Iron-Sulfur Cluster Assembly

Abstract

The NifU protein is a homodimer that is proposed to provide a molecular scaffold for the assembly of [Fe-S] clusters uniquely destined for the maturation of the nitrogenase catalytic components. There are three domains contained within NifU, with the N-terminal domain exhibiting a high degree of primary sequence similarity to a related family of [Fe-S] cluster biosynthetic scaffolds designated IscU. The C-terminal domain of NifU exhibits sequence similarity to a second family of proposed [Fe-S] cluster biosynthetic scaffolds designated Nfu. Genetic experiments described here involving amino acid substitutions within the N-terminal and C-terminal domains of NifU indicate that both domains can separately participate in nitrogenase-specific [Fe-S] cluster formation, although the N-terminal domain appears to have the dominant function. These in vivo experiments were supported by in vitro [Fe-S] cluster assembly and transfer experiments involving the activation of an apo-form of the nitrogenase Fe protein. The NifU protein is a homodimer that is proposed to provide a molecular scaffold for the assembly of [Fe-S] clusters uniquely destined for the maturation of the nitrogenase catalytic components. There are three domains contained within NifU, with the N-terminal domain exhibiting a high degree of primary sequence similarity to a related family of [Fe-S] cluster biosynthetic scaffolds designated IscU. The C-terminal domain of NifU exhibits sequence similarity to a second family of proposed [Fe-S] cluster biosynthetic scaffolds designated Nfu. Genetic experiments described here involving amino acid substitutions within the N-terminal and C-terminal domains of NifU indicate that both domains can separately participate in nitrogenase-specific [Fe-S] cluster formation, although the N-terminal domain appears to have the dominant function. These in vivo experiments were supported by in vitro [Fe-S] cluster assembly and transfer experiments involving the activation of an apo-form of the nitrogenase Fe protein. The Azotobacter vinelandii NifU protein is an ∼60-kDa homodimer proposed to provide a molecular scaffold for formation of [Fe-S] clusters or Fe-S cluster precursors required for full activation of the nitrogenase catalytic components (1Fu W. Jack R.F. Morgan T.V. Dean D.R. Johnson M.K. Biochemistry. 1994; 33: 13455-13463Crossref PubMed Scopus (129) Google Scholar, 2Yuvaniyama P. Agar J.N. Cash V.L. Johnson M.K. Dean D.R. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 599-604Crossref PubMed Scopus (277) Google Scholar). Primary sequence comparisons among NifU homologs indicate that it is a modular protein having three distinct domains (3Agar J.N. Yuvaniyama P. Jack R.F. Cash V.L. Smith A.D. Dean D.R. Johnson M.K. J. Biol. Inorg. Chem. 2000; 5: 167-177Crossref PubMed Scopus (107) Google Scholar, 4Hwang D.M. Dempsey A. Tan K.T. Liew C.C. J. Mol. Evol. 1996; 43: 536-540Crossref PubMed Scopus (52) Google Scholar). The N-terminal domain includes three cysteine residues conserved among all known NifU and NifU-like proteins. Incubation of the recombinantly expressed NifU N-terminal domain with NifS (a cysteine desulfurase), l-cysteine, and Fe2+ results in formation of labile [2Fe-2S] clusters on the NifU fragment, providing evidence that this domain could provide a scaffold for assembly of “transient” [Fe-S] clusters destined for nitrogenase activation (2Yuvaniyama P. Agar J.N. Cash V.L. Johnson M.K. Dean D.R. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 599-604Crossref PubMed Scopus (277) Google Scholar). In support of this hypothesis biochemical and genetic studies established that individual substitution of any of the three cysteine residues contained in the N-terminal domain of NifU impaired but did not eliminate the physiological maturation of the nitrogenase component proteins (3Agar J.N. Yuvaniyama P. Jack R.F. Cash V.L. Smith A.D. Dean D.R. Johnson M.K. J. Biol. Inorg. Chem. 2000; 5: 167-177Crossref PubMed Scopus (107) Google Scholar). It was subsequently shown that the N-terminal NifU domain is highly homologous to a small protein designated IscU (5Zheng L. Cash V.L. Flint D.H. Dean D.R. J. Biol. Chem. 1998; 273: 13264-13272Abstract Full Text Full Text PDF PubMed Scopus (575) Google Scholar). IscU also contains three conserved cysteines and can provide a scaffold for the sequential in vitro assembly of [2Fe-2S] and [4Fe-4S] clusters when incubated with IscS (a NifS cysteine desulfurase homolog), l-cysteine, and Fe2+ (6Agar J.N. Krebs C. Frazzon J. Huynh B.H. Dean D.R. Johnson M.K. Biochemistry. 