Isolation and characterization of MOD5, a gene required for isopentenylation of cytoplasmic and mitochondrial tRNAs of Saccharomyces cerevisiae.

The mod5-1 mutation is a nuclear mutation in Saccharomyces cerevisiae that reduces the biosynthesis of N6-(delta 2-isopentenyl)adenosine in both cytoplasmic and mitochondrial tRNAs to less than 1.5% of wild-type levels. The tRNA modification enzyme, delta 2-isopentenyl pyrophosphate:tRNA isopentenyl transferase, cannot be detected in vitro with extracts from mod5-1 cells. A characterization of the MOD5 gene would help to determine how the same enzyme activity in different cellular compartments can be abolished by a single nuclear mutation. To that end we have cloned the MOD5 gene and shown that it restores delta 2-isopentenyl pyrophosphate:tRNA isopentenyl transferase activity and N6-(delta 2-isopentenyl)adenosine to tRNA in both the mitochondria and the nucleus/cytoplasm compartments of mod5-1 yeast cells. That MOD5 sequences are expressed in Escherichia coli and can complement an N6-(delta 2-isopentenyl)-2-methylthioadenosine-deficient E. coli mutant leads us to conclude that MOD5 is the structural gene for delta 2-isopentenyl pyrophosphate:tRNA isopentenyl transferase.     

Identification of yeast tRNA Um(44) 2'-O-methyltransferase (Trm44) and demonstration of a Trm44 role in sustaining levels of specific tRNA(Ser) species

A characteristic feature of tRNAs is the numerous modifications found throughout their sequences, which are highly conserved and often have important roles. Um(44) is highly conserved among eukaryotic cytoplasmic tRNAs with a long variable loop and unique to tRNA(Ser) in yeast. We show here that the yeast ORF YPL030w (now named TRM44) encodes tRNA(Ser) Um(44) 2'-O-methyltransferase. Trm44 was identified by screening a yeast genomic library of affinity purified proteins for activity and verified by showing that a trm44-delta strain lacks 2'-O-methyltransferase activity and has undetectable levels of Um(44) in its tRNA(Ser) and by showing that Trm44 purified from Escherichia coli 2'-O-methylates U(44) of tRNA(Ser) in vitro. Trm44 is conserved among metazoans and fungi, consistent with the conservation of Um(44) in eukaryotic tRNAs, but surprisingly, Trm44 is not found in plants. Although trm44-delta mutants have no detectable growth defect, TRM44 is required for survival at 33 degrees C in a tan1-delta mutant strain, which lacks ac(4)C12 in tRNA(Ser) and tRNA(Leu). At nonpermissive temperature, a trm44-delta tan1-delta mutant strain has reduced levels of tRNA(Ser(CGA)) and tRNA(Ser(UGA)), but not other tRNA(Ser) or tRNA(Leu) species. The trm44-delta tan1-delta growth defect is suppressed by addition of multiple copies of tRNA(Ser(CGA)) and tRNA(Ser(UGA)), directly implicating these tRNA(Ser) species in this phenotype. The reduction of specific tRNA(Ser) species in a trm44-delta tan1-delta mutant underscores the importance of tRNA modifications in sustaining tRNA levels and further emphasizes that tRNAs undergo quality control.     

