Cloverleaf Model Research Articles

NMR evidence for common tertiary structure base pairs in yeast and E. coli tRNA.

ABOUT ten years ago Holley1 and coworkers sequenced the first tRNA and proposed the now famous cloverleaf secondary structure. Since that time about sixty additional tRNAs have been sequenced and all fit the cloverleaf model2. This homology constitutes a strong case for the cloverleaf model and there is now compelling evidence from nuclear magnetic resonance (NMR) studies3 that the clover-leaf model does provide a correct description of the secondary structure of tRNA molecules in solution.

Assignment of the hydrogen bonded proton resonances in (Escherichia coli) tRNAGlu by sequential melting.

The 300 MHz nuclear magnetic resonance peaks from the hydrogen bonded protons in (Escherichia coli) tRNA(Glu) in Mg(++) solutions have been measured at temperatures between 25 degrees and 74 degrees . Between 25 degrees and 49 degrees four resonances broaden, three of which are assigned to the hU arm and one to a G.C pair of the tertiary structure. By 64 degrees these four resonances have disappeared and the nuclear magnetic resonance spectrum is fitted very well by a computer simulation based upon resonances from the acceptor, T-Psi-C, and anticodon arms. At 66 degrees the resonances from the T-Psi-C arm are lost and at 74 degrees only those from the anticodon are left. All 20 resonances expected from the cloverleaf model have been assigned by comparing the calculated positions of resonances in a particular arm with the stepwise loss of intensity with temperature. The root mean square error between calculated and observed positions is 0.17 ppm. Resonances at the end of helical regions which are sensitive to stacking beyond the helix allow us to conclude that the acceptor arm is stacked upon the T-Psi-C in a regular helix and that G [unk]73 is stacked upon base pair [unk]1-72 but the hU and anticodon arms are not stacked in a regular helix upon the intervening base A [unk]27.

Selective modification of cytidine, undine, guanosine and pseudouridine residues in Escherichia coli leucine transfer ribonucleic acid

The specificity of methoxyamine for the cytidine residues in an Escherichia coli leuoine transfer RNA (tRNA 1 leu is described in detail. Of the six non-hydrogen-bonded cytidine residues in the clover-leaf model of this tRNA, four are very reactive (C-35, 53, 85 and 86) and two are unreactive (C-67 and 79). The specificity of l-cyclohexyl-3-[2-morpholino-(4)-ethyl]carbodiimide methotosylate for the uridine, guanosine and pseudouridine residues in the leucine tRNA was also investigated. The carbodiimide completely modified four uridine residues (U-33, 34, 50 and 51) and partially modified G-37 and Ψ-39. For technical reasons, the sites of partial modification in loop I of the tRNA were difficult to establish. There was no modification of base residues in loop IV nor of U-59 at the base of stem e of the tRNA. The modification patterns described for the leucine tRNA are compared with those observed for the E. coli initiator tRNA 1 met and su + III tyrosine tRNA. Several general conclusions regarding tRNA conformation are made. In particular, the evidence supporting a diversity of anticodon loop structures amongst tRNAs is discussed.

High resolution proton NMR study of an isolated fragment of R17 bacteriophage mRNA.

HIGH resolution nuclear magnetic resonance (NMR) spectra of the hydrogen-bonded protons in helical regions of nucleic acids are an informative monitor of the base pairing1. Studies of about 10 purified tRNAs and of model systems have shown that the ring NH protons of uracil and guanine, when involved in Watson-Crick base pairing, give resonances between −15 and −11 p.p.m. downfield from DSS1,2. Since there is only one such proton in each base pair, the intensity in this spectral region is a direct measure of the number of base pairs. Simple rules have been derived to predict the NMR spectra originating from these protons on the basis of ring currents from the adjacent bases and for tRNA the comparison between predicted and observed spectra has strongly supported the clover leaf model of base pairing. In addition, ring current shifts from bases adjacent to the helical region provided information about the structure at the end of the helix. Here we apply this approach to the secondary structure of the R17 fragment discussed in the preceding paper3. We prepared 0.5 mg of the R17 fragment according to a somewhat modified method of Bernardi and Spahr4, and measured the NMR spectrum of a 25 µl sample.

