Sulfur Transfer Research Articles

TusA Is a Versatile Protein That Links Translation Efficiency to Cell Division in Escherichia coli.

To enable accurate and efficient translation, sulfur modifications are introduced posttranscriptionally into nucleosides in tRNAs. The biosynthesis of tRNA sulfur modifications involves unique sulfur trafficking systems for the incorporation of sulfur atoms in different nucleosides of tRNA. One of the proteins that is involved in inserting the sulfur for 5-methylaminomethyl-2-thiouridine (mnm5s2U34) modifications in tRNAs is the TusA protein. TusA, however, is a versatile protein that is also involved in numerous other cellular pathways. Despite its role as a sulfur transfer protein for the 2-thiouridine formation in tRNA, a fundamental role of TusA in the general physiology of Escherichia coli has also been discovered. Poor viability, a defect in cell division, and a filamentous cell morphology have been described previously for tusA-deficient cells. In this report, we aimed to dissect the role of TusA for cell viability. We were able to show that the lack of the thiolation status of wobble uridine (U34) nucleotides present on Lys, Gln, or Glu in tRNAs has a major consequence on the translation efficiency of proteins; among the affected targets are the proteins RpoS and Fis. Both proteins are major regulatory factors, and the deregulation of their abundance consequently has a major effect on the cellular regulatory network, with one consequence being a defect in cell division by regulating the FtsZ ring formation.IMPORTANCE More than 100 different modifications are found in RNAs. One of these modifications is the mnm5s2U modification at the wobble position 34 of tRNAs for Lys, Gln, and Glu. The functional significance of U34 modifications is substantial since it restricts the conformational flexibility of the anticodon, thus providing translational fidelity. We show that in an Escherichia coli TusA mutant strain, involved in sulfur transfer for the mnm5s2U34 thio modifications, the translation efficiency of RpoS and Fis, two major cellular regulatory proteins, is altered. Therefore, in addition to the transcriptional regulation and the factors that influence protein stability, tRNA modifications that ensure the translational efficiency provide an additional crucial regulatory factor for protein synthesis.

Desulfurization Behavior of Fe-18Cr-18Mn Alloy during the Pressurized Electroslag Remelting with Different Atmospheres and Na2O-containing Slags

Pressurized electroslag remelting (PESR) has been widely used for preparation of high-quality high nitrogen steels. However, desulfurization might be limited by the pressurized-nitrogen atmosphere in the process. In this paper, the desulfurization behaviors of electroslag remelting in air and nitrogen pressure were investigated and further an appropriate method to improve the sulfur removal in PESR was explored. The results show that the ESR in air maintained relatively high and stable desulfurization ability, but the PESR provided worse sulfur removal owing to the lack of sulfur gasifying oxidation under N2 pressure. With the decline of sulfur distribution coefficient (LS) and the enrichment of sulfur in slag during the remelting, the desulfurization in PESR was gradually weakened. The level of gas pressure had little effect on sulfur removal. The Na2O addition in conventional CaO-Al2O3-CaF2 slags significantly increased the activity of desulfurizers and LS, and slightly improved sulfur mass transfer in slags, then alleviated the negative effect brought by the reduction of desulfurization ability in PESR and achieved higher desulfurization degree. In addition, desulfurization in the remelting was mainly achieved at the liquid metal film of electrode tips, which was benefited from the great reaction specific surface area and high initial sulfur content.

Sulfur Analysis and Sulfur Transfer Rule During Simulated Thermal Processing of Heavy Oil

The distribution and transfer of sulfide in the thermal reaction process are studied. It has found that different types of sulfur have different distribution patterns in the refining process. Thiophene derivatives with better thermal stability are mainly distributed in the heavy components, while mercaptans and small molecular thioethers with poor thermal stability are mainly distributed in the light components. During the thermal reaction, part of the sulfide will decompose or react to produce sulfide with a smaller molecular weight, which will be transferred to light components or gases. The thermal reaction temperature has a certain effect on the product yield and sulfur content. As the reaction temperature increases, the liquid yield increases and the sulfur content decreases, but the thermal reaction temperature has little effect on the sulfide type distribution.

