Occurrence of selenium-containing tRNAs in mouse leukemia cells

Selenium Distribution and Speciation in Tissues from Rats Administered with Non-Native Selenotrisulfides

Selenotrisulfides (STS, R-S-Se-S-R) are metabolic intermediates in the bioconversion of inorganic Se species to organoselenium compounds. These Se species are reactive with a variety of endogenous molecules, particularly thiol-containing proteins, with this reactivity facilitating Se transport and subsequent utilization within the body. In this study, X-ray fluorescence microscopy (XFM) and high energy resolution fluorescence detected X-ray absorption spectroscopy (HERFD-XAS) were applied to investigate Se distribution and speciation in vivo. Male rats administered with 1 mg Se/kg b.w. as selenious acid (SA), L-penicillamine selenotrisulfide (PenSSeSPen) or selenenyl penicillamine bound to rat serum albumin (RSA-SSeSPen) showed statistically significant elevations in Se concentrations in the kidney, liver, and blood after 48 h treatment; however, no change in Se concentration was observed in the testes. Notably, XFM revealed a strong colocalization of Se and Cu in the renal cortex, a phenomenon previously observed in cultured cells and in rats fed diets supplemented with 5 mg Se/kg as selenite. Linear combination and principal component analyses of Se Kα1 HERFD-XAS spectra revealed marked differences in Se speciation between the renal cortex and medulla and between red blood cells and plasma for all groups, including the control. STS were identified in linear combination fits of spectra from all tissues, except the testes. These results highlight the vital roles of STS in the intracellular reduction and transport of Se throughout the bloodstream and various tissues.

C5-Substituted 2-Selenouridines Ensure Efficient Base Pairing with Guanosine; Consequences for Reading the NNG-3' Synonymous mRNA Codons

5-Substituted 2-selenouridines (R5Se2U) are post-transcriptional modifications present in the first anticodon position of transfer RNA. Their functional role in the regulation of gene expression is elusive. Here, we present efficient syntheses of 5-methylaminomethyl-2-selenouridine (1, mnm5Se2U), 5-carboxymethylaminomethyl-2-selenouridine (2, cmnm5Se2U), and Se2U (3) alongside the crystal structure of the latter nucleoside. By using pH-dependent potentiometric titration, pKa values for the N3H groups of 1–3 were assessed to be significantly lower compared to their 2-thio- and 2-oxo-congeners. At physiological conditions (pH 7.4), Se2-uridines 1 and 2 preferentially adopted the zwitterionic form (ZI, ca. 90%), with the positive charge located at the amino alkyl side chain and the negative charge at the Se2-N3-O4 edge. As shown by density functional theory (DFT) calculations, this ZI form efficiently bound to guanine, forming the so-called “new wobble base pair”, which was accepted by the ribosome architecture. These data suggest that the tRNA anticodons with wobble R5Se2Us may preferentially read the 5′-NNG-3′ synonymous codons, unlike their 2-thio- and 2-oxo-precursors, which preferentially read the 5′-NNA-3′ codons. Thus, the interplay between the levels of U-, S2U- and Se2U-tRNA may have a dominant role in the epitranscriptomic regulation of gene expression via reading of the synonymous 3′-A- and 3′-G-ending codons.

Does the Selenium (SE) Level and Se-Dependent Enzyme Activity in Blood Plasma Correlate with Human Lymphocyte Subpopulations and Function?

Recent literature data on the effects of Se on subpopulations of T lymphocytes, on autologous mixed lymphocyte reaction (AMLR) and on natural killer (NK) cell cytotoxicity are limited or poorly defined. In healthy volunteers we have estimated se levels, glutathione peroxidase (GSH-Px) activity and lipid peroxide levels in human plasma and simultaneously, the subpopulations of T lymphocytes, proliferation in AMLR, and activity of NK cells.