On the biogenesis of cytokinins in roots of Phaseolus vulgaris

Roots of intact bean plants were supplied with [¹⁴C]adenine by pulse-chase experiments. The rate of incorporation of radioactivity into tRNA and oligonucleotides of roots as well as the content of radioactive labeled cytokinin nucleotides in these RNA fractions were determined. On the average, 1/70 of the radioactivity incorporated into tRNA was localized in N⁶(²isopentenyl)adenosine. The half life of tRNA was estimated to be 65–70 h. Shortly after the pulse period, oligonucleotides contained zeatin riboside at a ratio of 1:800, on the basis of radioactivity. The half life of these oligonucleotides was determined to be about 8 h. The main free radioactive cytokinin of roots and leaves was zeatin. Comparing the rate of degradation of ¹⁴C-labeled tRNA and the oligonucleotides of roots and the rate of appearance of radioactive cytokinins in roots and leaves, we found strong indications for their dependency. The results contradict the hypothesis of de novo synthesis of cytokinins in roots of intact bean plants.Abbreviations AMP adenosine monophosphate - IPA N⁶(²isopentenyl)adenosine - IPAde N⁶(²isopentenyl)adenosine - Z zeatin - ZR zeatinriboside - TLC thin-layer chromatography - HPLC high performance liquid chromatography Part of the doctoral thesis, Bonn 1980     

Temperature jump relaxation studies on the interactions between transfer RNAs with complementary anticodons. The effect of modified bases adjacent to the anticodon triplet

We have used the temperature-jump relaxation technique to determine the kinetic and thermodynamic parameters for the association between the following tRNAs pairs having complementary anticodons: tRNA(Ser) with tRNA(Gly), tRNA(Cys) with tRNA(Ala) and tRNA(Trp) with tRNA(Pro). The anticodon sequence of E. coli tRNA(Ser), GGA, is complementary to the U*CC anticodon of E. coli tRNA(Gly(2] (where U* is a still unknown modified uridine base) and A37 is not modified in none of these two tRNAs. E. coli tRNA(Ala) has a VGC anticodon (V is 5-oxyacetic acid uridine) while tRNA(Cys) has the complementary GCA anticodon with a modified adenine on the 3' side, namely 2-methylthio N6-isopentenyl adenine (mS2i6A37) in E. Coli tRNA(Cys) and N6-isopentenyl adenine (i6A37) in yeast tRNA(Cys). The brewer yeast tRNA(Trp) (anticodon CmCA) differs from the wild type E. coli tRNA(Trp) (anticodon CCA) in several positions of the nucleotide sequence. Nevertheless, in the anticodon loop, only two interesting differences are present: A37 is not modified while C34 at the first anticodon position is modified into a ribose 2'-O methyl derivative (Cm). The corresponding complementary tRNA is E.coli tRNA(Pro) with the VGG anticodon. Our results indicate a dominant effect of the nature and sequence of the anticodon bases and their nearest neighbor in the anticodon loop (particularly at position 37 on the 3' side); no detectable influence of modifications in the other tRNA stems has been detected. We found a strong stabilizing effect of the methylthio group on i6A37 as compared to isopentenyl modification of the same residue. We have not been able so far to assess the effect of isopentenyl modification alone in comparison to unmodified A37. The results obtained with the complex yeast tRNA(Trp)-E.coli tRNA(Pro) also suggest that a modification of C34 to Cm34 does not significantly increase the stability of tRNA(Trp) association with its complementary anticodon in tRNA(Pro). The observations are discussed in the light of inter- and intra-strand stacking interactions among the anticodon triplets and with the purine base adjacent to them, and of possible biological implications.     

Natural occurrence of an inhibitor of mammalian cell growth in human and mouse cells of normal and tumor origin.