Anion Intercalation of VS4 Triggers Atomic Sulfur Transfer to Organic Disulfide in Rechargeable Lithium Battery

AbstractTransition metal polysulfides MSx (x > 2) have attracted much attention because of their high theoretical capacities as electrode materials for lithium batteries. Among them, VS4 has a linear chain structure in which van der Walls forces exist between chains. Previous reports found lithium ions can be intercalated into VS4 in the discharge, and elemental V and Li2S are formed when discharged to low voltage (e.g., <1.0 V vs Li/Li+). In this study is found that bis(trifluoromethanesulfonyl)imide (TFSI−) anions can be intercalated into VS4 when charged to 3.0 V or above after discharged to 1.8 V in rechargeable lithium battery, triggering sulfur evolution. To improve its cycling reversibility diphenyl disulfide (DPDS) is used as a sulfur receptor in the composite electrode consisting of nanoscaled VS4 particles and carbon nanotubes (CNTs). In the charge, sulfur atoms are transferred from VS4 to DPDS forming diphenyl trisulfide (DPTS). This strategy enables improved capacity retention and material utilization. The electrode shows a reversible capacity of 130.7 mAh g−1 after 300 cycles at the current density of 240 mA g−1.

Characterization and Reconstitution of Human Lipoyl Synthase (LIAS) Supports ISCA2 and ISCU as Primary Cluster Donors and an Ordered Mechanism of Cluster Assembly.

Lipoyl synthase (LIAS) is an iron–sulfur cluster protein and a member of the radical S-adenosylmethionine (SAM) superfamily that catalyzes the final step of lipoic acid biosynthesis. The enzyme contains two [4Fe–4S] centers (reducing and auxiliary clusters) that promote radical formation and sulfur transfer, respectively. Most information concerning LIAS and its mechanism has been determined from prokaryotic enzymes. Herein, we detail the expression, isolation, and characterization of human LIAS, its reactivity, and evaluation of natural iron–sulfur (Fe–S) cluster reconstitution mechanisms. Cluster donation by a number of possible cluster donor proteins and heterodimeric complexes has been evaluated. [2Fe–2S]-cluster-bound forms of human ISCU and ISCA2 were found capable of reconstituting human LIAS, such that complete product turnover was enabled for LIAS, as monitored via a liquid chromatography–mass spectrometry (LC–MS) assay. Electron paramagnetic resonance (EPR) studies of native LIAS and substituted derivatives that lacked the ability to bind one or the other of LIAS’s two [4Fe–4S] clusters revealed a likely order of cluster addition, with the auxiliary cluster preceding the reducing [4Fe–4S] center. These results detail the trafficking of Fe–S clusters in human cells and highlight differences with respect to bacterial LIAS analogs. Likely in vivo Fe–S cluster donors to LIAS are identified, with possible connections to human disease states, and a mechanistic ordering of [4Fe–4S] cluster reconstitution is evident.

Subduction zone sulfur mobilization and redistribution by intraslab fluid–rock interaction