We found a significantly positive correlation between the selenium level and GSH-Px activity. The proliferative response in AMLR significantly correlated with plasma selenium levels but not with GSH-Px activity. NK cytotoxicity, subpopulations of T lymphocytes, and lipid peroxide levels did not correlate with both selenium concentration and GSH-Px activity. We suppose that the effect of Se on the proliferation of suppressor T lymphocytes (Ts) in AMLR is not mediated through GSH-Px activity and fluctuations of Se concentration within a physiological range in healthy persons do not affect NK cytotoxicity.

Why Nature Chose Selenium

The authors were asked by the Editors of ACS Chemical Biology to write an article titled "Why Nature Chose Selenium" for the occasion of the upcoming bicentennial of the discovery of selenium by the Swedish chemist Jöns Jacob Berzelius in 1817 and styled after the famous work of Frank Westheimer on the biological chemistry of phosphate [Westheimer, F. H. (1987) Why Nature Chose Phosphates, Science 235, 1173-1178]. This work gives a history of the important discoveries of the biological processes that selenium participates in, and a point-by-point comparison of the chemistry of selenium with the atom it replaces in biology, sulfur. This analysis shows that redox chemistry is the largest chemical difference between the two chalcogens. This difference is very large for both one-electron and two-electron redox reactions. Much of this difference is due to the inability of selenium to form π bonds of all types. The outer valence electrons of selenium are also more loosely held than those of sulfur. As a result, selenium is a better nucleophile and will react with reactive oxygen species faster than sulfur, but the resulting lack of π-bond character in the Se-O bond means that the Se-oxide can be much more readily reduced in comparison to S-oxides. The combination of these properties means that replacement of sulfur with selenium in nature results in a selenium-containing biomolecule that resists permanent oxidation. Multiple examples of this gain of function behavior from the literature are discussed.

Selenomodification of tRNA in archaea requires a bipartite rhodanese enzyme

5-Methylaminomethyl-2-selenouridine (mnm(5)Se(2)U) is found in the first position of the anticodon in certain tRNAs from bacteria, archaea and eukaryotes. This selenonucleoside is formed in Escherichia coli from the corresponding thionucleoside mnm(5)S(2)U by the monomeric enzyme YbbB. This nucleoside is present in the tRNA of Methanococcales, yet the corresponding 2-selenouridine synthase is unknown in archaea and eukaryotes. Here we report that a bipartite ybbB ortholog is present in all members of the Methanococcales. Gene deletions in Methanococcus maripaludis and in vitro activity assays confirm that the two proteins act in trans to form in tRNA a selenonucleoside, presumably mnm(5)Se(2)U. Phylogenetic data suggest a primal origin of seleno-modified tRNAs.

tRNA structural and functional changes induced by oxidative stress

Oxidatively damaged biomolecules impair cellular functions and contribute to the pathology of a variety of diseases. RNA is also attacked by reactive oxygen species, and oxidized RNA is increasingly recognized as an important contributor to neurodegenerative complications in humans. Recently, evidence has accumulated supporting the notion that tRNA is involved in cellular responses to various stress conditions. This review focuses on the intriguing consequences of oxidative modification of tRNA at the structural and functional level.

Genetic analysis of selenocysteine biosynthesis in the archaeon Methanococcus maripaludis

In Archaea selenocysteine (Sec) is synthesized in three steps. First seryl-tRNA synthetase acylates tRNA(Sec) with serine to generate Ser-tRNA(Sec). Then phosphoseryl-tRNA(Sec) kinase (PSTK) forms Sep-tRNA(Sec) , which is converted to Sec-tRNA(Sec) by Sep-tRNA:Sec-tRNA synthase (SepSecS) in the presence of selenophosphate produced by selenophosphate synthetase (SelD). A complete in vivo analysis of the archaeal Sec biosynthesis pathway is still unavailable, and the existence of a redundant pathway or of a rescue mechanism based on the conversion of Sep-tRNA(Sec) to Cys-tRNA(Sec) during selenium starvation, cannot be excluded. Here we present a mutational analysis of Sec biosynthesis in Methanococcus maripaludis strain Mm900. Sec formation is abolished upon individually deleting the genes encoding SelD, PSTK or SepSecS; the resulting mutant strains could no longer grow on formate while growth with H(2) + CO(2) remained unaffected. However, deletion of the PSTK and SepSecS genes was not possible unless the selenium-free [NiFe]-hydrogenases Frc and Vhc were expressed. This required the prior deletion of either the gene encoding SelD or that of HrsM, a LysR-type regulator suppressing transcription of the frc and vhc operons in the presence of selenium. These results show that M. maripaludis Mm900 is facultatively selenium-dependent with a single pathway of Sec-tRNA(Sec) formation.

