The place of tachykinin NK2 receptor antagonists in the treatment diarrhea-predominant irritable bowel syndrome.

Tachykinins act as neurotransmitters and neuromodulators in the central and peripheral nervous system. Preclinical studies and clinical trials showed that inhibition of the tachykinin receptors, mainly NK2 may constitute a novel attractive option in the treatment of irritable bowel syndrome (IBS). In this review, we focused on the role of tachykinins in physiology and pathophysiology of gastrointestinal (GI) tract. Moreover, we summed up recent data on tachykinin receptor antagonists in the therapy of IBS. Ibodutant is a novel drug with an interesting pharmacological profile, which exerted efficacy in women with diarrhea-predominant IBS (IBS-D) in phase II clinical trials. The promising results were not replicable and confirmed in phase III of clinical trials. Ibodutant is not ready to be introduced in the pharmaceutical market and further studies on alternative NK2 antagonist are needed to make NK2 antagonists useful tools in IBS-D treatment.

Modified nucleoside dependent Watson-Crick and wobble codon binding by tRNALysUUU species.

Nucleoside modifications are important to the structure of all tRNAs and are critical to the function of some tRNA species. The transcript of human tRNA(Lys3)(UUU) with a UUU anticodon, and the corresponding anticodon stem and loop domain (ASL(Lys3)(UUU)), are unable to bind to poly-A programmed ribosomes. To determine if specific anticodon domain modified nucleosides of tRNA(Lys) species would restore ribosomal binding and also affect thermal stability, we chemically synthesized ASL(Lys) heptadecamers and site-specifically incorporated the anticodon domain modified nucleosides pseudouridine (Psi(39)), 5-methylaminomethyluridine (mnm(5)U(34)) and N6-threonylcarbamoyl-adenosine (t(6)A(37)). Incorporation of t(6)A(37) and mnm(5)U(34) contributed structure to the anticodon loop, apparent by increases in DeltaS, and significantly enhanced the ability of ASL(Lys3)(UUU) to bind poly-A programmed ribosomes. Neither ASL(Lys3)(UUU)-t(6)A(37) nor ASL(Lys3)(UUU)-mnm(5)U(34) bound AAG programmed ribosomes. Only the presence of both t(6)A(37) and mnm(5)U(34) enabled ASL(Lys3)(UUU) to bind AAG programmed ribosomes, as well as increased its affinity for poly-A programmed ribosomes to the level of native Escherichia coli tRNA(Lys). The completely unmodified anticodon stem and loop of human tRNA(Lys1,2)(CUU) with a wobble position-34 C bound AAG, but did not wobble to AAA, even when the ASL was modified with t(6)A(37). The data suggest that tRNA(Lys)(UUU) species require anticodon domain modifications in the loop to impart an ordered structure to the anticodon for ribosomal binding to AAA and require a combination of modified nucleosides to bind AAG.

Functional anticodon architecture of human tRNALys3 includes disruption of intraloop hydrogen bonding by the naturally occurring amino acid modification, t6A.

