Genetic dissection of the bacterial Fe-S protein biogenesis machineries.

Iron­sulfur (Fe-S) clusters are one of the most ancient and versatile inorganic cofactors present in the three domains of life. Fe-S clusters are essential cofactors for the activity of a large variety of metalloproteins that play crucial physiological roles. Fe-S protein biogenesis is a complex process that starts with the acquisition of the elements (iron and sulfur atoms) and their assembly into an Fe-S cluster that is subsequently inserted into the target proteins. The Fe-S protein biogenesis is ensured by multiproteic systems conserved across all domains of life. Here, we provide an overview on how bacterial genetics approaches have permitted to reveal and dissect the Fe-S protein biogenesis process in vivo.

A Diverged Transcriptional Network for Usage of Two Fe-S Cluster Biogenesis Machineries in the Delta-Proteobacterium Myxococcus xanthus.

Myxococcus xanthus possesses two Fe-S cluster biogenesis machineries, ISC (iron-sulfur cluster) and SUF (sulfur mobilization). Here, we show that in comparison to the phylogenetically distant Enterobacteria, which also have both machineries, M. xanthus evolved an independent transcriptional scheme to coordinately regulate the expression of these machineries. This transcriptional response is directed by RisR, which we show to belong to a phylogenetically distant and biochemically distinct subgroup of the Rrf2 transcription factor family, in comparison to IscR that regulates the isc and suf operons in Enterobacteria. We report that RisR harbors an Fe-S cluster and that holo-RisR acts as a repressor of both the isc and suf operons, in contrast to Escherichia coli, where holo-IscR represses the isc operon whereas apo-IscR activates the suf operon. In addition, we establish that the nature of the cluster and the DNA binding sites of RisR, in the isc and suf operons, diverge from those of IscR. We further show that in M. xanthus, the two machineries appear to be fully interchangeable in maintaining housekeeping levels of Fe-S cluster biogenesis and in synthesizing the Fe-S cluster for their common regulator, RisR. We also demonstrate that in response to oxidative stress and iron limitation, transcriptional upregulation of the M. xanthus isc and suf operons was mediated solely by RisR and that the contribution of the SUF machinery was greater than the ISC machinery. Altogether, these findings shed light on the diversity of homeostatic mechanisms exploited by bacteria to coordinately use two Fe-S cluster biogenesis machineries. IMPORTANCE Fe-S proteins are ubiquitous and control a wide variety of key biological processes; therefore, maintaining Fe-S cluster homeostasis is an essential task for all organisms. Here, we provide the first example of how a bacterium from the Deltaproteobacteria branch coordinates expression of two Fe-S cluster biogenesis machineries. The results revealed a new model of coordination, highlighting the unique and common features that have independently emerged in phylogenetically distant bacteria to maintain Fe-S cluster homeostasis in response to environmental changes. Regulation is orchestrated by a previously uncharacterized transcriptional regulator, RisR, belonging to the Rrf2 superfamily, whose members are known to sense diverse environmental stresses frequently encountered by bacteria. Understanding how M. xanthus maintains Fe-S cluster homeostasis via RisR regulation revealed a strategy reflective of the aerobic lifestyle of this organsim. This new knowledge also paves the way to improve production of Fe-S-dependent secondary metabolites using M. xanthus as a chassis.

OrpR is a σ54 -dependent activator using an iron-sulfur cluster for redox sensing in Desulfovibrio vulgaris Hildenborough.

Enhancer binding proteins (EBPs) are key players of σ54 -regulation that control transcription in response to environmental signals. In the anaerobic microorganism Desulfovibrio vulgaris Hildenborough (DvH), orp operons have been previously shown to be coregulated by σ54 -RNA polymerase, the integration host factor IHF and a cognate EBP, OrpR. In this study, ChIP-seq experiments indicated that the OrpR regulon consists of only the two divergent orp operons. In vivo data revealed that (i) OrpR is absolutely required for orp operons transcription, (ii) under anaerobic conditions, OrpR binds on the two dedicated DNA binding sites and leads to high expression levels of the orp operons, (iii) increasing the redox potential of the medium leads to a drastic down-regulation of the orp operons expression. Moreover, combining functional and biophysical studies on the anaerobically purified OrpR leads us to propose that OrpR senses redox potential variations via a redox-sensitive [4Fe-4S]2+ cluster in the sensory PAS domain. Overall, the study herein presents the first characterization of a new Fe-S redox regulator belonging to the σ54 -dependent transcriptional regulator family probably advantageously selected by cells adapted to the anaerobic lifestyle to monitor redox stress conditions.

Making iron-sulfur cluster: structure, regulation and evolution of the bacterial ISC system.