Biochemistry of selenium-derivatized naturally occurring and unnatural nucleic acids

Selenium (Se) can provide unique biochemical and biological functions, and properties to macromolecules, including protein and RNA. Although Se has not yet been found in DNA, identification of the presence of Se in natural tRNAs has led to discovery of the naturally occurring 2-selenouridine and 5-[(methylamino)methyl]-2-selenouridine (mnm(5)se(2)U). The Se-atoms at C(2) of the modified uridines are introduced by 2-selenouridine synthase via displacement of the S-atoms in the corresponding 2-thiouridine nucleotides of the tRNAs, and selenophosphate is used as the Se donor. The research indicated that mnm(5)se(2)U is located at the first or wobble position of the anticodons in several bacterial tRNAs, including tRNA(Lys), tRNA(Glu), and tRNA(Gln). The 2-seleno functionality on this modified nucleotide probably improves the translation accuracy and/or efficiency. These observations in vivo suggest that the presence of Se can provide natural RNAs with useful properties to better function and survival. To further investigate the biochemical and structural properties of Se-derivatized nucleic acids (SeNA), we have pioneered chemical and enzymatic synthesis of Se-derivatized nucleic acids, and introduced Se into both RNA and DNA at a variety of positions by atom-specific replacement of oxygen. This review outlines the recent advancements in chemical and biochemical syntheses, and studies of SeNAs, and their potential applications in structural and functional investigation of nucleic acids and their protein complexes.

Trace 5-methylaminomethyl-2-selenouridine in bovine tRNA and the selenouridine synthase activity in bovine liver

We measured the amount of Se in bovine liver tRNA. tRNA was chromatographed on a BD-cellulose column and Se-rich tRNA was eluted from the column in front of a main tRNA peak. There was 0.3 mmol Se/mol of tRNA and this level is about one tenth that of Escherichia coli tRNA. This suggests the presence of an enzyme that modifies tRNA with Se in bovine liver. We isolated the activity of this enzyme (selenouridine synthase) by chromatography of bovine liver extracts on a DEAE-cellulose column. ATP and selenophosphate synthetase, as well as selenouridine synthase and tRNA, were necessary for the reaction. 75Se was used to label the reaction products, which were analyzed by TLC after digestion with ribonuclease T2. The position of the 75Se-nucleotide on a TLC plate was identical to that of the Se-nucleotide, 5-methylaminomethyl-2-seleno-Up, prepared from 75Se-tRNA in E. coli.

Selenocysteine-containing proteins in mammals

Since the recent discovery of selenocysteine as the 21st amino acid in protein, the field of selenium biology has rapidly expanded. Twelve mammalian selenoproteins have been characterized to date and each contains selenocysteine that is incorporated in response to specific UGA code words. These selenoproteins have different cellular functions, but in those selenoproteins for which the function is known, selenocysteine is located at the active center. The presence of selenocysteine at critical sites in naturally occurring selenoproteins provides an explanation for the important role of selenium in human health and development. This review describes known mammalian selenoproteins and discusses recent developments and future directions in the selenium field.