N6-(delta2-isopentenyl)adenosine was found both as a component of tRNA and as the cytoplasmic mononucleotide in human leukemic lymphoblasts and myeloblasts from peripheral blood and bone marrow samples. This hypermodified nucleotide was also found in the tRNA and as a mononucleotide in human (MRC-5 and KB) and mouse (A9, FLV, LM, and RAG) cell lines. The relative amounts of this hypermodified nucleotide in the tRNA of the cell lines and the human leukemias were similar (the mean value being 0.06 +/- 0.03 mole % of the total tRNA nucleotide content); whereas the amounts occurring as the free cytoplasmic mononucleotide were more varied but still comparable (the mean value being 0.53 +/- .09 mole % of all cytoplasmic nucleotides) for all cells investigated with the notable exception of all normal, diploid cell lines under study (0.04 mole%). A possible relationship of the free cytoplasmic mononucleotide with the nucleotide in the tRNA for control of mammalian cell protein synthesis in vivo was investigated by addition of N6-(delta2-isopentenyl)adenosine to the culture medium. The exogenously added nucleoside caused inhibition of cell growth within 3 h and cell death within 36 h at concentrations as low as 0.4 muM. No comparable effects were seen when adenosine, adenine, or N6-(delta2-isopentenyl)-adenine were added to the cultures. The simultaneous presence of adenosine in cultures containing N6-(delta2-isopentenyl)adenosine did not alter the detrimental effects of the hypermodified nucleoside on cell growth even when the concentration of adenosine was 50-fold that of N6-(delta2-isopentenyl)adenosine. Addition of N6-(delta2-isopentenyl)adenosine to cell cultures caused within the first 6 h a significant reduction in the rates of RNA and protein synthesis; whereas DNA synthesis continued at a rate comparable to control and adenosine-treated cells for 18 h before decreasing.     

Antibodies to N6-(delta2-isopentenyl) adenosine and its nucleotide: interaction with purified tRNAs and with bases, nucleosides and nucleotides of the isopentenyladenosine family.

The interaction of antibodies directed toward N6-(delta2-isopentenyl)adenosine, i6Ado, or its nucleotide with related bases, nucleosides, nucleotides and purified tRNAs is described. The selectivity of the antibody preparation was tested in inhibition experiments utilizing a sensitive radioimmunoassay to quantitate the binding of [3H]i6Ado to the antibody. Purified tRNAs containing various modified nucleosides adjacent to the 3'-end of the anticodon were tested to provide information about the selectivity of the antibody preparation toward nucleotides in this position of the tRNA chain. Antibodies directed against the nucleotide hapten were used to purify tRNAs which contain i6Ado and to quantitate the amount of that nucleotide. The same order of selectivity was expressed whether the nucleotides were free or in a tRNA molecule. Interaction of the antibody with compounds from the i6Ado family demonstrated dominance of the hydrophobic isopentenyl group and the importance of positional differences of modifications.     

Function of Modified Nucleosides 7-Methylguanosine, Ribothymidine, and 2-Thiomethyl-N6-(Isopentenyl)adenosine in Procaryotic Transfer Ribonucleic Acid

To elucidate subtle functions of transfer ribonucleic acid (tRNA) modifications in protein synthesis, pairs of tRNA's that differ in modifications at specific positions were prepared from Bacillus subtilis. The tRNA's differ in modifications in the anticodon loop, the extra arm, and the TUC loop. The functional properties of these species were compared in aminoacylation, as well as in initiation and peptide bond formation, at programmed ribosomes. These experiments demonstrated the following. (i) In tRNA(f) (Met) the methylation of guanosine 46 in the extra arm to 7-methylguanosine by the 7-methylguanosine-forming enzyme from Escherichia coli changes the aminoacylation kinetics for the B. subtilis methionyl-tRNA synthetase. In repeated experiments the V(max) value is decreased by one-half. (ii) tRNA(f) (Met) species with ribothymidine at position 54 (rT54) or uridine at position 54 (U54) were obtained from untreated or trimethoprim-treated B. subtilis. The formylated fMet-tRNA(f) (Met) species with U54 and rT54, respectively, function equally well in an in vitro initiation system containing AUG, initiation factors, and 70s ribosomes. The unformylated Met-tRNA(t) (Met) species, however, differ from each other: "Met-tRNA(f) (Met) rT" is inactive, whereas the U54 counter-upart effectively forms the initiation complex. (iii) Two isoacceptors, tRNA(1) (Phe) and tRNA(2) (Phe), were obtained from B. subtilis. tRNA(1) (Phe) accumulates only under special growth conditions and is an incompletely modified precursor oftRNA(2) (Phe): in the first position of the anticodon, guanosine replaces Gm, and next to the 3' end of the anticodon (isopentenyl)adenosine replaces 2-thiomethyl-N(6)-(isopentenyl)adenosine. Both tRNA's behave identically in aminoacylation kinetics. In the factor-dependent AUGU(3)-directed formation of fMet-Phe, the undermodified tRNA(1) (Phe) is always less efficient at Mg(2+) concentrations between 5 and 15 mM than its mature counterpart.     

