NifK gene

The biosynthesis of the MoFe protein is extremely complex. Here we will first describe the biosynthesis of FeMoco, then that of the apo-MoFe protein, encoded by the nifD and nifK genes and containing the P clusters, and finally we will summarize what is known about the combination of FeMoco with the apo-MoFe protein to form active MoFe protein. 

The MoFe protein is an aifli tetramer of Mr 220 kDa, and its a and subunits are encoded by the nifD and nifK gene, respectively (Figure 1(a) and Table 1). It contains, in preparations with the highest activity, 2 molybdenum (Mo), 30 to 34 iron (Fe), and an approximately equivalent number of acid-labile sulfur (S ) atoms (Table 1). This metal content is consistent with the presence of two different types of unique metal clusters in the protein, that is, the [8Fe-7S] cluster (P cluster), which is bridged between each a/3 subunit pair, and the [Mo-7Fe-9S-homocitrate] cluster (FeMo cofactor or FeMoco), which is located within each a subunit (Figure la). ... 

The MoFe-protein is an a2 2 tetramer (with the subunits coded by the nifD and nifK genes, respectively), with a total molecular weight of 240,000. The two subunits are of similar size for example, the isolated a and /3 subunits of A. vinelandii MoFe-protein have 491 and 522 amino acids, respectively (46). In general, the amino acid sequences of MoFe-proteins are less well conserved than are Fe-protein sequences, so that the MoFe-protein sequences from A. vinelandii and C. pasteuria-num are only 36% identical (47). Associated with the MoFe-protein tetramer are approximately 2 molybdenum atoms, 30 iron atoms, and 30 sulfur atoms that are organized into two types of metal centers the FeMo-cofactor and the P-cluster pair. The structures and properties of these centers have been extensively probed by a wide variety of techniques. 

Fani, R., GaUo, R., and Lio, P. (2000). Molecular evolution of nitrogen fixation The evolutionary history of the nifD, nifK, nifE, and nifN genes. J. Mol. Evol. 51, 1—11. 

Table 1). The exact stoichiometry between the a, p, and 8 subunits has not been established in this case, and the suboptimal subunit composition was speculated to be a result of the weak interactions between the subunits. Other than the subunit composition, the VFe proteins from A. vinelandii And A. chroococcum have similar properties, which is consistent with the sequence homology of their structural genes. The a and p subunits of the VFe protein, encoded by vnfD and vnfKgenes, respectively, are homologous to the /j/TD-encoded a and nifK-encoded p subunits of the MoFe protein whereas the additional small 8 subunit of the VFe protein is encoded by vnfG (Table 3).5,8,16,31-36 ype protein... 

Although muddled by the unreliability of SDS electrophoresis, the evidence, as outlined earlier, for an 02 subunit structure is substantial. It is strengthened by genetic studies with K. pneumoniae. Nif K and nif D are structural genes for the a and )8 subunits of Kpl (Roberts et al., 1978 Merrick eta/., 1978). Although both subunits are absent in many nifK andnifD mutants, one nif K strain showed only the j3 subunit and another showed a change in the electrophoretic mobility of the a subunit. Nine nif D strains... 

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