Title: <scp>FeS</scp> Cluster Assembly: <scp>NIF</scp> System in Nitrogen‐Fixing Bacteria
Abstract: Abstract Biological nitrogen fixation is restricted to a specialized group of microorganisms, called diazotrophs, and is catalyzed by the complex metalloenzyme nitrogenase. All diazotrophs recognized so far produce a molybdenum‐containing nitrogenase. However, some diazotrophs also produce paralogous nitrogenases that utilize a transition metal other than Mo as a constituent of the active site cofactor called FeMo‐cofactor. Early genetic studies established that proteins required to produce, activate, and sustain nitrogenase are not limited to the catalytic components but also include other proteins involved in the assembly of nitrogenase‐associated metal‐containing cofactors, protein folding, electron transfer, and regulation. The number of genes associated with nitrogen fixation varies among different species, and the involvement of accessory proteins depends on the metabolic and environmental conditions encountered during growth. To date, nearly all nitrogen‐fixing species having sequenced genomes contain a minimum of six genes dedicated to nitrogen fixation. These include genes encoding the nitrogenase catalytic components, nifH , nifD , nifK , and three genes, nifE , nifN , and nifB , whose products are required for FeMo‐cofactor formation. In some instances, genes associated with nitrogen fixation encode proteins that mirror other related cellular process. For example, the products of nifU and nifS are specifically involved in the assembly of [Fe–S] clusters dedicated to nitrogenase activation, whereas the structurally and functionally related housekeeping genes iscU and iscS are involved in [Fe–S] cluster formation that supports other Fe–S proteins. The best‐studied diazotrophic organism is the strict aerobic microbe Azotobacter vinelandii . This organism is capable of producing three genetically distinct, but mechanistically similar, nitrogenases: Mo‐, V‐, and Fe‐only‐dependent enzymes. Under conditions of diazotrophy, A . vinelandii specifically elevates the expression of more than 60 genes. Among those, NifU and NifS are known to affect the synthesis and accumulation of active nitrogenase catalytic components in all three systems. In this chapter, we describe the initial assembly of [Fe–S] clusters necessary for the formation of both simple and complex [Fe–S] clusters associated with nitrogen fixation. This minimal toolkit for [Fe–S] cluster assembly includes a cysteine desulfurase, encoded by nifS , and an [Fe–S] cluster assembly scaffold, encoded by nifU . Studies on the nitrogenase specific system have also indicated that electron transfer reactions are required for effective [Fe–S] cluster formation and that preformed [Fe–S] clusters are likely trafficked to client proteins by specific carrier proteins. All of these features have provided a framework for studies on the general process of [Fe–S] cluster assembly that appears to be conserved throughout nature.
Publication Year: 2017
Publication Date: 2017-09-15
Language: en
Type: other
Indexed In: ['crossref']
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Cited By Count: 1
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