Immunospecific binding of tRNA precursors containing N6-(delta 2-isopentenyl) adenosine

Antibodies specific for N6-(delta 2-isopentenyl) adenosine (i6A) were immobilized on Sepharose and this adsorbent (Sepharose-anti-i6A) was used to selectively isolate bacteriophage T4 tRNA precursors containing i6A/ms2i6A from an unfractionated population of 32P-labeled T4 RNAs. The results showed that antibodies to i6A selectively bound only those tRNA precursors containing i6A/ms2i6A. Binding of tRNA precursors by antibody and specificity of the binding was assessed by membrane binding using 32P-labeled tRNA precursor. Binding was highly specific for i6A/ms2i6A residues in the tRNA precursors. This binding can be used to separate modified from unmodified precursor RNAs and to study the biosynthetic pathways of tRNA precursors.     

The ms2io6A37 Modification of tRNA in Salmonella typhimurium Regulates Growth on Citric Acid Cycle Intermediates

The modified nucleoside 2-methylthio-N-6-isopentenyl adenosine (ms²i⁶A) is present in position 37 (adjacent to and 3′ of the anticodon) of tRNAs that read codons beginning with U except tRNA _I,V^Ser in Escherichia coli. In Salmonella typhimurium, 2-methylthio-N-6-(cis-hydroxy)isopentenyl adenosine (ms²io⁶A; also referred to as 2-methylthio cis-ribozeatin) is found in tRNA, most likely in the species that have ms²i⁶A in E. coli. Mutants (miaE) of S. typhimurium in which ms²i⁶A hydroxylation is blocked are unable to grow aerobically on the dicarboxylic acids of the citric acid cycle. Such mutants have normal uptake of dicarboxylic acids and functional enzymes of the citric acid cycle and the aerobic respiratory chain. The ability of S. typhimurium to grow on succinate, fumarate, and malate is dependent on the state of modification in position 37 of those tRNAs normally having ms²io⁶A37 and is not due to a second cellular function of tRNA (ms²io⁶A37)hydroxylase, the miaE gene product. We suggest that S. typhimurium senses the hydroxylation status of the isopentenyl group of the tRNA and will grow on succinate, fumarate, or malate only if the isopentenyl group is hydroxylated.     

Rapid tRNA decay can result from lack of nonessential modifications

The biological role of many nonessential tRNA modifications outside of the anticodon remains elusive despite their evolutionary conservation. We show here that m7G46 methyltransferase Trm8p/Trm82p acts as a hub of synthetic interactions with several tRNA modification enzymes, resulting in temperature-sensitive growth. Analysis of three double mutants indicates reduced levels of tRNA(Val(AAC)), consistent with a role of the corresponding modifications in maintenance of tRNA levels. Detailed examination of a trm8-delta trm4-delta double mutant demonstrates rapid degradation of preexisting tRNA(Val(AAC)) accompanied by its de-aminoacylation. Multiple copies of tRNA(Val(AAC)) suppress the trm8-delta trm4-delta growth defect, directly implicating this tRNA in the phenotype. These results define a rapid tRNA degradation (RTD) pathway that is independent of the TRF4/RRP6-dependent nuclear surveillance pathway. The degradation of an endogenous tRNA species at a rate typical of mRNA decay demonstrates a critical role of nonessential modifications for tRNA stability and cell survival.     