The structure of the human tRNA(Lys3) anticodon stem and loop domain (ASL(Lys3)) provides evidence of the physicochemical contributions of N6-threonylcarbamoyladenosine (t(6)A(37)) to tRNA(Lys3) functions. The t(6)A(37)-modified anticodon stem and loop domain of tRNA(Lys3)(UUU) (ASL(Lys3)(UUU)- t(6)A(37)) with a UUU anticodon is bound by the appropriately programmed ribosomes, but the unmodified ASL(Lys3)(UUU) is not [Yarian, C., Marszalek, M., Sochacka, E., Malkiewicz, A., Guenther, R., Miskiewicz, A., and Agris, P. F., Biochemistry 39, 13390-13395]. The structure, determined to an average rmsd of 1.57 +/- 0.33 A (relative to the mean structure) by NMR spectroscopy and restrained molecular dynamics, is the first reported of an RNA in which a naturally occurring hypermodified nucleoside was introduced by automated chemical synthesis. The ASL(Lys3)(UUU)-t(6)A(37) loop is significantly different than that of the unmodified ASL(Lys3)(UUU), although the five canonical base pairs of both ASL(Lys3)(UUU) stems are in the standard A-form of helical RNA. t(6)A(37), 3'-adjacent to the anticodon, adopts the form of a tricyclic nucleoside with an intraresidue H-bond and enhances base stacking on the 3'-side of the anticodon loop. Critically important to ribosome binding, incorporation of the modification negates formation of an intraloop U(33).A(37) base pair that is observed in the unmodified ASL(Lys3)(UUU). The anticodon wobble position U(34) nucleobase in ASL(Lys3)(UUU)-t(6)A(37) is significantly displaced from its position in the unmodified ASL and directed away from the codon-binding face of the loop resulting in only two anticodon bases for codon binding. This conformation is one explanation for ASL(Lys3)(UUU) tendency to prematurely terminate translation and -1 frame shift. At the pH 5.6 conditions of our structure determination, A(38) is protonated and positively charged in ASL(Lys3)(UUU)-t(6)A(37) and the unmodified ASL(Lys3)(UUU). The ionized carboxylic acid moiety of t(6)A(37) possibly neutralizes the positive charge of A(+)(38). The protonated A(+)(38) can base pair with C(32), but t(6)A(37) may weaken the interaction through steric interference. From these results, we conclude that ribosome binding cannot simply be an induced fit of the anticodon stem and loop, otherwise the unmodified ASL(Lys3)(UUU) would bind as well as ASL(Lys3)(UUU)-t(6)A(37). t(6)A(37) and other position 37 modifications produce the open, structured loop required for ribosomal binding.

Synthesis and properties of uniquely modified oligoribonucleotides: yeast tRNA(Phe) fragments with 6-methyluridine and 5,6-dimethyluridine at site-specific positions.

The phosphoramidites of 6-methyluridine and 5,6-dimethyluridine were synthesized and the modified uridines site-selectively incorporated into heptadecamers corresponding in sequence to the yeast tRNA(Phe) anticodon and TpsiC domains. The oligoribonucleotides were characterized by NMR, MALDI-TOF MS and UV-monitored thermal denaturations. The 6-methylated uridines retained the syn conformation at the polymer level and in each sequence location destabilized the RNAs compared to that of the unmodified RNA. The decrease in RNA duplex stability is predictable. However, loss of stability when the modified uridine is in a loop is sequence context dependent, and can not, at this time, be predicted from the location in the loop.

Modified constructs of the tRNA TPsiC domain to probe substrate conformational requirements of m(1)A(58) and m(5)U(54) tRNA methyltransferases.

The TPsiC stem and loop (TSL) of tRNA contains highly conserved nucleoside modifications, m(5)C(49), T(54), Psi(55)and m(1)A(58). U(54)is methylated to m(5)U (T) by m(5)U(54)methyltransferase (RUMT); A(58)is methylated to m(1)A by m(1)A(58)tRNA methyltransferase (RAMT). RUMT recognizes and methylates a minimal TSL heptadecamer and RAMT has previously been reported to recognize and methylate the 3'-half of the tRNA molecule. We report that RAMT can recognize and methylate a TSL heptadecamer. To better understand the sensitivity of RAMT and RUMT to TSL conformation, we have designed and synthesized variously modified TSL constructs with altered local conformations and stabilities. TSLs were synthesized with natural modifications (T(54)and Psi(55)), naturally occurring modifications at unnatural positions (m(5)C(60)), altered sugar puckers (dU(54)and/or dU(55)) or with disrupted U-turn interactions (m(1)Psi(55)or m(1)m(3)Psi(55)). The unmodified heptadecamer TSL was a substrate of both RAMT and RUMT. The presence of T(54)increased thermal stability of the TSL and dramatically reduced RAMT activity toward the substrate. Local conformation around U(54)was found to be an important determinant for the activities of both RAMT and RUMT.

Role of modified nucleosides of yeast tRNA(Phe) in ribosomal binding.