Iron sulfur (Fe-S) clusters rank among the most ancient and conserved prosthetic groups. Fe-S clusters containing proteins are present in most, if not all, organisms. Fe-S clusters containing proteins are involved in a wide range of cellular processes, from gene regulation to central metabolism, via gene expression, RNA modification or bioenergetics. Fe-S clusters are built by biogenesis machineries conserved throughout both prokaryotes and eukaryotes. We focus mostly on bacterial ISC machinery, but not exclusively, as we refer to eukaryotic ISC system when it brings significant complementary information. Besides covering the structural and regulatory aspects of Fe-S biogenesis, this review aims to highlight Fe-S biogenesis facets remaining matters of discussion, such as the role of frataxin, or the link between fatty acid metabolism and Fe-S homeostasis. Last, we discuss recent advances on strategies used by different species to make and use Fe-S clusters in changing redox environmental conditions.

When cobalt-mediated [2 + 2 + 2] cycloaddition reaction dares go astray: synthesis of unprecedented cobalt(iii)-complexes.

In sharp contrast with the standard [2 + 2 + 2] cycloaddition reaction of diyne/ene, cobalt-mediated cycloadditions with γ-alkylidenebutenolide led to unprecedented cobalt(iii) polycyclic complexes. A plausible mechanism supported by a computational study based on an unusual fragmentation of the butenolide moiety was postulated to account for this original reaction.

The bacterial MrpORP is a novel Mrp/NBP35 protein involved in iron-sulfur biogenesis.

Despite recent advances in understanding the biogenesis of iron-sulfur (Fe-S) proteins, most studies focused on aerobic bacteria as model organisms. Accordingly, multiple players have been proposed to participate in the Fe-S delivery step to apo-target proteins, but critical gaps exist in the knowledge of Fe-S proteins biogenesis in anaerobic organisms. Mrp/NBP35 ATP-binding proteins are a subclass of the soluble P-loop containing nucleoside triphosphate hydrolase superfamily (P-loop NTPase) known to bind and transfer Fe-S clusters in vitro. Here, we report investigations of a novel atypical two-domain Mrp/NBP35 ATP-binding protein named MrpORP associating a P-loop NTPase domain with a dinitrogenase iron-molybdenum cofactor biosynthesis domain (Di-Nase). Characterization of full length MrpORP, as well as of its two domains, showed that both domains bind Fe-S clusters. We provide in vitro evidence that the P-loop NTPase domain of the MrpORP can efficiently transfer its Fe-S cluster to apo-target proteins of the ORange Protein (ORP) complex, suggesting that this novel protein is involved in the maturation of these Fe-S proteins. Last, we showed for the first time, by fluorescence microscopy imaging a polar localization of a Mrp/NBP35 protein.

Obligate sugar oxidation in Mesotoga spp., phylum Thermotogae, in the presence of either elemental sulfur or hydrogenotrophic sulfate-reducers as electron acceptor.

Mesotoga prima strain PhosAc3 is a mesophilic representative of the phylum Thermotogae comprising only fermentative bacteria so far. We show that while unable to ferment glucose, this bacterium is able to couple its oxidation to reduction of elemental sulfur. We demonstrate furthermore that M. prima strain PhosAc3 as well as M. prima strain MesG1 and Mesotoga infera are able to grow in syntrophic association with sulfate-reducing bacteria (SRB) acting as hydrogen scavengers through interspecies hydrogen transfer. Hydrogen production was higher in M. prima strain PhosAc3 cells co-cultured with SRB than in cells cultured alone in the presence of elemental sulfur. We propose that the efficient sugar-oxidizing metabolism by M. prima strain PhosAc3 in syntrophic association with a hydrogenotrophic sulfate-reducing bacterium can be extrapolated to all members of the Mesotoga genus. Genome comparison of Thermotogae members suggests that the metabolic difference between Mesotoga and Thermotoga species (sugar oxidation versus fermentation) is mainly due to the absence of the bifurcating [FeFe]-hydrogenase in the former. Such an obligate oxidative process for using sugars, unusual within prokaryotes, is the first reported within the Thermotogae. It is hypothesized to be of primary ecological importance for growth of Mesotoga spp. in the environments that they inhabit.

Niobium-Catalyzed Intramolecular Addition of O-H and N-H Bonds to Alkenes: A Tool for Hydrofunctionalization.

A convenient, versatile, and easy to handle intramolecular hydrofunctionalization of alkenes (C-O and C-N bonds formation) is reported using a novel niobium-based catalytic system. This atom economic and eco-friendly methodology provides an additional synthetic tool for the straightforward formation of valuable building blocks enabling molecular complexity. Various pyran, furan, pyrrolidine, piperidine, lactone, and lactam derivatives as well as spirocyclic compounds are produced in high yields and selectivities.

Regio- and Stereoselective Hydrosilylation of Unsymmetrical Alkynes Catalyzed by a Well-Defined, Low-Valent Cobalt Catalyst.

