NifS gene

In enteric bacteria, expression of the nif genes is under general nitrogen regulation by the ntr system. This system consists of essentially a sensor-activator pair of proteins, encoded by ntrB and ntr(d. When fixed nitrogen is limiting, the sensor, NtrB, phosphorylates (and so activates) the activator,... 

Fig. 7. The nitrogen fixation (nif) genes of Klebsiella pneumoniae Eire contiguous on ca. 23 kb of chromosomal DNA. The EU rowheads indicate transcription stEuts.

Zheng L, White RH, Cash VL, Dean DR. 1994. A mechanism for the desulfurization of L-cysteine catalyzed by the nifS gene prodnct. Biochemistry 33 4714-20. 

Figure 1. Nif genes required for nitrogen fixation as arranged in Klebsiella pneumoniae. The numbers in the top row are molecular weights in kilodaltons of the protein gene products of the respective nif genes whose letters are shown below them.

Figure 24-4 Sequence of nif genes of Klebsiella pneumoniae.56 These precede the his operon directly at the right side. The nitrogenase structural genes are marked with green.

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. 

This has been isolated from a range of nitrogen-fixing organisms, and appears to be identical in all cases except when an inactive form is produced by mutants deficient in certain of the nif genes. It differs from the molybdenum cofactor found in a number of molybdoenzymes (Section 62.1.9). 

Figure 1. Map of nif genes in Klebsiella pneumonia. The designation, (act), represents the molybdenum co-factor. Component II (llox) is reduced by the nif electron transfer system. IIrcd then binds and reduces lox(act), and the resulting complex with ATP can reduce nitrogen to ammonia.

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