Iron-sulfur (Fe-S) cluster assembly: the SufBCD complex is a new type of Fe-S scaffold with a flavin redox cofactor

Enzyme catalyzed formation of radicals from S-adenosylmethionine and inhibition of enzyme activity by the cleavage products

A large superfamily of enzymes have been identified that make use of radical intermediates derived by reductive cleavage of S-adenosylmethionine. The primary nature of the radical intermediates makes them highly reactive and potent oxidants. They are used to initiate biotransformations by hydrogen atom abstraction, a process that allows a particularly diverse range of substrates to be functionalized, including substrates with relatively inert chemical structures. In the first part of this review, we discuss the evidence supporting the mechanism of radical formation from S-adenosylmethionine. In the second part of the review, we examine the potential of reaction products arising from S-adenosylmethionine to cause product inhibition. The effects of this product inhibition on kinetic studies of 'radical S-adenosylmethionine' enzymes are discussed and strategies to overcome these issues are reviewed. This article is part of a Special Issue entitled: Radical SAM enzymes and Radical Enzymology.

Inference of the transcriptional regulatory network in Staphylococcus aureus by integration of experimental and genomics-based evidence

Transcriptional regulatory networks are fine-tuned systems that help microorganisms respond to changes in the environment and cell physiological state. We applied the comparative genomics approach implemented in the RegPredict Web server combined with SEED subsystem analysis and available information on known regulatory interactions for regulatory network reconstruction for the human pathogen Staphylococcus aureus and six related species from the family Staphylococcaceae. The resulting reference set of 46 transcription factor regulons contains more than 1,900 binding sites and 2,800 target genes involved in the central metabolism of carbohydrates, amino acids, and fatty acids; respiration; the stress response; metal homeostasis; drug and metal resistance; and virulence. The inferred regulatory network in S. aureus includes ∼320 regulatory interactions between 46 transcription factors and ∼550 candidate target genes comprising 20% of its genome. We predicted ∼170 novel interactions and 24 novel regulons for the control of the central metabolic pathways in S. aureus. The reconstructed regulons are largely variable in the Staphylococcaceae: only 20% of S. aureus regulatory interactions are conserved across all studied genomes. We used a large-scale gene expression data set for S. aureus to assess relationships between the inferred regulons and gene expression patterns. The predicted reference set of regulons is captured within the Staphylococcus collection in the RegPrecise database (http://regprecise.lbl.gov).

Enterococcus faecalis sufCDSUB complements Escherichia coli sufABCDSE

Iron-sulfur [Fe-S] clusters are inorganic prosthetic groups that play essential roles in all living organisms. Iron and sulfur mobilization, formation of [Fe-S] clusters, and delivery to its final protein targets involves a complex set of specific protein machinery. Proteobacteria has three systems of [Fe-S] biogenesis, designated NIF, ISC, and SUF. In contrast, the Firmicutes system is not well characterized and has only one system, formed mostly by SUF homologs. The Firmicutes phylum corresponds to a group of pathological bacteria, of which Enterococcus faecalis is a clinically relevant representative. Recently, the E. faecalis sufCDSUB [Fe-S] cluster biosynthetic machinery has been identified, although there is no further information available about the similarities and/or variations of Proteobacteria and Firmicutes systems. The aim of the present work was to compare the ability of the different Proteobacteria and Firmicutes systems to complement the Azotobacter vinelandii and Escherichia coli ISC and SUF systems. Indeed, E. faecalis sufCDSUB is able to complement the E. coli SUF system, allowing viable mutants of both sufABCDSE and iscRSU-hscBA-fdx systems. The presence of all E. faecalis SUF factors enables proper functional interactions, which would not otherwise occur in proteins from different systems.

Ancient and essential: the assembly of iron-sulfur clusters in plants

In plants iron-sulfur (Fe-S) proteins are found in the plastids, mitochondria, cytosol and nucleus, where they are essential for numerous physiological and developmental processes. Recent mutant studies, mostly in Arabidopsis thaliana, have identified three pathways for the assembly of Fe-S clusters. The plastids harbor the SUF (sulfur mobilization) pathway and operate independently, whereas cluster assembly in the cytosol depends on the emerging CIA (cytosolic iron-sulfur cluster assembly) pathway and mitochondria. The latter organelles use the ISC (iron-sulfur cluster) assembly pathway. In all three pathways the assembly process can be divided into a first stage where S and Fe are combined on a scaffold protein, and a second stage in which the Fe-S cluster is transferred to a target protein. The second stage might involve different carrier proteins with specialized functions.

The apicomplexan plastid and its evolution

Protistan species belonging to the phylum Apicomplexa have a non-photosynthetic secondary plastid-the apicoplast. Although its tiny genome and even the entire nuclear genome has been sequenced for several organisms bearing the organelle, the reason for its existence remains largely obscure. Some of the functions of the apicoplast, including housekeeping ones, are significantly different from those of other plastids, possibly due to the organelle's unique symbiotic origin.