Herein, the use of a well-defined low-valent cobalt(I) catalyst [HCo(PMe3)4] capable of performing the highly regio- and stereoselective hydrosilylation of internal alkynes is reported. The reaction can be applied to a variety of hydrosilanes, symmetrical and unsymmetrical alkynes, giving in many cases a single hydrosilylation isomer. Experimental and theoretical studies suggest the key step to be a hydro-cobaltation and that the reaction proceeds through a classical Chalk-Harrod mechanism.

Chiral Phosphate in Rhodium-Catalyzed Asymmetric [2+2+2] Cycloaddition: Ligand, Counterion, or Both?

Investigations based on NMR spectroscopy, mass spectrometry, and DFT calculations shed light on the metallic species generated in the rhodium-catalyzed asymmetric [2+2+2] cycloaddition reaction between diynes and isocyanates with the chiral phosphate TRIP. The catalytic mixture comprising [{Rh(cod)Cl}2 ], 1,4-diphenylphosphinobutane (dppb), and Ag(S)-TRIP actually gives rise to two species, both having an effect on the stereoselectivity. One is a rhodium(I) complex in which TRIP is a weakly coordinating counterion, whereas the other is a bimetallic Rh/Ag complex in which TRIP is a strongly coordinating X-type ligand.

C2-Alkylation and Alkenylation of Indoles Catalyzed by a Low-Valent Cobalt Complex in the Absence of Reductant.

Herein an extremely versatile, well-defined, low-valent cobalt catalyst [Co(PMe3)4] capable of intermolecular and intramolecular imine-directed C2-alkylation and alkenylation of indoles is reported. The reaction proceeds in the absence of reducing agents or additives, affording a range of substituted indoles and dihydropyrroloindoles in high yields and regioselectivities. With the aid of deuterium labeling studies and DFT (Density Functional Theory) calculations, a mechanism is proposed that is based on a Ligand-to-Ligand Hydrogen Transfer pathway.

Orange protein from Desulfovibrio alaskensis G20: insights into the Mo-Cu cluster protein-assisted synthesis.

A novel metalloprotein containing a unique [S2MoS2CuS2MoS2](3-) cluster, designated as Orange Protein (ORP), was isolated for the first time from Desulfovibrio gigas, a sulphate reducer. The orp operon is conserved in almost all sequenced Desulfovibrio genomes and in other anaerobic bacteria, however, so far D. gigas ORP had been the only ORP characterized in the literature. In this work, the purification of another ORP isolated form Desulfovibrio alaskensis G20 is reported. The native protein is monomeric (12443.8 ± 0.1 Da by ESI-MS) and contains also a MoCu cluster with characteristic absorption bands at 337 and 480 nm, assigned to S-Mo charge transfer bands. Desulfovibrio alaskensis G20 recombinant protein was obtained in the apo-form from E. coli. Cluster reconstitution studies and UV-visible titrations with tetrathiomolybdate of the apo-ORP incubated with Cu ions indicate that the cluster is incorporated in a protein metal-assisted synthetic mode and the protein favors the 2Mo:1Cu stoichiometry. In Desulfovibrio alaskensis G20, the orp genes are encoded by a polycistronic unit composed of six genes whereas in Desulfovibrio vulgaris Hildenborough the same genes are organized into two divergent operons, although the composition in genes is similar. The gene expression of ORP (Dde_3198) increased 6.6 ± 0.5 times when molybdate was added to the growth medium but was not affected by Cu(II) addition, suggesting an involvement in molybdenum metabolism directly or indirectly in these anaerobic bacteria.

Single-Cell Analysis of Growth and Cell Division of the Anaerobe Desulfovibrio vulgaris Hildenborough.

Recent years have seen significant progress in understanding basic bacterial cell cycle properties such as cell growth and cell division. While characterization and regulation of bacterial cell cycle is quite well-documented in the case of fast growing aerobic model organisms, no data has been so far reported for anaerobic bacteria. This lack of information in anaerobic microorganisms can mainly be explained by the absence of molecular and cellular tools such as single cell microscopy and fluorescent probes usable for anaerobes and essential to study cellular events and/or subcellular localization of the actors involved in cell cycle. In this study, single-cell microscopy has been adapted to study for the first time, in real time, the cell cycle of a bacterial anaerobe, Desulfovibrio vulgaris Hildenborough (DvH). This single-cell analysis provides mechanistic insights into the cell division cycle of DvH, which seems to be governed by the recently discussed so-called incremental model that generates remarkably homogeneous cell sizes. Furthermore, cell division was reversibly blocked during oxygen exposure. This may constitute a strategy for anaerobic cells to cope with transient exposure to oxygen that they may encounter in their natural environment, thereby contributing to their aerotolerance. This study lays the foundation for the first molecular, single-cell assay that will address factors that cannot otherwise be resolved in bulk assays and that will allow visualization of a wide range of molecular mechanisms within living anaerobic cells.