2000; 39: 7856-7862Crossref PubMed Scopus (386) Google Scholar, 7Agar J.N. Zheng L. Cash V.L. Dean D.R. Johnson M.K. J. Am. Chem. Soc. 2000; 122: 2136-2137Crossref Scopus (116) Google Scholar). The IscS and IscU proteins together with a suite of other proteins, IscA, HscB, HscA, and Fdx, whose corresponding genes are clustered on both the A. vinelandii and Escherichia coli genomes, have been shown by genetic experiments using E. coli to be involved in the maturation of a variety of Fe-S cluster-containing proteins involved in intermediary metabolism, such as aconitase and glutamate synthase (8Schwartz C.J. Djaman O. Imlay J.A. Kiley P.J. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 9009-9014Crossref PubMed Scopus (258) Google Scholar, 9Tokumoto U. Takahashi Y. J. Biochem. (Tokyo). 2001; 130: 63-71Crossref PubMed Scopus (215) Google Scholar, 10Nakamura M. Saeki K. Takahashi Y. J. Biochem. (Tokyo). 1999; 126: 10-18Crossref PubMed Scopus (168) Google Scholar, 11Takahashi Y. Nakamura M. J. Biochem. (Tokyo). 1999; 126: 917-926Crossref PubMed Scopus (227) Google Scholar). In the case of A. vinelandii, the iscSUAhscBAfdx gene cluster cannot be deleted because this gene cluster is essential under growth conditions for which the nif-specific genes are either expressed or not expressed (5Zheng L. Cash V.L. Flint D.H. Dean D.R. J. Biol. Chem. 1998; 273: 13264-13272Abstract Full Text Full Text PDF PubMed Scopus (575) Google Scholar). Thus, under nitrogen fixing conditions NifU is apparently unable to effectively substitute for IscU function. Similarly, NifU deletion strains are severely impaired in their capacity for diazotrophic growth, indicating that IscU cannot effectively replace the function of NifU (12Jacobson M.R. Cash V.L. Weiss M.C. Laird N.F. Newton W.E. Dean D.R. Mol. Gen. Genet. 1989; 219: 49-57Crossref PubMed Scopus (247) Google Scholar). In contrast, the iscSUAhscBAfdx genes are not essential in E. coli because there is some redundant function provided by another [Fe-S] cluster assembly apparatus encoded by sufABCDSE. Elegant genetic studies have demonstrated that the E. coli iscSUAhscBAfdx genes are essential in the absence of sufABCDS and vice versa (13Takahashi Y. Tokumoto U. J. Biol. Chem. 2002; 277: 28380-28383Abstract Full Text Full Text PDF PubMed Scopus (352) Google Scholar). Although the A. vinelandii genome encodes proteins homologous to shown to a cysteine desulfurase S. L. E. M. PubMed Scopus Google Scholar, L. S. L. M. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar, J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google it not homologs to which the gene cluster is essential in this The domain of NifU contains conserved cysteines that have been shown to cluster NifU (3Agar J.N. Yuvaniyama P. Jack R.F. Cash V.L. Smith A.D. Dean D.R. Johnson M.K. J. Biol. Inorg. Chem. 2000; 5: 167-177Crossref PubMed Scopus (107) Google Scholar). These clusters have been designated clusters because not to be precursors destined for nitrogenase it been proposed that clusters could have a function or some other related to the physiological formation or of clusters on the N-terminal domain (2Yuvaniyama P. Agar J.N. Cash V.L. Johnson M.K. Dean D.R. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 599-604Crossref PubMed Scopus (277) Google Scholar, J.N. Yuvaniyama P. Jack R.F. Cash V.L. Smith A.D. Dean D.R. Johnson M.K. J. Biol. Inorg. Chem. 2000; 5: 167-177Crossref PubMed Scopus (107) Google Scholar). In with this hypothesis substitution of any of cysteine residues also results in a capacity of A. vinelandii for diazotrophic The C-terminal domain of NifU contains conserved cysteine and studies demonstrated that could be by with on the physiological maturation of nitrogenase (3Agar J.N. Yuvaniyama P. Jack R.F. Cash V.L. Smith A.D. Dean D.R. Johnson M.K. J. Biol. Inorg. Chem. 2000; 5: 167-177Crossref PubMed Scopus (107) Google Scholar). small proteins exhibiting sequence similarity to the C-terminal domain of NifU, of the cysteine have been shown of providing in vitro for the assembly of labile [2Fe-2S] or [4Fe-4S] clusters Huynh B.H. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar, K. M. J. Biol. Chem. 2000; Full Text Full Text PDF PubMed Scopus Google Scholar, S. C. S. Biochem. J. PubMed Scopus Google Scholar, C. E. Proc. Natl. Acad. Sci. U. S. A. 1999; PubMed Scopus Google Scholar). These results that the C-terminal NifU domain provide a second cluster assembly and could strains deleted for are severely impaired in their capacity for diazotrophic growth, strains for cysteines contained in the N-terminal of diazotrophic In the biochemical and genetic experiments were to the of the N-terminal and C-terminal domains of NifU in nitrogenase this the capacity for diazotrophic growth of strains having of substitutions for conserved cysteines within the N-terminal and C-terminal domains was an for in vitro activation of an of the nitrogenase Fe protein was and the capacity for in vitro activation of protein using of NifU having of substitutions for conserved cysteines within the N-terminal and C-terminal domains was also and that A. vinelandii and genes M.R. Cash V.L. J. Newton W.E. Dean D.R. J. 1989; PubMed Google were to for in E. coli or for of expressed NifU or was using a and were by sequence by the of and an was the C. PubMed Scopus Google and with and to of and genes under the of the and In to and contains which was by and to and by were for the of and for biochemical L. Cash V.L. Jack R.F. Dean D.R. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google was for of in the absence of of a for which were and was by a the of the of A. vinelandii the were in in the genome and the as described in (12Jacobson M.R. Cash V.L. Weiss M.C. Laird N.F. Newton W.E. Dean D.R. Mol. Gen. Genet. 1989; 219: 49-57Crossref PubMed Scopus (247) Google Scholar, M.R. Cash V.L. J. Newton W.E. Dean D.R. J. 1989; PubMed Google Scholar). in this are not of in A. vinelandii, and are in A. vinelandii were in a the of a nitrogen J. PubMed Scopus Google in this in a E. coli was as the for of of and or of of and The for were as described the individual of a described L. Cash V.L. Jack R.F. Dean D.R. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google was that the of under of the experiments was the and E. coli was as the the were in of with of in a of genes was by the of of or of on the were for and by and the were NifU and NifS protein were under conditions using either or a with nitrogen were in of for of and by by for The was to a with and The was with of the and a of was NifU or NifU and NifS were and in nitrogen NifS was as described L. Cash V.L. Jack R.F. Dean D.R. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar). NifU or NifS were using the J. Chem. PubMed Scopus Google the of NifU and NifS in was by PubMed Scopus Google using highly NifU and NifS as [Fe-S] [Fe-S] cluster assembly a that contained NifU, l-cysteine, and experiments described in the and in NifU, or was the assembly or an of NifS having the by was also described in the and in involved using of NifU having or cysteine residues by assembly were incubated on for and to a and with the was with of the which were using in and to the NifU and NifS were as described and was as described by and Chem. Scopus Google Scholar). a of [Fe-S] NifU that was NifS the described were for the of a NifS that a the of and for this was using of the assembly and as described the was to a with conditions [Fe-S] NifU the NifS to the of the nitrogenase in assembly this the NifS in the assembly an substitution for the cysteine cysteine desulfurase this the NifS in the assembly a the The NifS was the assembly using a activation of the protein as described under the of activation other activation experiments were this the NifS in the assembly an substitution for the cysteine cysteine desulfurase this the NifS in the assembly a the The NifS was the assembly using a activation of the protein as described under the of activation other activation experiments were in a A. vinelandii were using by for as described J. P.J. Dean D.R. Biochemistry. 