Naturally occurring nucleoside modifications are an intrinsic feature of transfer RNA (tRNA), and have been implicated in the efficiency, as well as accuracy-of codon recognition. The structural and functional contributions of the modified nucleosides in the yeast tRNA(Phe) anticodon domain were examined. Modified nucleosides were site-selectively incorporated, individually and in combinations, into the heptadecamer anticodon stem and loop domain, (ASL(Phe)). The stem modification, 5-methylcytidine, improved RNA thermal stability, but had a deleterious effect on ribosomal binding. In contrast, the loop modification, 1-methylguanosine, enhanced ribosome binding, but dramatically decreased thermal stability. With multiple modifications present, the global ASL stability was mostly the result of the individual contributions to the stem plus that to the loop. The effect of modification on ribosomal binding was not predictable from thermodynamic contributions or location in the stem or loop. With 4/5 modifications in the ASL, ribosomal binding was comparable to that of the unmodified ASL. Therefore, modifications of the yeast tRNA(Phe) anticodon domain may have more to do with accuracy of codon reading than with affinity of this tRNA for the ribosomal P-site. In addition, we have used the approach of site-selective incorporation of specific nucleoside modifications to identify 2'O-methylation of guanosine at wobble position 34 (Gm34) as being responsible for the characteristically enhanced chemical reactivity of C1400 in Escherichia coli 16S rRNA upon ribosomal footprinting of yeast tRNA(Phe). Thus, effective ribosome binding of tRNA(Phe) is a combination of anticodon stem stability and the correct architecture and dynamics of the anticodon loop. Correct tRNA binding to the ribosomal P-site probably includes interaction of Gm34 with 16S rRNA C1400.

Thermodynamic contribution of nucleoside modifications to yeast tRNA(Phe) anticodon stem loop analogs.

The determination of the structural and functional contributions of natural modified nucleosides to tRNA has been limited by lack of an approach that can systematically incorporate the modified units. We have produced a number of oligonucleotide analogs, of the anticodon of yeast tRNA(Phe) by, combining standard automated synthesis for the major nucleosides with specialty chemistries for the modified nucleosides. In this study, both naturally occurring and unnatural modified nucleotides were placed in native contexts. Each oligonucleotide was purified and the nucleoside composition determined to validate the chemistry. The RNAs were denatured and analyzed to determine the van't Hoff thermodynamic parameters. Here, we report the individual thermodynamic contributions for Cm, Gm, m1G, m5C, psi. In addition m5m6U, m1psi, and m3psi, were introduced to gain additional understanding of the physicochemical contribution of psi and m5C at an atomic level. These oligonucleotides demonstrate that modifications have measurable thermodynamic contributions and that loop modifications have global contributions.

Structural and functional roles of the N1- and N3-protons of psi at tRNA's position 39.

Pseudouridine at position 39 (Psi(39)) of tRNA's anticodon stem and loop domain (ASL) is highly conserved. To determine the physicochemical contributions of Psi(39)to the ASL and to relate these properties to tRNA function in translation, we synthesized the unmodified yeast tRNA(Phe)ASL and ASLs with various derivatives of U(39)and Psi(39). Psi(39)increased the thermal stability of the ASL (Delta T (m)= 1.3 +/- 0.5 degrees C), but did not significantly affect ribosomal binding ( K (d)= 229 +/- 29 nM) compared to that of the unmodified ASL (K (d)= 197 +/- 58 nM). The ASL-Psi(39)P-site fingerprint on the 30S ribosomal subunit was similar to that of the unmodified ASL. The stability, ribosome binding and fingerprint of the ASL with m(1)Psi(39)were comparable to that of the ASL with Psi(39). Thus, the contribution of Psi(39)to ASL stability is not related to N1-H hydrogen bonding, but probably is due to the nucleoside's ability to improve base stacking compared to U. In contrast, substitutions of m(3)Psi(39), the isosteric m(3)U(39)and m(1)m(3)Psi(39)destabilized the ASL by disrupting the A(31)-U(39)base pair in the stem, as confirmed by NMR. N3-methylations of both U and Psi dramatically decreased ribosomal binding ( K (d)= 1060 +/- 189 to 1283 +/- 258 nM). Thus, canonical base pairing of Psi(39)to A(31)through N3-H is important to structure, stability and ribosome binding, whereas the increased stability and the N1-proton afforded by modification of U(39)to Psi(39)may have biological roles other than tRNA's binding to the ribosomal P-site.

