The Genetics of Nitrogen Fixation

A brief outline of the genetics of nitrogen fixation, relevant to the structure of the three types of nitrogenases and the types of redox centers they contain, is considered below. 

The component proteins of Mo nitrogenase are MoFe protein, an 2)32 tetramer encoded by the nifDK genes, and an Fe protein, a y2 dimer encoded by nifH. The formation of an active enzyme requires, in addition to these structural genes, the functions of several other nif genes. The nif M in some unknown way activates the Fe protein polypeptide and is essential for its function. A number of nif genes are involved in FeMoco biosynthesis (nifHBENVQ) and are therefore essential for the synthesis of an active MoFe protein. 

Mutant strains of A. chroococcum carrying deletions of both Mo and V nitrogenase structural genes are unable to grow on N2 ill). In contrast, comparable strains of A. vinelandii can grow provided neither Mo nor V is added to the growth medium (12). This ability is due to the presence of the third nitrogenase in A. vinelandii. 

The presence of an open reading frame between vnfD and vnfK led to the identification of an additional small subunit type (8, encoded by vnfG) of the VFe protein it has the hexameric structure 012 2 2 ill). A homologous gene exists between anfD and anfK (13a) and encodes a small subunit of the third nitrogenase (R. Eady and R. Pau, unpublished). 

These genetic data support the suggestion of a parallel route for the synthesis of cofactors of Mo-independent nitrogenase function involving some early steps in common with FeMoco biosynthesis. 

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Ludden, P.W. and G.P. Roberts. The biochemistry and genetics of nitrogen fixation by photosynthetic bacteria. In Anoxygenic Photosynthetic Bacteria, Kluwer Academic Publishers, Dordrecht, The Netherlands,... 

The Unit of Nitrogen Fixation at the University of Sussex became the foremost laboratory in the world for the study of many aspects of the biological nitrogen fixation problem. Perhaps the areas most successfully exploited were the chemical see Section E) and the genetic. However, the biological work was considerably broader than the genetical, and it says much for both Chatt and for his Deputy Director and later Director of the Unit) that such a diverse research programme could be successfully carried out with such success. 

A large part of the supply of nitrogen already comes from biological fixation of nitrogen from air and soil. Extensive research has been carried out for more than 20 years [5] to develop genetic varieties of basic crops that can fix nitrogen directly from the air, The successes are limited to a few species however, even in the case of success in this field, the testing of the environmental impacts and reproduction of the new species and their distribution worldwide may take considerable time. 

C. Kennedy and P. E. Bishop, In W. Klipp, B. Masepohl, J. R. Gallon, and W. E. Newton, eds.. Genetics and Regulation of Nitrogen Fixation in Free-Living Bacteria. Kluwer/Springer, Dordrecht, the Netherlands, 2004, pp. 27—52. ... 

Jones, R. Haselkorn, R. The DNA sequence of the Rhodobacter capsulatus ntrA, ntrB and ntrC gene analogues required for nitrogen fixation. Mol. Gen. Genet., 215, 507-516 (1989)... 

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