Subduction zones mediate the sulfur exchange between Earth’s interior and surface, and play a critical role in the long-term global sulfur cycle. Dehydration of the subducted slab releases aqueous fluids, which in turn interact with the surrounding wall-rocks during reactive flow through the slab. Fluids are regarded as the key agent for sulfur transfer within the slab and subsequently into the mantle wedge. However, the mechanisms underlying sulfur mobilization and redistribution during fluid–rock interaction are not fully understood. Here, we investigate three high-pressure blueschist/eclogite–selvage–vein systems from the Southwestern Tianshan metamorphic belt in northwestern China to explore sulfur behavior during fluid–rock interaction along channelized fluid pathways and within their alteration halos. Petrologic observations on mineral parageneses and δ34S compositions of sulfides reveal two different scenarios of sulfur mobilization and redistribution during fluid–rock interaction at HP conditions. Fe-rich and medium-fS2 (near the pyrite–pyrrhotite buffer) fluids favor pyrrhotite stability over pyrite and can trigger the pyrite–to–pyrrhotite transition in the reaction halo. In this case the bulk-rock sulfur budget may not vary but the sulfur isotope exchange is still effective, producing a δ34S mixing trend in the metasomatic zones. In contrast, Fe-poor and high-fS2 fluids cause conversion of pyrrhotite to pyrite, significant sulfur addition, and sulfur isotope exchange in alteration selvages. Medium-fS2 fluids can transport sulfur and thus are effective in transferring sulfur out of the slab, whereas high-fS2 fluids cause sulfur sequestration along fluid pathways and considerably reduce long-distance sulfur transfer. Multiple fluid infiltration events with different fluid sources result in several pyrite overgrowth sequences, recorded by contrasting Co–Ni element distributions and in-situ δ34S zoning of pyrite. The heaviest δ34S value (+25‰) thus far reported in HP rocks has been found in vein pyrite, suggesting that a seawater sulfate-derived δ34S signature can be transported to great depths in a subduction zone, even though the timing and mechanism of sulfate–to–sulfide reduction in the subduction zone remain unconstrained. Our study provides natural evidence for fluid-mediated sulfide transformation, sulfur transport/storage, and sulfur isotope exchange during fluid–rock interaction, and illuminates the role these processes play in sulfur release from the subducting slab and sulfur subduction into the deep mantle.

Accelerated reduction and solubilization of elemental sulfur by 1,2-aminothiols.

Nucleophilic 1,2-aminothiol compounds readily reduce typically-insoluble elemental sulfur to polysulfides in both water and nonpolar organic solvents. The resulting anionic polysulfide species are stabilized through hydrogen-bonding interactions with the proximal amine moieties. These interactions can facilitate sulfur transfer to alkenes.

TRNA Modifications as a Readout of S and Fe-S Metabolism.

Iron-Sulfur (Fe-S) clusters function as core prosthetic groups known to modulate the activity of metalloenzymes, act as trafficking vehicles for biological iron and sulfur, and participate in several intersecting metabolic pathways. The formation of these clusters is initiated by a class of enzymes called cysteine desulfurases, whose primary function is to shuttle sulfur from the amino acid L-cysteine to a variety of sulfur transfer proteins involved in Fe-S cluster synthesis as well as in the synthesis of other thiocofactors. Thus, sulfur and Fe-S cluster metabolism are connected through shared enzyme intermediates, and defects in their associated pathways cause a myriad of pleiotropic phenotypes, which are difficult to dissect. Post-transcriptionally modified transfer RNA (tRNA) represents a large class of analytes whose synthesis often requires the coordinated participation of sulfur transfer and Fe-S enzymes. Therefore, these molecules can be used as biologically relevant readouts for cellular Fe and S status. Methods employing LC-MS technology provide a valuable experimental tool to determine the relative levels of tRNA modification in biological samples and, consequently, to assess genetic, nutritional, and environmental factors modulating reactions dependent on Fe-S clusters. Herein, we describe a robust method for extracting RNA and analytically evaluating the degree of Fe-S-dependent and -independent tRNA modifications via an LC-MS platform.

Study on the production of low sulfur petroleum coke

ABSTRACT High-quality low sulfur petroleum coke (LSPC) (sulfur content <0.5 wt.%) can be used to produce graphite electrode, which has high market demand and economic value. In this work, the sulfur distribution of delayed coking (DC) reaction of vacuum residue (VR) and ebullated-bed hydrotreating unconverted oil (UCO) was studied. The effects of different hydrotreating catalysts and different hydrotreating depths on the sulfur transfer of the coking reaction of UCO were investigated. The economic index of producing low sulfur petroleum coke was put forward, and the equation for predicting the sulfur content of petroleum coke was established. According to the economic index, it is suitable to use the catalyst with high desulfurization performance and low ring-opening performance to hydrogenate VR to produce LSPC. Meanwhile, it is not economical to adopt higher hydrogenation depth. According to the prediction equation, for UCO with a Conradson carbon residue (CCR) content of 25 wt.%, its sulfur content should be less than 0.25 wt% to produce high-quality low sulfur petroleum coke.