1998; PubMed Scopus Google Scholar). The was to a with and The was with of the The Fe protein was with a with the Fe protein was under an to using an the [4Fe-4S] cluster the Fe protein of and was and the was incubated on for in a in the to of this were on a with and with the The protein was of the protein activation contained NifU, and in a experiments the of NifU and NifS were as shown in NifU and NifS were and Fe protein was did not the of indicating that [Fe-S] cluster transfer NifU to protein is Fe protein was by the nitrogenase described K. Dean D.R. J. Biol. Chem. 1994; Full Text PDF PubMed Google Scholar). were under an in contained of activation corresponding to of Fe protein and of protein in a of experiments were to that under conditions was with to Fe protein were incubated for in a and the was by the of of was using a with a molecular and a conditions Fe protein in this a of of of Fe protein. of activation of protein were as a of this the conditions described here the protein is unable to support any of the N-terminal and C-terminal of it was shown that deletion of the A. vinelandii gene diazotrophic growth capacity (12Jacobson M.R. Cash V.L. Weiss M.C. Laird N.F. Newton W.E. Dean D.R. Mol. Gen. Genet. 1989; 219: 49-57Crossref PubMed Scopus (247) Google Scholar). In contrast, substitution of for any of the three N-terminal cysteines proposed to be involved in providing for [Fe-S] cluster assembly diazotrophic capacity but not as severely as for the deletion (3Agar J.N. Yuvaniyama P. Jack R.F. Cash V.L. Smith A.D. Dean D.R. Johnson M.K. J. Biol. Inorg. Chem. 2000; 5: 167-177Crossref PubMed Scopus (107) Google Scholar). These were also in the There are three for of cysteine within the proposed N-terminal assembly scaffold not eliminate [Fe-S] cluster assembly within the N-terminal a second [Fe-S] cluster assembly is within NifU, or NifU some other function involving nitrogenase These were by the on diazotrophic growth capacity as a of substitution of or all three cysteines within the N-terminal The of for cysteine substitutions are shown in and the of substitutions on diazotrophic growth is shown in These and results that the diazotrophic growth results as a of substitution for any or all three of the N-terminal indicating that cluster assembly for nitrogenase maturation not a N-terminal NifU studies that a of small proteins, designated primary sequence when with the C-terminal domain of NifU, of the cysteines within this and can be as in vitro scaffolds for the assembly of [2Fe-2S] or [4Fe-4S] clusters Huynh B.H. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar, K. M. J. Biol. Chem. 2000; Full Text Full Text PDF PubMed Scopus Google Scholar). also shown that substitution of either of cysteines within NifU on the capacity for diazotrophic growth for the strains (3Agar J.N. Yuvaniyama P. Jack R.F. Cash V.L. Smith A.D. Dean D.R. Johnson M.K. J. Biol. Inorg. Chem. 2000; 5: 167-177Crossref PubMed Scopus (107) Google Scholar). In the a was all three cysteines within the N-terminal domain cysteine within the C-terminal domain is by a capacity for diazotrophic growth either the or strains having of substitutions within the N-terminal Although a having substitutions within both the N-terminal and C-terminal domains is severely impaired in diazotrophic growth it is to a a that a deletion in These results are the as for a (12Jacobson M.R. Cash V.L. Weiss M.C. Laird N.F. Newton W.E. Dean D.R. Mol. Gen. Genet. 