A distinctive RNA fold: the solution structure of an analogue of the yeast tRNAPhe T Psi C domain.

The structure of an analogue of the yeast tRNAPhe T Psi C stem-loop has been determined by NMR spectroscopy and restrained molecular dynamics. The molecule contained the highly conserved modification ribothymidine at its naturally occurring position. The ribothymidine-modified T Psi C stem-loop is the product of the m5U54-tRNA methyltransferase, but is not a substrate for the m1A58-tRNA methyltransferase. Site-specific substitutions and 15N labels were used to confirm the assignment of NOESY cross-peaks critical in defining the global fold of the molecule. The structure is unusual in that the loop folds far over into the major groove of the curved stem. This conformation is stabilized by both stacking interactions and hydrogen bond formation. Furthermore, this conformation appears to be unique among RNA hairpins of similar size. There is, however, a considerable resemblance to the analogous domain in the crystal structure of the full-length yeast tRNAPhe. We believe, therefore, that the structure we have determined may represent an intermediate in the folding pathway during the maturation of tRNA.

The uridine in "U-turn": contributions to tRNA-ribosomal binding.

"U-turns" represent an important class of structural motifs in the RNA world, wherein a uridine is involved in an abrupt change in the direction of the polynucleotide backbone. In the crystal structure of yeast tRNAPhe, the invariant uridine at position 33 (U33), adjacent to the anticodon, stabilizes the exemplar U-turn with three non-Watson-Crick interactions: hydrogen bonding of the 2'-OH to N7 of A35 and the N3-H to A36-phosphate, and stacking between C32 and A35-phosphate. The functional importance of each noncanonical interaction was determined by assaying the ribosomal binding affinities of tRNAPhe anticodon stem and loop domains (ASLs) with substitutions at U33. An unsubstituted ASL bound 30S ribosomal subunits with an affinity (Kd = 140+/-50 nM) comparable to that of native yeast tRNAPhe (Kd = 100+/-20 nM). However, the binding affinities of ASLs with dU-33 (no 2'-OH) and C-33 (no N3-H) were significantly reduced (2,930+/-140 nM and 2,190+/-300 nM, respectively). Surprisingly, the ASL with N3-methyluridine-33 (no N3-H) bound ribosomes with a high affinity (Kd = 220+/-20 nM). In contrast, ASLs constructed with position 33 uridine analogs in nonstacking, nonnative, and constrained conformations, dihydrouridine (C2'-endo), 6-methyluridine (syn) and 2'O-methyluridine (C3'-endo) had almost undetectable binding. The inability of ASLs with 6-methyluridine-33 and 2'O-methyluridine-33 to bind ribosomes was not attributable to any thermal instability of the RNAs. These results demonstrate that proton donations by the N3-H and 2'OH groups of U33 are not absolutely required for ribosomal binding. Rather, the results suggest that the overall uridine conformation, including a dynamic (C3'-endo > C2'-endo) sugar pucker, anti conformation, and ability of uracil to stack between C32 and A35-phosphate, are the contributing factors to a functional U-turn.

Single atom modification (O-->S) of tRNA confers ribosome binding.