SufD and SufC ATPase activity are required for iron acquisition during in vivo Fe-S cluster formation on SufB

In vivo biogenesis of Fe-S cluster cofactors requires complex biosynthetic machinery to limit release of iron and sulfide, to protect the Fe-S cluster from oxidation, and to target the Fe-S cluster to the correct apoenzyme. The SufABCDSE pathway for Fe-S cluster assembly in Escherichia coli accomplishes these tasks under iron starvation and oxidative stress conditions that disrupt Fe-S cluster metabolism. Although SufB, SufC, and SufD are all required for in vivo Suf function, their exact roles are unclear. Here we show that SufB, SufC, and SufD, coexpressed with the SufS-SufE sulfur transfer pair, purify as two distinct complexes (SufBC(2)D and SufB(2)C(2)) that contain Fe-S clusters and FADH(2). These studies also show that SufC and SufD are required for in vivo Fe-S cluster formation on SufB. Furthermore, while SufD is dispensable for in vivo sulfur transfer, it is absolutely required for in vivo iron acquisition. Finally, we demonstrate for the first time that the ATPase activity of SufC is necessary for in vivo iron acquisition during Fe-S cluster assembly.

A new role for heme, facilitating release of iron from the bacterioferritin iron biomineral

Bacterioferritin (BFR) from Escherichia coli is a member of the ferritin family of iron storage proteins and has the capacity to store very large amounts of iron as an Fe(3+) mineral inside its central cavity. The ability of organisms to tap into their cellular stores in times of iron deprivation requires that iron must be released from ferritin mineral stores. Currently, relatively little is known about the mechanisms by which this occurs, particularly in prokaryotic ferritins. Here we show that the bis-Met-coordinated heme groups of E. coli BFR, which are not found in other members of the ferritin family, play an important role in iron release from the BFR iron biomineral: kinetic iron release experiments revealed that the transfer of electrons into the internal cavity is the rate-limiting step of the release reaction and that the rate and extent of iron release were significantly increased in the presence of heme. Despite previous reports that a high affinity Fe(2+) chelator is required for iron release, we show that a large proportion of BFR core iron is released in the absence of such a chelator and further that chelators are not passive participants in iron release reactions. Finally, we show that the catalytic ferroxidase center, which is central to the mechanism of mineralization, is not involved in iron release; thus, core mineralization and release processes utilize distinct pathways.

An in vivo method for characterization of protein interactions within sulfur trafficking systems of E. coli

Sulfur trafficking systems are multiprotein systems that synthesize sulfur-containing cofactors such as iron-sulfur clusters. The sulfur is derived enzymatically from cysteine and transferred between nucleophilic cysteine residues within proteins until incorporation into the relevant cofactor. As these systems are poorly understood, we have developed an in vivo method for characterizing these interactions and have applied our method to the SUF system of Escherichia coli, which is responsible for iron-sulfur cluster biogenesis under oxidative stress and iron limitation. Proteins that interact covalently with SufE were trapped in vivo, purified, and identified by mass spectrometry. We identified SufE-SufS and SufE-SufB interactions, interactions previously demonstrated in vitro, indicating that our method has the ability to identify physiologically relevant interactions. The sulfur acceptor function of SufE is likely due to the low pK(a) of its active site C51, which we determined to be 6.3 ± 0.7. We found that SufE interacts with several Fe-S cluster proteins, further supporting the validity of the method, and with tryptophanase, glutaredoxin-3, and glutaredoxin-4, possibly suggesting a role for these enzymes in iron-sulfur biogenesis by the SUF system. Our results indicate that this method could serve as a general tool for the determination of sulfur trafficking mechanisms.

Iron-sulfur world in aerobic and hyperthermoacidophilic archaea Sulfolobus

The general importance of the Fe-S cluster prosthetic groups in biology is primarily attributable to specific features of iron and sulfur chemistry, and the assembly and interplay of the Fe-S cluster core with the surrounding protein is the key to in-depth understanding of the underlying mechanisms. In the aerobic and thermoacidophilic archaea, zinc-containing ferredoxin is abundant in the cytoplasm, functioning as a key electron carrier, and many Fe-S enzymes are produced to participate in the central metabolic and energetic pathways. De novo formation of intracellular Fe-S clusters does not occur spontaneously but most likely requires the operation of a SufBCD complex of the SUF machinery, which is the only Fe-S cluster biosynthesis system conserved in these archaea. In this paper, a brief introduction to the buildup and maintenance of the intracellular Fe-S world in aerobic and hyperthermoacidophilic crenarchaeotes, mainly Sulfolobus, is given in the biochemical, genetic, and evolutionary context.