1989; 219: 49-57Crossref PubMed Scopus (247) Google which a small deletion a of that encodes of the N-terminal and In the the having a deletion was that it could be all the of NifU were and the cannot be to a on gene of results is that the domain within NifU, which includes NifU have some function that to the physiological formation, or of [Fe-S] clusters that are on or both of the proposed [Fe-S] cluster assembly Although the function of the domain to be studies have established that substitution for any of the cysteine residues assembly of the cluster but not have a on the capacity for diazotrophic growth (3Agar J.N. Yuvaniyama P. Jack R.F. Cash V.L. Smith A.D. Dean D.R. Johnson M.K. J. Biol. Inorg. Chem. 2000; 5: 167-177Crossref PubMed Scopus (107) Google Scholar). In the genetic a redundant function for the N-terminal and C-terminal domains in the assembly of [Fe-S] clusters for nitrogenase maturation and that the domain could have a function that to the formation or of [Fe-S] clusters nitrogenase of an in for of the Fe of the NifU primary sequence as as genetic studies have been to indicate that NifU could have some function in the maturation of nitrogenase proteins related to assembly of their [Fe-S] clusters (12Jacobson M.R. Cash V.L. Weiss M.C. Laird N.F. Newton W.E. Dean D.R. Mol. Gen. Genet. 1989; 219: 49-57Crossref PubMed Scopus (247) Google Scholar, J. A. M. M. Cash Dean J. PubMed Google Scholar). The formation of labile [Fe-S] on NifU when incubated with NifS (a cysteine desulfurase), and subsequently to the hypothesis that NifU and the related IscU family of proteins provide molecular scaffolds for the assembly of [Fe-S] clusters (2Yuvaniyama P. Agar J.N. Cash V.L. Johnson M.K. Dean D.R. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 599-604Crossref PubMed Scopus (277) Google Scholar, 7Agar J.N. Zheng L. Cash V.L. Dean D.R. Johnson M.K. J. Am. Chem. Soc. 2000; 122: 2136-2137Crossref Scopus (116) Google Scholar). the this hypothesis not been in the case of NifU by using in vitro [Fe-S] cluster assembly and transfer There is a that [2Fe-2S] clusters on the IscU scaffold can be to an J.A. Biochemistry. 2002; PubMed Scopus Google but this is and a of IscU to the protein. could be related to a for the of molecular in the of [Fe-S] cluster assembly and there are for in vitro activation for any of proteins that have catalytic and have also been shown by biochemical and genetic experiments to be physiological There are the which some of the involved in the of an for in vitro [Fe-S] protein there are known protein NifU and that are required for nitrogenase-specific [Fe-S] cluster genetic and biochemical experiments have established that the nitrogenase Fe protein is a physiological for [Fe-S] assembly (12Jacobson M.R. Cash V.L. Weiss M.C. Laird N.F. Newton W.E. Dean D.R. Mol. Gen. Genet. 1989; 219: 49-57Crossref PubMed Scopus (247) Google Scholar). there is a established for Fe protein Fe protein can be to a for in vitro [Fe-S] cluster transfer because [4Fe-4S] cluster can be by in the of and Biochemistry. PubMed Scopus Google Scholar). a for the in vitro activation of the nitrogenase a was that the high of A. vinelandii NifU and NifS in E. coli NifU and NifS are and their appears to be this the of NifU and NifS in the in A. vinelandii for in vitro [Fe-S] cluster NifU and NifS were using in this were incubated with Fe2+ and to in vitro [Fe-S] cluster of the assembly conditions are under assembly was the was to a that includes and and the was with NifS and NifU were with high experiments established that any by NifS or to the and Fe2+ not NifU to the the were with protein protein and NifU were in in the activation and for Fe protein the of the NifU activation were to conditions for [Fe-S] cluster assembly and Fe protein of Fe2+ and were to the assembly which that activation was with to with activation Fe2+ and l-cysteine, shown in activation NifU, l-cysteine, or Fe2+ was the assembly but a high of activation was with the there was activation an of having the cysteine by was in of NifS in the [Fe-S] cluster assembly was under any which could be the of some that of the protein. In experiments