitrogenase; D005974:Glutamate-Ammonia Ligase; D009584:Nitrogen