Influence of Electro-Emulsification on Desulfurization of Rejected Electrolytic Manganese Metal in Electroslag Remelting Process

A transient three-dimensional comprehensive numerical model was developed to clarify the effect of electro-emulsification on sulfur mass transfer during the electroslag remelting process of rejected electrolytic manganese metal scrap. The magnetohydrodynamic thermal two-phase flow and solidification behavior were numerically clarified. The kinetics module was established to assess the sulfur mass transfer rate between molten manganese metal and molten slag. Furthermore, variations of the alternating current (AC) magnitude and direction were taken into account. A reasonable agreement between the experimental and simulated results was observed. The surface area of molten droplets was shown to enlarge and it regularly fluctuates under the Lorentz force action. At the constant frequency of 50 Hz, the desulfurization rate rises from 54.7 to 71.0 pct when the AC root mean square (RMS) value of the AC is increased from 3000 to 4000 A, while at the constant AC RMS value of 3500 A, it dropped from 63.4 to 58.5 pct with the frequency increases from 5 to 500 Hz.

Sulfur isotope behavior during metamorphism and anatexis of Archean sedimentary rocks: A case study from the Ghost Lake batholith, Ontario, Canada

Recycling of surface-derived sulfur into the deep earth can impart distinct sulfur isotope signatures to magmas. The details of sulfur transfer from sedimentary rocks to magmas (and ultimately igneous rocks) through metamorphism and devolatilization and/or partial melting, however, is difficult to trace. To understand this process in detail we studied multiple-sulfur isotope compositions of sulfides in the Archean (c. 2685 Ma) Ghost Lake batholith (GLB) and its surrounding host metasedimentary rocks of the Superior Craton (Ontario, Canada) by high spatial resolution secondary ion mass spectrometry, complemented by high-precision gas source isotope ratio mass spectrometry measurements. The GLB comprises strongly peraluminous biotite+cordierite, biotite+muscovite, and muscovite+garnet+tourmaline granites to leucogranites, which are thought to represent the partial melts of surrounding metagreywackes and metapelites. The metasedimentary rocks display a range of metamorphic grades increasing from biotite-chlorite (280-380°C) at ∼5 km away from the GLB to sillimanite-K-feldspar grade (∼660°C) immediately adjacent to the batholith, thus providing a natural experiment to understand sulfur isotope variations from low- to high-grade Archean sedimentary rocks, as well as granites representative of their partial melts.We find that metasedimentary sulfide δ34S values increase with progressive metamorphism at most 2-3‰ (from −1‰ up to +1 to +2‰). An increase in δ34S values in pyrrhotite during prograde metamorphism can be explained through Rayleigh fractionation during pyrite desulfidation reactions. Pyrite from all but one of the granite samples preserve δ34S values similar to that of the high-grade metasedimentary rocks, indicating that partial melting did not result in significant fractionation of δ34S. The exception to this is one granite sample from a part of the batholith characterized by abundant metasedimentary inclusions. This sample contains pyrite with heterogeneous and low δ34S values (down to −16‰) which likely resulted from incomplete homogenization of sulfur between the granitic melt and metasedimentary inclusions. Small (several tenths of a permil), mostly positive Δ33S are observed in both the metasedimentary rocks and granites.Our results suggest that Archean strongly peraluminous granites could be a high-fidelity archive to quantify the bulk sulfur isotope composition of the Archean siliciclastic sediments. Further, our findings indicate that subduction of reduced sulfur-bearing sediments in the Archean with δ34S at or near 0‰ should result in release of sulfur-bearing fluids in the mantle wedge with similar values (within a few permil). S-MIF (if initially present in Archean surface material) may be preserved during this process. However, the absence of S-MIF in igneous rocks does not preclude assimilation of Archean sedimentary material as either S-MIF may not be originally present in the Archean sedimentary sulfur and/or homogenization or dilution could obscure any S-MIF originally present in assimilated Archean sediments.

