Genetics of nitrogen fixation : (nitrogenase, molybdenum, ammonium excretion, legumes)
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Recent advances in understanding the genetics of nitrogen fixation have provided ideas for novel application of nitrogenase and nitrogen-fixing organisms. Mutant strains have been useful for identifying the active site of nitrogenase and for identifying other factors, besides nitrogenase, that are specifically required for an organism to fix nitrogen. These strains have been important tools for assaying such factors during their purification. A fine-structure map of Klebsiella pneumoniae was obtained by deletion mapping of many nif (nitrogen fixation) mutations. Transformation between Nif[superscript minus sign] mutant strains of Azoto bactervinelandii has shown that the nif genes are scattered around the chromosome, unlike the situation in K. pneumoniae in which nif genes are clustered. Regulatory mutations have been useful for constructing derepressed ammonium-excreting strains in A. vinelandii. Such strains can fertilize the roots of cereal plants. The regulation of nitrogen fixation seems to be quite complex since ammonium, oxygen, and molybdenum all play a role in nitrogenase synthesis in K. pneumoniae. The Rhizobium-legume symbiosis has been studied with mutant strains of Rhizobium that are unable to form root nodules and mutant strains which form root nodules unable to fix nitrogen. Several strains that do not infect the host plant lack a surface polysaccharide that is present in wild-type cells. Some mutant strains of Rhizobium cause leghemoglobin deficient nodules to be formed. Another mutant phenotype causes the plant to fix more nitrogen than the wild type. Such strains might have potential use in agriculture.