In vitro effects of sodium selenite on nuclear 3,5,3'-triiodothyronine (T3) receptor gene expression in rat pituitary GH4C1 cells

The present study was undertaken in order to investigate the effects of sodium selenite on: 1. The growth of rat pituitary GH4C1 cells; 2. The nuclear T3 receptor gene expression; 3. The cytoplasmic protein phosphorylation; and 4. The prolactin secretion in rat pituitary GH4C1 cell line. Sodium selenite (up to 2.5 microM) has no inhibitory effect on GH4C1 cell proliferation as well as the prolactin secretion. On the other hand, 0.5 microM sodium selenite significantly decreases the rate of mRNA synthesis and/or degradation of both, the alpha 1 form of the T3 receptor (TR alpha 1) and the alpha 2 isoform of the T3 receptor. At 1 microM of sodium selenite, significant changes in the electrophoretic profile of low molecular mass cytoplasmic proteins were found, moreover, sodium selenite (1 microM) also considerably affects phosphorylation of a higher molecular mass proteins. The results based on the in vitro experiments suggest that sodium selenite may affect specific processes at the pretranslational level as well as it may also take part in processes of posttranslational modification of protein(s), the cell vitality and the cell growth remaining unchanged.

Summary: the modified nucleosides of RNA

A comprehensive listing is made of posttranscriptionally modified nucleosides from RNA reported in the literature through mid-1994. Included are chemical structures, common names, symbols, Chemical Abstracts registry numbers (for ribonucleoside and corresponding base), Chemical Abstracts Index Name, phylogenetic sources, and initial literature citations for structural characterization or occurrence, and for chemical synthesis. The listing is categorized by type of RNA: tRNA, rRNA, mRNA, snRNA, and other RNAs. A total of 93 different modified nucleosides have been reported in RNA, with the largest number and greatest structural diversity in tRNA, 79; and 28 in rRNA, 12 in mRNA, 11 in snRNA and 3 in other small RNAs.

Recognition of the mRNA selenocysteine insertion sequence by the specialized translational elongation factor SELB

In Escherichia coli the unusual amino acid selenocysteine is incorporated cotranslationally at an in-frame UGA codon. Incorporation of selenocysteine relies, in part, on the interaction between a specialized elongation factor, the SELB protein, and a cis-acting element within the mRNA. Boundary and toeprint experiments illustrate that the SELB-GTP-Sec-tRNA(Sec) ternary complex binds to the selenoprotein encoding mRNAs fdhF and fdnG, serving to increase the concentration of SELB and Sec-tRNA(Sec) on these mRNAs in vivo. Moreover, toeprint experiments indicate that SELB recognizes the ribosome-bound message and that, upon binding, SELB may protrude out of the ribosomal-mRNA track so as to approach the large ribosomal subunit. The results place the mRNA-bound SELB-GTP-Sec-tRNA(Sec) ternary complex at the selenocysteine codon (as expected) and suggest a mechanism to explain the specificity of selenocysteine insertion. Cis-acting mRNA regulatory elements can tether protein factors to the translation complex during protein synthesis.

Biosynthesis of selenium-modified tRNAs in Methanococcus vannielii

Selenium-containing nucleosides are natural components of several tRNA species in Methanococcus vannielii. In the present study, the incorporation of selenium from 75SeO3(2-) into these macromolecules was investigated in sonic extracts of M. vannielii. Nucleoside analysis of the 75Se-labeled tRNAs from these in vitro reaction mixtures demonstrated that the selenium was present in 75Se-labeled nucleosides identical to the two naturally occurring 2-selenouridines produced in vivo. Incorporation of selenium into these nucleosides was ATP-dependent and was maximal after 20 min. Addition of O-acetylserine enhanced the activity 2- to 3-fold, implicating a role for selenocysteine in the reaction. Added L-selenocysteine could function as a selenium donor, but the D isomer and DL-selenomethionine were inactive. RPC-5 chromatography of bulk tRNA isolated from M. vannielii grown on 75SeO3(2-) separated five major species of seleno-tRNAs. The amino acid-accepting activity of these tRNAs was investigated.