it was that of in the activation was not although there was a in activation to results that is not required for cluster transfer NifU to the but a could be required for the of protein in a to in vitro protein Although is in the [Fe-S] cluster biosynthetic there is evidence to indicate that are required for [Fe-S] cluster is in the activation because it is required for of the Fe protein [4Fe-4S] cluster using and it is also required for of nitrogenase It that is required for in vivo [Fe-S] cluster Fe protein maturation because of Fe protein in or have a full of the [4Fe-4S] cluster Dean D.R. PubMed Scopus Google Scholar). The to NifU was by of the assembly the which contained within any [Fe-S] clusters within NifU as as the [2Fe-2S] that NifU contained of NifU cluster assembly and of NifU cluster These the of three for both with of and NifU the of the clusters and the required for it is that the NifU homodimer a of the that can be experiments described here provide the and of the clusters be to and of an [Fe-S] assembly to the described These experiments are in and be of protein was also by the of and which the of and that could be by NifU in the activation as described The of and Fe2+ in Similarly, the of a of and Fe2+ in the for These results indicate that activation by the transfer of an [Fe-S] cluster the NifU by a by the of Fe2+ and with within the protein. was to or not the activation the of NifU and both of which are in the activation or NifU is to These experiments were as described that the recombinantly expressed NifS contained a the Genetic experiments were to that of a this on the capacity for diazotrophic growth, indicating that the of NifS full physiological function. [Fe-S] cluster assembly and of using the as described under NifS was NifU using a with of NifS and the [Fe-S] NifU was by of the this was with activation was Although this is not as high as the of activation in other the of activation as the of clusters on there is some of [Fe-S] clusters the NifU scaffold the required for of NifU and NifS is not required to transfer [Fe-S] clusters have been on the NifU In of the by of it was established that a high of in vitro activation of the protein could be the of this was by experiments the of NifU and protein was The results a NifU activation is an of NifU and protein. It is not known the activation NifU is although this is In experiments the of was using of NifU for which the three N-terminal cysteines are by or for which residues and a cysteine in the C-terminal domain are all by The results of experiments that of the proposed N-terminal scaffold but not the capacity for in vitro activation of protein of activation was also when the of the NifU and protein was The results were when of NifU for which any of the three conserved cysteines within the N-terminal domain have been by were not In contrast, of assembly within both the N-terminal and C-terminal domains by amino acid substitution in vitro protein activation The the in vitro on of both proposed [Fe-S] cluster biosynthetic when with the in vivo are of results is that the in in vitro experiments that an biosynthetic which in is not in the in vitro The results of and biochemical experiments described here provide evidence that the NifU family of proteins have the capacity to provide [Fe-S] cluster assembly scaffold activation experiments indicate that the proposed assembly scaffold within the N-terminal domain is a for in vitro protein and this is with genetic experiments that a for the C-terminal domain under the physiological conditions in the In this and other that a physiological with of the C-terminal domain function is when the N-terminal domain is also The results with the are in with with it was shown that a of and is required to a in that C. E. Proc. Natl. Acad. Sci. U. S. A. 1999; PubMed Scopus Google Scholar). The gene encodes a protein to the N-terminal domain within NifU, and the gene encodes a protein to the C-terminal domain of Genetic experiments described here also indicate there is other scaffold in to contained within NifU that can be a for [Fe-S] cluster of both the N-terminal and C-terminal domains of NifU not eliminate nitrogenase have is that the of a gene designated which also been shown to be of as an [Fe-S] cluster assembly scaffold in vitro C. Agar J.N. Smith A.D. Frazzon J. Dean D.R. Huynh B.H. Johnson M.K. Biochemistry. 