Genes encoding nitrogenase

A preparation of the third nitrogenase from A. vinelandii, isolated from a molybdenum-tolerant strain but lacking the structural genes for the molybdenum and vanadium nitrogenases, was discovered to contain FeMoco 194). The 8 subunit encoded by anfG was identified in this preparation, which contained 24 Fe atoms and 1 Mo atom per mol. EPR spectroscopy and extraction of the cofactor identified it as FeMoco. The hybrid enzyme could reduce N2 to ammonia and reduced acetylene to ethylene and ethane. The rate of formation of ethane was nonlinear and the ethane ethylene ratio was strongly dependent on the ratio of nitrogenase components. 

Yet another enzyme able to release or transfer sulfur in the S° oxidation state is the PLP-dependent cysteine desulfurase that is encoded by the nifS gene of the nitrogenase gene cluster shown in Fig. 24-4. 

Also required for the vanadium and iron-only nitrogenases, which are encoded by vnf and anf genes, respectively. 

The a and (3 subunits of nitrogenase, encoded by the M. thermolithotrophicus niJD and niJK genes, are also predicted to contain amino-acid sequences conserved in the a and (3 subunits of the MoFe nitrogenases encoded by niJD and niJK genes in bacteria [180]. 

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. 

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gene exists between anfD and anfK (13a) and encodes a small subunit of the third nitrogenase (R. Eady and R. Pau, unpublished). 

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