In vitro incorporation of selenium into tRNAs of Salmonella typhimurium

Broken-cell preparations of Salmonella typhimurium rapidly incorporated 75Se from 75SeO3(2-) into tRNA by an ATP-dependent process. Selenium incorporation in the presence of 50 microM 75SeO3(2-) (0.8-1 pmol per A260 unit) was enhanced by the selenocysteine precursor, O-acetyl-L-serine (to 3.7 pmol per A260 unit). This increase in incorporation was a function of O-acetyl-L-serine concentration. Neither O-acetyl-L-homoserine nor O-phospho-L-serine stimulated the incorporation of selenium into tRNA. The incorporation of 75Se from 75SeO3(2-) was decreased by adding L-selenocysteine but not by adding the D isomer. When homologous bulk tRNA was added to the broken-cell preparations, an increased rate of 75Se labeling was observed. The supernatant fraction of the broken-cell preparation contained all of the enzymes required for this process. Reversed-phase HPLC analysis of labeled bulk tRNA digested to nucleosides showed the presence of a labeled compound that coeluted with authentic 5-methylaminomethyl-2-selenouridine.

Class-specific effects of selenium on PWM-driven human antibody synthesis in vitro

The influence of inorganic and organic forms of selenium (Se) on human antibody production was studied in a Pokeweed Mitogen (PWM)-driven in vitro system. Mitogen-stimulated peripheral blood mononuclear cells (PBMC) of eight healthy donors were cultured with different Se compounds at concentrations between 10(-3) and 10(-9) M. At high Se levels (10(-3)-10(-4) M), IgM and IgG production of all donors were strongly inhibited owing to reduced cell viability. However, in five of eight donors, low levels of Se enhanced IgG secretion. This was most effective in the presence of inorganic Se, whereas selenomethionine and selenocystine were less effective. In contrast to IgG, IgM synthesis was significantly reduced by low Se levels in five donors. No significant correlation between donor serum Se levels and antibody production in vitro was found. The addition of low levels of Se to PBMC, stimulated with PHA or PWM, showed no effect on proliferation, whereas a high concentration (5 x 10(-3) M) of sodium selenite and selenocystine suppressed proliferation owing to reduced cell viability. Thus, the present results show that Se supplementation can enhance human antibody production and, moreover, suggest some selectivity of Se action on human immune responses that may result in increased switching from IgM to IgG production.

The conversion of phosphoserine residues to selenocysteine residues on an opal suppressor tRNA and casein

This study has been undertaken in order to elucidate the mechanisms of incorporation of Se into glutathione peroxidase (GSHPx), in which selenocysteine corresponds to the opal termination codon UGA on the mRNA. We studied the above mechanisms using an opal suppressor tRNA, prepared from bovine liver, and casein as a model protein for the GSHPx apo-enzyme which might contain phosphoserine. The results showed that opal suppressor tRNA did not accept selenocysteine (lower than 0.1 mmol/mol) under the standard conditions. A trace amount of phosphoseryl-tRNA was converted to selenocysteyl-tRNA by incubation with H2Se and some enzymes. Meanwhile, a number of phosphoserine residues in casein were converted to selenocysteine residues by incubation with H2Se and enzymes. These results suggest that opal suppressor tRNA plays a role in synthesizing GSHPx via co- and/or post-translational mechanisms.

A selenium-containing nucleoside at the first position of the anticodon in seleno-tRNAGlu from Clostridium sticklandii

In previous studies, the single selenonucleoside component of a selenium-containing tRNAGlu isolated from Clostridium sticklandii has been shown to be 5-methyl-aminomethyl-2-selenouridine. Here, we show that this selenonucleoside is most likely located at the "wobble" position of the anticodon of the clostridial seleno-tRNAGlu. Nuclease T1 digestion of this seleno-tRNAGlu generated one major selenium-containing oligonucleotide (25 bases long). The selenium-containing residue within this oligonucleotide was located by sequence analysis of the oligonucleotide before and after removal of selenium by treatment with cyanogen bromide. The sequence of this oligonucleotide, A-A-C-C-G-C-C-C-U-U+-U-C-A+C-G-G-C-G-G-U-A-A-C-A-G, is homologous to that of the Escherichia coli tRNAGlu2 from residues 27 to 50, including the anticodon region and the variable loop, except that the E. coli tRNA has 5-methylaminomethyl-2-thiouridine instead of the selenonucleoside.