Mechanochemical Synthesis of Short DNA Fragments.

We demonstrate the first mechanochemical synthesis of DNA fragments by ball milling, enabling the synthesis of oligomers of controllable sequence and length using multi-step, one-pot reactions, without bulk solvent or the need to isolate intermediates. Mechanochemistry allowed for coupling of phosphoramidite monomers to the 5'-hydroxyl group of nucleosides, iodine/water oxidation of the resulting phosphite triester linkage, and removal of the 5'-dimethoxytrityl (DMTr) protecting group in situ in good yields (up to 60 % over three steps) to produce DNA dimers in a one-pot manner. H-Phosphonate chemistry under milling conditions enabled coupling and protection of the H-phosphonate linkage, as well as removal of the 5'-DMTr protecting group in situ, enabling a one-pot process with good yields (up to 65 % over three steps, or ca. 87 % per step). Sulfurization of the internucleotide linkage was possible using elemental sulfur (S8) or sulfur transfer reagents, yielding the target DNA phosphorothioate dimers in good yield (up to 80 % over two steps). This work opens the door to creation of solvent-free synthesis methodologies for DNA and RNA therapeutics.

In situ determination of sulfur speciation and partitioning in aqueous fluid-silicate melt systems

Current knowledge of sulfur behaviour in magmas is based exclusively on ex situ analyses. Here we report the first in situ measurement of sulfur speciation and partitioning between coexisting aqueous fluids and silicate melts. These data were acquired using Raman spectroscopy in a diamond anvil cell at 700 °C, 0.3–1.5 GPa, and oxygen fugacity in the vicinity of the sulfide-sulfate transition, conditions relevant to subduction zone magmatism. Results show that sulfate and sulfide are dominant in the studied systems, together with the and radical ions that are absent in quenched fluid and silicate glass products. The derived fluid/melt partition coefficients for sulfide and sulfate are consistent with those from available ex situ data for shallow crust conditions (<10 km), but indicate stronger partitioning of these sulfur species into the silicate melt phase with increasing depth. In contrast, both radical ions partition at least 10 times more than sulfate and sulfide into the fluid phase. Thus, by enhancing the transfer of sulfur and associated chalcophile metals from melt into fluid upon magma degassing, and may be important players in the formation of economic metal resources within the redox window of the sulfate-sulfide transition in subduction zone settings.

Sulfur and Oxygen Effects on High-Si Ductile Iron Casting Skin Formation

The main objective of the present paper is to evaluate by thermal (cooling curve) analysis the solidification pattern and the occurrence of the layer of degenerate graphite at the surface of ductile iron castings (3.15% Si, typically as 450-18 grade, ISO 1563/2011), with or without a mold coating, including S or O, and different agents (carbonic material, iron powder), supposed to act to block their diffusion into the iron melt. It is found that the mold coating materials temperately influence the parameters of the solidification cooling curves and, more visibly, the occurrence and the thickness of the undesired skin layer. Different graphite morphologies comparable to the casting body are present, at a large range of thicknesses, from 50 up to 200 µm. The sulfur presence in the mold coating will promote a higher skin thickness compared to oxygen (up to 50% by oxygen and 2.5–3.3 times for sulfur action), despite the fact that in the casting body, the graphite nodularity undergoes a limited decrease (from 85% up to 82%–83% level). Carbonic material or iron powder supplementary addition decreases these undesired effects, but the solidification undercooling compared to the equilibrium system is increased. It is found that carbonic material is more efficient at limiting oxygen than iron powder is at limiting the negative effects of sulfur on the casting skin thickness. More experiments are necessary to quantify their capacity to block the oxygen or sulfur transfer into the iron melt.