Escherichia coli tRNALysSUU, as well as human tRNALys3SUU, has 2-thiouridine derivatives at wobble position 34 (s2U*34). Unlike the native tRNALysSUU, the full-length, unmodified transcript of human tRNALys3UUU and the unmodified tRNALys3UUU anticodon stem/loop (ASLLys3UUU) did not bind AAA- or AAG-programmed ribosomes. In contrast, the completely unmodified yeast tRNAPhe anticodon stem/loop (ASLPheGAA) had an affinity (Kd = 136+/-49 nM) similar to that of native yeast tRNAPheGmAA (Kd = 103+/-19 nM). We have found that the single, site-specific substitution of s2U34 for U34 to produce the modified ASLLysSUU was sufficient to restore ribosomal binding. The modified ASLLysSUU bound the ribosome with an affinity (Kd = 176+/-62 nM) comparable to that of native tRNALysSUU (Kd = 70+/-7 nM). Furthermore, in binding to the ribosome, the modified ASLLys3SUU produced the same 16S P-site tRNA footprint as did native E. coli tRNALysSUU, yeast tRNAPheGmAA, and the unmodified ASLPheGAA. The unmodified ASLLys3UUU had no footprint at all. Investigations of thermal stability and structure monitored by UV spectroscopy and NMR showed that the dynamic conformation of the loop of modified ASLLys3SUU was different from that of the unmodified ASLLysUUU, whereas the stems were isomorphous. Based on these and other data, we conclude that s2U34 in tRNALysSUU and in other s2U34-containing tRNAs is critical for generating an anticodon conformation that leads to effective codon interaction in all organisms. This is the first example of a single atom substitution (U34-->s2U34) that confers the property of ribosomal binding on an otherwise inactive tRNA.

Modified RNAs as potential drug targets.

Bleomycin (BLM) is a natural antibiotic that is effective in treatment of selected cancers. Although the exact therapeutic mechanism of bleomycin is not known, its target is thought to be a nucleic acid. Besides cleaving DNA, in vitro, Fe-bleomycin cleaves the anticodon of yeast tRNA(Phe) specifically. Using CD and fluorescence spectroscopy we have found that apo-bleomycin binds to synthetic RNA analogs of the anticodon of yeast tRNA(Phe) with an affinity similar to that previously reported for DNA. In order to understand BLM's selectivity, the role magnesium ions play in RNA recognition should be explained. Many RNA substrates for Fe-BLM, including yeast tRNA(Phe), are not cleaved by the drug when the Mg2+ concentration exceeds 1 mM. Competition experiments with anticodon analogs provide insight into the role of magnesium ions in RNA recognition by BLM. These simple modified RNAs may be useful as model systems for investigating BLM/RNA recognition and development of highly selective drugs toward RNA targets.

Unconventional structure of tRNA(Lys)SUU anticodon explains tRNA's role in bacterial and mammalian ribosomal frameshifting and primer selection by HIV-1.

Transfer RNA(Lys)SUU, with a 5-modified-2-thiouridine at wobble position 34, facilitates -1 frameshifts for correct translation of the Escherichia coli DNA polymerase gamma subunit and retroviral polymerases. Peptidyl-tRNA(Lys)SUU prematurely terminates translation more often than other tRNAs. In order to determine if the anticodon structures of bacterial and mammalian tRNA(Lys)SUU species explain these observations, oligonucleotides corresponding to the anticodon regions of mammalian and E. coli tRNA(Lys)SUU were synthesized and their physicochemical properties compared with that of E. coli tRNA(Glu)SUC. The anticodon region of tRNA(Lys)SUU was stabilized by an unusual interaction between the side chains of the 5-modified-s(2)U34 and N-6-threonylcarbamoyl-adenosine-37 (t(6)A37), a combination of modified nucleosides unique to tRNA(Lys)SUU species. This first observation of modified nucleoside side-chain interactions is analogous to the interactions of amino acid side chains in proteins. The tRNA(Lys)SUU anticodon structure was determined from NMR restraints on model oligonucleotides. With only two of three anticodon bases available for codon pairing, this unconventional anticodon structure is a reasonable explanation for the bacterial and mammalian tRNA(Lys)SUU tendency to frameshift. A two-out-of-three reading of coding triplets also explains the increased rate at which peptidyl-tRNA(Lys)SUU prematurely terminates translation. In addition, modified nucleoside interaction distorts the anticodon loop. The distorted loop is a possible structural determinant for the preferential selection of tRNA(Lys3)SUU as primer of HIV-1 reverse transcriptase in vivo.

Ribosomal binding of modified tRNA anticodons related to thermal stability.