Methyldeficient mammalian 4S RNA: evidence for L-ethionine-induced inhibition of N6-dimethyladenosine synthesis in rat liver tRNA

The nucleotide composition of 4s RNA from livers of rats fed with a diet containing 0.3% D-ethionine was found to be identical with that from untreated animals. In contrast, one single modified nucleotide was absent in 4s RNA from livers of rats fed with a 0.3% L-ethionine diet. The minor nucleo=tide was also absent in liver 4s RNA from rats fed with a 0.3% L-ethionine diet followed by ten days of normal food. It was identified after dephosphorylation by ultraviolet absorption spectra, cochromatography with authentic material and mass spectra as N(6)-dimethyladenosine. It is concluded that S-adenosylethionine, the primary product of L-ethionine in the liver, causes strong and selective inhibition of the specific RNA-methylase responsible for adenosine to N(6)-dimethyl=adenosine methylation in rat liver 4s RNA. Compared to the strong inhibition of N(6)-dimethyladenosine formation described here, L-ethionine-dependent ethylation of liver 4s RNA is far less efficient. The quantitation of l-methyladenosine, ribothymidine and 3'-terminal adenosine in this 4s RNA as well as its aminoacid acceptor activity is typical for tRNA; hence it may be concluded that N(6)-dimethyladenosine is a component of rat liver tRNA. This may demonstrate the first evidence for the existence of specifically methyl-deficient mammalian tRNA. A possible correlation between the activity of L-ethionine as a liver carcinogen and its ability to induce the formation of methyl-deficient tRNA by selectively inhibiting the synthesis of N(6)-dimethyladenosine on the tRNA level in the same organ is discussed.     

Characierization of monoclonal antibodies specific for isopentenyl adenosine derivatives occurring in transfer RNA

A family of isopentenyl adenosine derivatives are naturally occurring components of transfer RNA and are involved in several different functional roles in the cell. To facilitate the study of the biochemistry of these modified nucleosides we have raised monoclonal antibodies to N⁶-(Δ²-isopentenyl)adenosine and N⁶-(4-hydroxy-3-methyl-but-2-enyl)adenosine. The antibodies show considerable specificity and three characteristic types are distinguishable. The first type have the hydroxylated derivative as the preferred antigen, the second type have isopentenyl adenosine as the preferred antigen and a third type show a specificity for all isopentenyl-containing derivatives.     

Localization of an oligodeoxynucleotide complementing 16S ribosomal RNA residues 520-531 on the small subunit of Escherichia coli ribosomes: electron microscopy of ribosome-cDNA-antibody complexes.

The oligodeoxynucleotide dACCGCGGCTGCT, complementary to Escherichia coli small ribosomal subunit RNA residues 520-531, has been used to probe subunit conformation and to localize the sequence in the subunit. Conditions for binding of the cDNA to 30S subunits were optimized and specificity of the interaction was demonstrated by RNase H cleavage. Three kinds of terminal modification of this cDNA were used to allow its localization by immune electron microscopy. A solid phase support with 5'-dimethoxytrity-N6-delta 2-isopentenyl-adenosine linked to controlled pore glass was synthesized, and used to prepare oligomer with an added 3'-terminal residue of isopentenyl adenosine. cDNA with a 5' primary amine substituent was modified with 1-fluoro-2,4-dinitrobenzene to prepare 5'-dinitrophenyl oligonucleotide, and both modifications together gave doubly-derivatized probes. Immune electron microscopy with antibodies to dinitrophenol, isopentenyl adenosine, or both, was used to place the cDNA on 30S subunits. In each case the probe was placed at a single site at the junction of the head and body of the subunit, near the decoding site and the area in which elongation factor Tu is bound. It is proposed that this segment of ribosomal RNA functions in mRNA binding and orientation.     