Genomic sequence analysis of Dissulfurirhabdus thermomarina SH388 and proposed reassignment to Dissulfurirhabdaceae fam. nov.

Here, we report the draft genome sequence of Dissulfurirhabdus thermomarina SH388. Improved phylogenetic and taxonomic analysis of this organism using genome-level analyses supports assignment of this organism to a novel family within the phylum Desulfobacterota. Additionally, comparative genomic and phylogenetic analyses contextualize the convergent evolution of sulfur disproportionation and potential extracellular electron transfer in this organism relative to other members of the Desulfobacterota.

Biosynthesis of Sulfur-Containing Small Biomolecules in Plants.

Sulfur is an essential element required for plant growth. It can be found as a thiol group of proteins or non-protein molecules, and as various sulfur-containing small biomolecules, including iron-sulfur (Fe/S) clusters, molybdenum cofactor (Moco), and sulfur-modified nucleotides. Thiol-mediated redox regulation has been well investigated, whereas biosynthesis pathways of the sulfur-containing small biomolecules have not yet been clearly described. In order to understand overall sulfur transfer processes in plant cells, it is important to elucidate the relationships among various sulfur delivery pathways as well as to investigate their interactions. In this review, we summarize the information from recent studies on the biosynthesis pathways of several sulfur-containing small biomolecules and the proteins participating in these processes. In addition, we show characteristic features of gene expression in Arabidopsis at the early stage of sulfate depletion from the medium, and we provide insights into sulfur transfer processes in plant cells.

Flotation mechanism of sulphide melt on vapour bubbles in partially molten magmatic systems

The motion of dense Fe-rich immiscible sulphide liquids is generally supposed to be dominated by gravitational settling in crustal magma chambers, however they may become buoyant by attachment to low-density vapour bubbles to form compound drops, possibly contributing to the upward transfer of sulphur and transition metals in degassing magma bodies. Here, using numerical models, we consider constraints on the flotation of compound drops, and find a wide range of morphologies that would be expected to be capable of migrating through magmatic systems, including both melt- and mush-dominated domains, but also some situations where the compound drops will spontaneously separate to allow the vapour to rise while the sulphide phase is sequestered in the mush. The stability of compound drops is empirically related to a function f(Bo)=|σMV−σMS|σMS⋅1/BoS+|σVS−σMS|σMS⋅1/BoV, where M, V, and S denote silicate melt, vapour, and sulphide liquid, σ is surface tension (N⋅m−1), and Bo is the Bond number. Over a range of plausible conditions in magmas, f(Bo) must exceed values of 3 to 4 for capillary forces to overcome buoyancy forces tending to pull them apart. Using published thermodynamic models for vapour and sulphide solubility in silicate melts, we show that in many magmas the second boiling of vapour occurs synchronously with the saturation of sulphide liquid, increasing the potential for coupling of vapour and sulphide droplets. Diffusive coarsening produces mm-sized bubbles within 100-500 years, generating enough buoyancy to trigger the migration of compound drops. During compaction-driven expulsion of interstitial melt, compound drops can pass through constrictions between crystals in mush. The flotation of sulphide-vapour aggregates is likely to occur in crustal magma reservoirs and provides a feasible mechanism for the removal of sulphide liquid from crystal mushes, promoting its ability to participate in the generation of magmatic sulphide and porphyry copper deposits, emissions of the metals during volcanic eruptions, and even the remobilization of chalcophile metals sequestered in deep arc cumulates to generate porphyry Cu-(Au) deposits.