The Primary Structure of Alanine Transfer Ribonucleic Acid 1 from <italic>Torulopsis utilis</italic><subtitle>II. Partial Digestion with Ribonuclease T<sub>1</sub> and Derivation of the Complete Sequence</subtitle>

Partial RNase T1[EC 2.7.7. 26] digestion of tRNAAla1(Torulopsis utilis) in the presence of Mg2+ yielded two half molecules by scission at the anticodon site. Several large oligonucleotides smaller than the halves were obtained by partial RNase T1 digestion of the tRNA in the absence of Mg2+. The sequences of these large fragments were analyzed. The complete sequence of this tRNA was derived from the results of these analyses and those described in the preceding paper. The cloverleaf model for the secondary structure was compared with that of Saccharomyces tRNAAla, as regards the aminoacyl-tRNA synthetase recognition sites.

The conformational transitions in yeast tRNAPhe as studied with tRNAPhe fragments.

Conformational transitions of the anticodon branch, the acceptor branch and the tertiary structure of yeast tRNAPhe were investigated using tRNAPhe fragments. Thermodynamic and kinetic methods were employed using the optical absorbance of the fragments and the fluorescence of the Y‐base. Related results of partial nuclease digestions on tRNAPhe fragments are also reported. Hypochromicity, reaction enthalpy and rate constants of helix‐coil transitions of the anti codon branch agree with the values expected from the cloverleaf model. The dissociation of the branch is accompanied by a quenching of the fluorescence of the Y‐base. In some isolated fragments the anticodon branch is more stable than in the intact tRNA. The destabilization in the intact tRNA is attributed to electrostatic effects. The anticodon half Phe 21–57, at low concentrations and temperatures, assumes a structure which is different from a segment of the cloverleaf model. A transition from this structure to the one of a segment of the cloverleaf is observed with increasing temperature. At higher fragment concentrations and in the presence of Mg2+ dimers are formed. Bimolecular recombinations are observed between fragments Phe 1–18 and Phe 21–76 reconstituting the acceptor and dihydrouridine stems, and between Phe 1–18 and Phe 38–76 where only the base pairs of the acceptor stem are newly formed. The mechanism of the recombination and the relationship of the fragment combination to the intact tRNA are discussed. The thermodynamic properties of the fragment Phe 1‐69/70 are very similar to those of the intact tRNA. It can be concluded that the tertiary structure is essentially unchanged anti that the terminal nucleotides 70–76 are not directly involved in its formation. The thermal denaturation process of intact tRNAPhe can now be interpreted as follows: at first the tertiary structure is converted to a cloverleaf‐like structure; this process is followed by the melting of the acceptor and anticodon stems; finally the ribosylthymine and dihydrouridine stems dissociate independently.

Ring-current shifts in the 300 MHz nuclear magnetic resonance spectra of six purified transfer RNA molecules

We present the 300 MHz high-resolution proton nuclear magnetic resonance spectra of the ring NH hydrogen-bonded protons of six purified tRNAs. Good agreement was obtained between the observed spectra and those computed on the assumption of the suitable cloverleaf models. In the computation it is assumed that the hydrogen-bonded ring NH in each type of base pair has an intrinsic position with respect to 2,2-dimethyl-2-silapentane-5-sulfonate, i.e. in A·U it is at −14·8 parts per million, in G·C at −13·7 parts per million and in A·Ψat −13·5 parts per million. The shifts of these resonances from these positions are calculated by including ring current fields from the nearest neighbors. The agreement is very good, adding support to our earlier findings that there is no evidence for additional Watson-Crick base pairs detected beyond those in the cloverleaf. In general, resolved resonances are fitted by the computed spectra to within ±0·2 part per million showing that there is no need for any additional physical mechanism to explain the nuclear magnetic resonance positions. Hence, the nuclear magnetic resonance spectra can be interpreted in terms of the structure of their neighbors and in a few important cases this has been particularly valuable in understanding the structure beyond the end of a helical region. In the tRNA Glu E.coli′ for example, the positions of the resonances in A·U no. 7 and A·U no. 49 at the interior ends of the acceptor and -T-Ψ-C- stems, respectively, strongly suggest that these two stems are in a continuous helix. Other structural effects at the ends of the helical regions are also suggested by the nuclear magnetic resonance spectra.