Studies on chemical modification of thionucleosides in the transfer ribonucleic acid of Escherichia coli

(35)S-labelled tRNA from Escherichia coli was treated with chemical reagents such as CNBr, H(2)O(2), NH(2)OH, I(2), HNO(2), KMnO(4) and NaIO(4), under mild conditions where the four major bases were not affected. Gel filtration of the treated tRNA showed desulphurization to various extents, depending on the nature of the reagent. The treated samples after conversion into nucleosides were chromatographed on a phosphocellulose column. NH(2)OH, I(2) and NaIO(4) reacted with all the four thionucleosides of E. coli tRNA, 4-thiouridine (s(4)U), 5-methylaminomethyl-2-thiouridine (mnm(5)s(2)U), 2-thiocytidine (s(2)C) and 2-methylthio-N(6)-isopentenyladenosine (ms(2)i(6)A), to various extents. CNBr, HNO(2) and NaHSO(3) reacted with s(4)U, mnm(5)s(2)U and s(2)C, but not with ms(2)i(6)A. KMnO(4) and H(2)O(2) were also found to react extensively with thionucleosides in tRNA. Iodine oxidation of (35)S-labelled tRNA showed that only 6% of the sulphur was involved in disulphide formation. Desulphurization of E. coli tRNA with CNBr resulted in marked loss of acceptor activities for glutamic acid, glutamine and lysine. Acceptor activities for alanine, arginine, glycine, isoleucine, methionine, phenylalanine, serine, tyrosine and valine were also affected, but to a lesser extent. Five other amino acids tested were almost unaffected. These results indicate the fate of thionucleosides in tRNA when subjected to various chemical reactions and the involvement of sulphur in aminoacyl-tRNA synthetase recognition of some tRNA species of E. coli.     

Selective inhibition of selenocysteine tRNA maturation and selenoprotein synthesis in transgenic mice expressing isopentenyladenosine-deficient selenocysteine tRNA

Selenocysteine (Sec) tRNA (tRNA([Ser]Sec)) serves as both the site of Sec biosynthesis and the adapter molecule for donation of this amino acid to protein. The consequences on selenoprotein biosynthesis of overexpressing either the wild type or a mutant tRNA([Ser]Sec) lacking the modified base, isopentenyladenosine, in its anticodon loop were examined by introducing multiple copies of the corresponding tRNA([Ser]Sec) genes into the mouse genome. Overexpression of wild-type tRNA([Ser]Sec) did not affect selenoprotein synthesis. In contrast, the levels of numerous selenoproteins decreased in mice expressing isopentenyladenosine-deficient (i(6)A(-)) tRNA([Ser]Sec) in a protein- and tissue-specific manner. Cytosolic glutathione peroxidase and mitochondrial thioredoxin reductase 3 were the most and least affected selenoproteins, while selenoprotein expression was most and least affected in the liver and testes, respectively. The defect in selenoprotein expression occurred at translation, since selenoprotein mRNA levels were largely unaffected. Analysis of the tRNA([Ser]Sec) population showed that expression of i(6)A(-) tRNA([Ser]Sec) altered the distribution of the two major isoforms, whereby the maturation of tRNA([Ser]Sec) by methylation of the nucleoside in the wobble position was repressed. The data suggest that the levels of i(6)A(-) tRNA([Ser]Sec) and wild-type tRNA([Ser]Sec) are regulated independently and that the amount of wild-type tRNA([Ser]Sec) is determined, at least in part, by a feedback mechanism governed by the level of the tRNA([Ser]Sec) population. This study marks the first example of transgenic mice engineered to contain functional tRNA transgenes and suggests that i(6)A(-) tRNA([Ser]Sec) transgenic mice will be useful in assessing the biological roles of selenoproteins.     