The physicochemical contributions of modified nucleosides to tRNA functions are not well understood. In order to determine the contributions of specific modifications to tRNA stability as well as to ribosomal binding, ten variously modified yeast tRNA(Phe) anticodon stems and loops (tRNA(Phe)AC) were synthesized. Thermal denaturation studies on these synthetic 17mers show dramatic stabilization (or destabilization) by the presence of the various naturally occurring nucleoside modifications. Adapting a novel molecular biology approach (initially pioneered by Moazed and Noller), the interactions of these variously modified anticodons with the E. coli 16S rRNA "P-site" residues are being quantitated. The binding (affinity) constant (kD) of the tRNA(Phe)AC to the 8 of the ten 16S rRNA nucleosides that interact with tRNA and synthetic anticodons are being examined. We postulate that the "stabilizing" modifications (m1G37, psi 39, and m5C40) in the presence of an "open loop" will dramatically increase the binding affinity of the tRNA(Phe)AC to the 30S E. coli ribosomal subunit when compared to unmodified tRNA(Phe)AC. On the other hand, "destabilizing" modifications are expected to reduce the binding affinity of the tRNA(Phe)AC to the E. coli 30S ribosomal subunit. The results from these experiments have demonstrated the importance of nucleoside modifications to tRNA stability and ribosomal binding affinity, and will relate the structural contributions of nucleoside modifications to tRNA function.

Site-selected introduction of modified purine and pyrimidine ribonucleosides into RNA by automated phosphoramidite chemistry.

The study of modified nucleoside contributions to RNA chemistry, structure and function has been thwarted by the lack of a site-selected method of incorporation which is both versatile and adaptable to present synthetic technologies. A reproducible and versatile site-selected incorporation of nine differently modified nucleosides into hepta- and octadecamer RNAs has been achieved with automated phosphoramidite chemistry. The 5'-O-(4,4'-dimethoxytrityl-2'-O-tert-butyldimethylsilyl-ribonucleoside- 3'-O-(2-cyanoethyl-N,N-diisopropyl)phosphoramidite syntheses of m5C, D, psi, riboT, s2U, mnm5U, m1G and m2A were designed for compatibility with the commercially available major and 2'OH methylated ribonucleoside phosphoramidites. The synthesis of the m5C phosphoramidite was uniquely designed, and the first syntheses and incorporation of the two modified purine ribonucleosides are reported in detail along with that of psi, s2U, and mnm5U. Cleavage of RNA product from the synthesis support column, deprotection of the RNA, its purification by HPLC and nucleoside composition analysis are described. Modified nucleoside-containing tRNA domains were synthesized and purified in mumol quantities required for biophysical, as well as biochemical, studies. The anticodon domain of yeast tRNA(Phe) was synthesized with modified nucleosides introduced at the native positions: Cm32, Gm34, m1G37 (precursor to Y), psi 39 and m5C40. The T loop and stem was synthesized with riboT54 and the D loop and stem with D16 and D17. The E coli tRNA(Glu2) anti-codon codon domain was synthesized with mnm5U at wobble position 34, but an attempt at incorporating s2U at the same position failed. The unprotected thio group was labile to the oxidation step of the cyclical process. Chemically synthesized anticodon and T domains have been used in assays of tRNA structure and function (Guenther et al (1994) Biochimie 76, 1143-1151).

Importance of modified nucleosides to the structure and function of tRNAs.

Although a few modifications are found in DNA, 93 modified nucleosides have been found in the various RNAs. For the most part, the chemistry and structure that modified nucleosides, individually and in combination, uniquely contribute to DNA or RNA function have yet to be explained. However, there are ten physicochemical contributions that can be attributed to modified nucleosides. Of particular interest is the increasingly documented relationship between the presence of modified nucleosides in tRNAs, and the site and affinity of Mg2+ binding to RNA and its effect on function.

Ribosome binding of DNA analogs of tRNA requires base modifications and supports the "extended anticodon".