Magnetically recoverable ferromagnetic 3D hierarchical core-shell Fe3O4@NiO/Co3O4 microspheres as an efficient and ligand-free catalyst for C–S bond formation in poly (ethylene glycol)

ABSTRACT A simple and efficient protocol for the synthesis of diaryl thioethers from the reaction of thiourea with a wide variety of aryl halides, including aryl iodides, aryl bromides and aryl chlorides in the presence of 3D hierarchical core-shell Fe3O4@NiO/Co3O4 microspheres has been described. This reaction enables the one-pot synthesis of diaryl thioethers in good to high yields using a non-toxic and magnetically separable catalyst in PEG-400 as an eco-friendly, safe, inexpensive and thermally stable solvent. Magnetic separation and reusability of catalyst for eight times without any significant loss of activity, the use of a commercially available, eco-friendly, cheap and chemically stable sulfur transfer agent and solvent, operational simplicity, environmentally benign, easier work-up procedure and cost efficiency make this method a promising candidate for potential applications in some organic reactions. The catalytic activity of Fe3O4@NiO/Co3O4 as a novel and inexpensive catalyst was investigated in the C-S cross coupling reaction.

Thioredoxin regulates human mercaptopyruvate sulfurtransferase at physiologically-relevant concentrations

3-Mercaptopyruvate sulfur transferase (MPST) catalyzes the desulfuration of 3-mercaptopyruvate (3-MP) and transfers sulfane sulfur from an enzyme-bound persulfide intermediate to thiophilic acceptors such as thioredoxin and cysteine. Hydrogen sulfide (H2S), a signaling molecule implicated in many physiological processes, can be released from the persulfide product of the MPST reaction. Two splice variants of MPST, differing by 20 amino acids at the N terminus, give rise to the cytosolic MPST1 and mitochondrial MPST2 isoforms. Here, we characterized the poorly-studied MPST1 variant and demonstrated that substitutions in its Ser-His-Asp triad, proposed to serve a general acid-base role, minimally affect catalytic activity. We estimated the 3-MP concentration in murine liver, kidney, and brain tissues, finding that it ranges from 0.4 μmol·kg-1 in brain to 1.4 μmol·kg-1 in kidney. We also show that N-acetylcysteine, a widely-used antioxidant, is a poor substrate for MPST and is unlikely to function as a thiophilic acceptor. Thioredoxin exhibits substrate inhibition, increasing the KM for 3-MP ∼15-fold compared with other sulfur acceptors. Kinetic simulations at physiologically-relevant substrate concentrations predicted that the proportion of sulfur transfer to thioredoxin increases ∼3.5-fold as its concentration decreases from 10 to 1 μm, whereas the total MPST reaction rate increases ∼7-fold. The simulations also predicted that cysteine is a quantitatively-significant sulfane sulfur acceptor, revealing MPST's potential to generate low-molecular-weight persulfides. We conclude that the MPST1 and MPST2 isoforms are kinetically indistinguishable and that thioredoxin modulates the MPST-catalyzed reaction in a physiologically-relevant concentration range.

The [4Fe-4S] cluster of sulfurtransferase TtuA desulfurizes TtuB during tRNA modification in Thermus thermophilus

TtuA and TtuB are the sulfurtransferase and sulfur donor proteins, respectively, for biosynthesis of 2-thioribothymidine (s2T) at position 54 of transfer RNA (tRNA), which is responsible for adaptation to high temperature environments in Thermus thermophilus. The enzymatic activity of TtuA requires an iron-sulfur (Fe-S) cluster, by which a sulfur atom supplied by TtuB is transferred to the tRNA substrate. Here, we demonstrate that the Fe-S cluster directly receives sulfur from TtuB through its inherent coordination ability. TtuB forms a [4Fe-4S]-TtuB intermediate, but that sulfur is not immediately released from TtuB. Further desulfurization assays and mutation studies demonstrated that the release of sulfur from the thiocarboxylated C-terminus of TtuB is dependent on adenylation of the substrate tRNA, and the essential residue for TtuB desulfurization was identified. Based on these findings, the molecular mechanism of sulfur transfer from TtuB to Fe-S cluster is proposed.