2001; PubMed Scopus Google could this function. this a was a deletion within (12Jacobson M.R. Cash V.L. Weiss M.C. Laird N.F. Newton W.E. Dean D.R. Mol. Gen. Genet. 1989; 219: 49-57Crossref PubMed Scopus (247) Google was in with substitutions that both the N-terminal and C-terminal domains of is not impaired in diazotrophic growth, it that to nitrogenase maturation under the conditions in the C. A. J. L. M. K. and is as the could some to nitrogenase but this not been because deletion of the A. vinelandii is (5Zheng L. Cash V.L. Flint D.H. Dean D.R. J. Biol. Chem. 1998; 273: 13264-13272Abstract Full Text Full Text PDF PubMed Scopus (575) Google Scholar). another is that an A. vinelandii which that encodes a primary sequence to the C-terminal domain of NifU could to nitrogenase-specific Fe-S cluster in the absence of NifU function. It is that the A. vinelandii gene and other gene also a of primary sequence within their when with appears that the gene NifU, is a modular although function is not It be that the C-terminal of the gene is also to the which been shown to provide a scaffold for Fe-S cluster in vitro Huynh B.H. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar). physiological for A. vinelandii a for nitrogenase [Fe-S] cluster assembly is related to the that the nitrogenase catalytic Fe protein and a high of the protein under conditions and both have [Fe-S] Thus, it that diazotrophic growth conditions a high on the of and required for the activation of the nitrogenase component proteins. The of the assembly of [Fe-S] proteins involved in intermediary metabolism, for for maturation of the nitrogenase component proteins is by the A. vinelandii encodes nif-specific and (5Zheng L. Cash V.L. Flint D.H. Dean D.R. J. Biol. Chem. 1998; 273: 13264-13272Abstract Full Text Full Text PDF PubMed Scopus (575) Google which are distinct the synthase required for cysteine synthase the in cysteine A. J. Gen. Google Scholar). there are scaffolds for nitrogenase-specific [Fe-S] cluster contained within NifU, is There are a of that on this is that scaffold in the of nitrogenase [Fe-S] another in the of [Fe-S] clusters that are second is that scaffolds could be under for or there are nif-specific proteins other the catalytic components that [Fe-S] are and a variety of Dean D.R. Chem. PubMed Scopus Google Scholar). Thus, [Fe-S] cluster scaffolds within NifU could be related to the maturation of the [Fe-S] proteins that participate in nitrogen there is such related to the N-terminal and C-terminal domains of NifU, the there also be some there could be some cluster assembly that on the N-terminal and C-terminal [Fe-S] clusters be required for the assembly a second or cluster transfer could to the assembly described here not among but the of genetic and biochemical together with can be to a for the and in vitro transfer of [Fe-S] clusters on the NifU scaffold to a known physiological protein been The results indicate in the case of nitrogenase molecular or other protein components not to be can be with a of in vitro [Fe-S] cluster which is and a of IscU to the protein J.A. Biochemistry. 2002; PubMed Scopus Google Scholar). Although such comparisons not indicate that [Fe-S] cluster transfer experiments are it that molecular and with the [Fe-S] cluster biosynthetic are to be required for transfer of [Fe-S] clusters to the proteins assembly of the [Fe-S] cluster is supported by genetic experiments that have shown an physiological for and in the maturation of [Fe-S] proteins such as aconitase and glutamate synthase U. Takahashi Y. J. Biochem. (Tokyo). 2001; 130: 63-71Crossref PubMed Scopus (215) Google Scholar, 11Takahashi Y. Nakamura M. J. Biochem. (Tokyo). 1999; 126: 917-926Crossref PubMed Scopus (227) Google Scholar).