The efficiency of translation depends on correct tRNA-ribosome interactions. The ability of chemically synthesized yeast tRNA(Phe) anticodon domains to effectively inhibit the binding of native yeast tRNA(Phe) to poly(U)-programmed Escherichia coli 30S ribosomal subunits was dependent on a Mg(2+)-stabilized stem and an open anticodon loop, both facilitated by base modifications. Analysis of tRNA sequences has revealed that base modifications which negate canonical hydrogen bonding are found in 95% of those tRNA anticodon loop sequences with the potential to form two Watson-Crick base pairs across the loop. Therefore, we postulated that a stable anticodon stem and an open loop are prerequisites for ribosome binding. To test this hypothesis, DNA analogs of the yeast tRNA(Phe) anticodon domain were designed to have modification-induced, Mg(2+)-stabilized stems and open loops. The unmodified DNA analog neither bound to poly(U)-programmed 30S ribosomal subunits nor inhibited the binding of native tRNA(Phe). However, specifically modified DNA analogs did bind to ribosomal subunits and effectively inhibited tRNA(Phe) from binding. Thus, modification-dependent Mg(2+)-stabilized anticodon domain structures with open loops have evolved as the preferred anticodon conformations for ribosome binding.

Aminoacyl-tRNA synthetase and U54 methyltransferase recognize conformations of the yeast tRNA(Phe) anticodon and T stem/loop domain.

The enzyme-catalyzed posttranscriptional modification of tRNA and the contributions of modified nucleosides to tRNA structure and function can be investigated with chemically synthesized domains of the tRNA molecule. Heptadecamer RNAs with and without modified nucleosides and DNAs designed as analogs to the anticodon and T stem/loop domains of yeast tRNA(Phe) were produced by automated chemical synthesis. The unmodified T stem/loop domain of yeast tRNA(Phe) was a substrate for the E coli m5U54-tRNA methyltransferase activity, RUMT. Surprisingly, the DNA analog of the T stem/loop domain composed of d(A,U,G,C) was also a substrate. In addition, the DNA analog inhibited the methylation of unfractionated, undermodified E coli tRNA lacking the U54 methylation. RNA anticodon domains and DNA analogs differentially and specifically affected aminoacylation of the wild type yeast tRNA(Phe). Three differentially modified tRNA(Phe) anticodon domains with psi 39 alone, m1G37 and m5C40, or psi 39 with m1G37 and m5C40,stimulated phenylalanyl-tRNA synthetase (FRS) activity. However, one anticodon domain, with m5C40 as the only modified nucleoside and a closed loop conformation, inhibited FRS activity. Modified and unmodified DNA analogs of the anticodon, tDNA(PheAC), inhibited FRS activity. Analysis of the enzyme activity in the presence of the DNA analog characterized the DNA/enzyme interaction as either partial or allosteric inhibition. The disparity of action between the DNA and RNA hairpins provides new insight into the potential allosteric relationship of anticodon binding and open loop conformational requirements for active site function of FRS and other aaRSs. The comparison of the stimulatory and inhibitory properties of variously modified RNA domains and DNA analogs demonstrates that conformation, in addition to primary sequence, is important for tRNA-protein interaction. The enzyme recognition of various DNA analogs as substrate and/or inhibitors of activity demonstrates that conformational determinants are not restricted to ribose and the standard A-form RNA structure.

Presence and coding properties of 2'-O-methyl-5-carbamoylmethyluridine (ncm5Um) in the wobble position of the anticodon of tRNA(Leu) (U*AA) from brewer's yeast.

The unknown modified nucleoside U* has been isolated by enzymatic and HPLC protocols from tRNA(Leu) (U*AA) recently discovered in brewer's yeast. The pure U* nucleoside has been characterized by electron impact mass spectroscopy, and comparison of its chromatographic and UV-absorption properties with those of appropriate synthetic compounds. The structure of U* was established as 2'-O-methyl-5-carbamoylmethyluridine (ncm5Um). The yeast tRNA(Leu) (U*AA) is the only tRNA so far sequenced which has been shown to contain ncm5Um. The location of such a modified uridine at the first position of the anticodon restricts the decoding property to A of the leucine UUA codon.

The synthesis of tRNA sup (UGA) anticodons with the 2-thiouridine derivatives as the 'first letter'.

The title oligoribonucleotides UpCpA; U = s2mcm5U, s2mnm6U, s2U have been synthesized by the condensations of the dimer 10 with phosphodiesters 4, 5 and 6, followed by the two-step deprotection of the fully blocked oligomers 11, 12 and 13, respectively.