The role of plant growth regulators on direct and indirect plant regeneration from various organs of <Emphasis Type="Italic">Leucaena leucocephala</Emphasis>

Prolific differentiation of shoot buds of Leucaena leucocephala was induced from the different plant parts viz. cotyledon, hypocotyl and leaf. Adventitious shoot bud formation was recorded with prudent application of N⁶-2- (isopentenyl) adenosine and 15% (v/v) coconut water. Coconut water alone was unable to produce any beneficial effect with regard to the shoot bud proliferation but the response was augmented with the increase in concentration of N⁶-2- (isopentenyl) adenosine. However supra-optimal level of N⁶-2-(isopentenyl) adenosine was inhibitory. Best response was recorded from the cotyledon explant at 2 mg dm⁻³ N⁶-2-(isopentenyl) adenosine compared to the other two explants. Comparative assessment was undertaken following the same experimental protocol in liquid shake culture. The regenerated shoot buds were subcultured in plant growth regulator-free medium where leafy shoot emergence was recorded. Optimum regeneration of roots was observed in these shoots in presence of 1 mg dm⁻³ α-naphthalene acetic acid. Plantlets were finally hardened following standard procedures before transplantation to the field. In another experimental set up, the de-embryonated cotyledons regenerated shoot buds via callus formation. The regenerated shoots and plantlets obtained through callus mediated organogenesis could be used for rapid multiplication and also for the genetic improvement of individual clones of Leucaena leucocephala.     

Synthesis and function of isopentenyl adenosine derivatives in tRNA

Isopentenyl adenosine derivatives can be found next to the anticodon (position 37) in tRNA from both the Bacteria and Eucarya domains. These modified nucleosides improve the efficiency of tRNA in translation, can increase and decrease translational fidelity, and make the tRNA less codon context sensitive. In bacteria the synthesis of isopentenyl adenosine derivatives seems to be linked to iron metabolism and central metabolic pathways.     

Maturation of a hypermodified nucleoside in transfer RNA.

E. coli C6 rel- met- cys- was cultured in a fully supplemented medium and in media lacking cysteine or methionine. tRNA isolated from the three cultures containted, respectively, a normal complement of modified nucleosides; a deficiency in thiolated nucleosides and a deficiency in methylated nucleosides. Both sulfur-deficient tRNA and methyl-deficient tRNA contained large amounts of N-6- (delta-2-isopentenyl) adenosine and small amounts of the 2-methylthio derivative. Methyl-deficient tRNA contained, in addition a large amount of a cytokinin active, differently modified nucleoside that is believed to be a sulfur derivative of N6-(delta-2-isopentenyl) adenosine. The structure of this compound is unknown. When methly-deficient tRNA and the precusor the tRNA-Tyr su3-+ A25 were enzymatically methylated in vitro, methyl groups were incorporated into derivatives of isopentenyladenosine. These results indicate that the biosynthesis of the 2-methylthio derivative of isopentenyladenosine may occur in a sequential manner, i.e., thiolation of isopentenyladenosine followed by methylation.     

A novel lysine-substituted nucleoside in the first position of the anticodon of minor isoleucine tRNA from Escherichia coli

A minor species of isoleucine tRNA (tRNA(minor Ile)) specific to the codon AUA has been isolated from Escherichia coli B and a modified nucleoside N+ has been found in the first position of the anticodon (Harada, F., and Nishimura, S. (1974) Biochemistry 13, 300-307). In the present study, tRNA(minor Ile)) was purified from E. coli A19, and nucleoside N+ was prepared, by high-performance liquid chromatography, in an amount (0.6) A260 units) sufficient for the determination of chemical structures. By 400 MHz 1H NMR analysis, nucleoside N+ was found to have a pyrimidine moiety and a lysine moiety, the epsilon amino group of which was involved in the linkage between these two moieties. From the NMR analysis together with mass spectrometry, the structure of nucleoside N+ was determined as 4-amino-2-(N6-lysino)-1-(beta-D-ribofuranosyl)pyrimidinium ("lysidine"), which was confirmed by chemical synthesis. Lysidine is a novel type of modified cytidine with a lysine moiety and has one positive charge. Probably because of such a unique structure, lysidine in the first position of anticodon recognizes adenosine but not guanosine in the third position of codon.