Double-Stereodifferentiation in Rhodium-Catalyzed [2 + 2 + 2] Cycloaddition: Chiral Ligand/Chiral Counterion Matched Pair.

The first enantioselective metal-catalyzed [2 + 2 + 2] cycloaddition involving a double asymmetric induction has been devised. It relies on the use of an in situ generated chiral cationic rhodium(I) catalyst with a matched chiral ligand/chiral counterion pair. Careful optimization of the catalytic system, as well as of the reaction conditions, led to atroposelective [2 + 2 + 2] pyridone cycloadducts with high ee's up to 96%. This strategy outperformed those previously described involving a chiral ligand only or a chiral counterion only.

Mild niobium-catalyzed [2 + 2 + 2] cycloaddition of sila-triynes: easy access to polysubstituted benzosilacyclobutenes.

A new and efficient synthesis of highly sensitive benzosilacyclobutenes has been developed. For the first time, these compounds can be synthesized in very high yields by a mild, unprecedented intramolecular niobium-catalyzed [2 + 2 + 2] cycloaddition of easily accessible tetrasubstituted sila-triynes. An easy access to highly functionalized benzosilacyclobutenes enlarging the number of potential applications in organic and material chemistry is described.

C-H activation/functionalization catalyzed by simple, well-defined low-valent cobalt complexes.

A facile C-H activation and functionalization of aromatic imines is presented using low-valent cobalt catalysts. Using Co(PMe3)4 as catalyst we have developed an efficient and simple protocol for the C-H/hydroarylation of alkynes with an anti selectivity. Deuterium-labeling experiments, DFT calculations coupled with the use of a well-defined catalyst have for the first time shed light on the elusive black box of cobalt catalyzed C-H functionalization.

Synthesis and photovoltaic performances in solution-processed BHJs of oligothiophene-substituted organocobalt complexes [(η4-C4(nT)4)Co(η5-C5H5)].

We describe an efficient synthetic route toward novel organocobalt complexes [(η(4)-C4(nT)4)Co(η(5)-C5H5)] with n = 1, 2, 3 thiophene rings. Solution-processed bulk heterojunctions solar cells based on CpCoCb(3T)4:PCBM blends achieve power conversion efficiencies of up to 2.1%.

IHF is required for the transcriptional regulation of the Desulfovibrio vulgaris Hildenborough orp operons.

Transcriptional activation of σ(54)-dependent promoters is usually tightly regulated in response to environmental cues. The high abundance of potential σ(54)-dependent promoters in the anaerobe bacteria, Desulfovibrio vulgaris Hildenborough, reflects the high versatility of this bacteria suggesting that σ(54) factor is the nexus of a large regulatory network. Understanding the key players of σ(54)-regulation in this organism is therefore essential to gain insights into the adaptation to anaerobiosis. Recently, the D. vulgaris orp genes, specifically found in anaerobe bacteria, have been shown to be transcribed by the RNA polymerase coupled to the σ(54) alternative sigma factor. In this study, using in vitro binding experiments and in vivo reporter fusion assays in the Escherichia coli heterologous host, we showed that the expression of the divergent orp promoters is strongly dependent on the integration host factor IHF. Bioinformatic and mutational analysis coupled to reporter fusion activities and mobility shift assays identified two functional IHF binding site sequences located between the orp1 and orp2 promoters. We further determined that the D. vulgaris DVU0396 (IHFα) and DVU1864 (IHFß) subunits are required to control the expression of the orp operons suggesting that they form a functionally active IHF heterodimer. Interestingly results obtained from the in vivo inactivation of DVU0396, which is required for orp operons transcription, suggest that several functionally IHF active homodimer or heterodimer are present in D. vulgaris.

Atroposelective [2+2+2] cycloadditions catalyzed by a rhodium(I)-chiral phosphate system.

Enantioselective cationic Rh(I)-catalyzed [2+2+2] cycloaddition reactions between diynes and isocyanates relying on the chiral anion strategy have been devised. In the presence of [Rh(cod)Cl]2, 1,4-bis(diphenylphosphino)butane, and the silver phosphate salt Ag(S)-TRIP as the unique source of chirality, axially chiral pyridones were isolated with ees up to 82%. This approach is novel in the field of chiral anion-mediated asymmetric catalysis since atroposelective transformations have so far remained unprecedented. It also proves to be complementary to the classical strategy based on chiral L-type ligands.

Highly enantioselective rhodium-catalyzed [2+2+2] cycloaddition of diynes to sulfonimines.

A new asymmetric [2+2+2] cycloaddition of diynes to sulfonimines under rhodium catalysis that provides the corresponding enantioenriched 1,2-dihydropyridines in good yields is described.