Assignment of the low field proton nuclear magnetic resonance spectrum of yeast phenylalanine transfer RNA to specific base pairs

High-resolution proton nuclear magnetic resonance spectra at 220 and 300 MHz have been used to investigate the base-pairing structure of fragments of yeast tRNA Phe, of chemically modified tRNA Phe and of intact tRNA Phe. To a very good approximation the positions of the fragment spectra are additive within 0·2 part per million, indicating that factors responsible for certain structural features in the intact molecule are already present in the smaller fragments (half molecules, hairpins and 3 4 molecules). A simple first-order ring-current shift theory taken in conjunction with the cloverleaf model for tRNA Phe (RajBhandary et al., 1967) has been used to predict the low-field (− 15 to −11 part per million) nuclear magnetic resonance spectra and make assignments of the resolved resonances to ring NH protons of specific base pairs. The general agreement between the predicted and observed spectra to within 0·2 part per million confirms in detail the cloverleaf model for the secondary structure of tRNA Phe in solution. It is also established that ring-current shifts are the principal factor responsible for the wide range of shifts observed in the low-field spectra. As a result it is evident that the resonances are very sensitive to small changes in the secondary structure and in some cases changes in the interbase distance as small as 0·2 Å could easily be detected. It is also clear from the analysis that certain of the resonances are sensitive to the tertiary structure of the molecule and specific examples are discussed. As with our previous study, we find no evidence for any strong Watson-Crick type base pairs beyond those predicted by the cloverleaf structure.

Determination of secondary and tertiary structural features of transfer RNA molecules in solution by nuclear magnetic resonance.

High-resolution 300-MHz proton nuclear magnetic resonance spectra of the hydrogen-bounded protons in three different purified tRNA molecules are presented. The resonances in the region between -11 and -15 ppm from 2,2-dimethyl-2-silapentane-5-sulfonate (DSS) are assigned to the ring NH protons of specific base pairs by two approaches. First, intrinsic positions of -14.8 ppm and -13.7 ppm are taken for the AU and GC ring NH protons, respectively, and the spectra are calculated by including ring current shifts from the nearest neighbors. The spectra calculated in this way on the basis of the cloverleaf are in good agreement with the observed. Second, fragments of yeast tRNA(Phe) were obtained, which helped in assignments of the spectrum of intact molecules. The close agreement strongly supports the cloverleaf model. Tertiary structural features were determined in a few cases where the ring currents at the terminal base pairs of helical regions depended upon stacking of the helices. In this way, we were able to show that in Escherichia coli tRNA(Glu) the CCA stem forms a continuous helix with the TPsiC stem, which is in accord with the preliminary x-ray structure of yeast tRNA(Phe), suggesting that this stacking is observed in solution and may be a general property of different tRNA molecules. Similar reasoning suggests that in E. coli tRNA(fMet) G-27 is stacked upon the dihydrouridine helix.

Selective modification of cytidine and uridine residues in Escherichia coli formylmethionine transfer ribonucleic acid

The specificity of methoxyamine for the cytidine residues in Escherichia coli formylmethionine tRNA is described in detail. Of the nine cytidine residues not involved in hydrogen-bonding in the clover leaf model of the tRNA, three are very reactive (C-1, 75 and 76), three less so (C-16, 17 and 35) and three unreactive (C-33, 49 and 57). Surprisingly, residue C-35 at the 3′ end of the anticodon triplet is not completely modified by methoxyamine. The specificity of 1-cyclohexyl 3-[2-morpholino (4)-ethyl] carbodiimide methotosylate for the uridine and guanosine residues of this tRNA is also described in detail. Of the twelve uridine and guanosine residues not involved in hydrogen-bonding in the secondary structure of the molecule, two are reactive (U-37 and48), one less so (U-18), one partially (U-34), and eightare unreactive (U-8 and 61; G-9, 15, 19, 20, 27 and 46). No guanosine residues in the tRNA are modified by the carbodiimide. The ribosylthymine and pseudouridine residues in loop IV are also unreactive. The extent and position of the carbodiimide modification as a function of time is also described. The importance of particular residues being modified or not under the reaction conditions used is discussed in terms of transfer RNA conformation. A reduction from 10 to 4 mm-magnesium ions in the modification experiments has no apparent effect on the extent and position of the carbodiimide or methoxyamine reactions.

On the structure of yeast tRNA Phe . Complementary-oligonucleotide binding studies.

The availability of (yeast)tRNAPhe for helix formation with complementary oligoribonucleotides has been explored by equilibrium dialysis studies. Oligoribonucleotides complementary to sequences which are double‐stranded, according to the cloverleaf model of tRNAPhe, did not bind. Two regions of tRNAPhe were fully acessible to oligonucleotide binding: (a) the 3′‐terminus; (b) the sequence (3′‐5′)U‐G‐A‐A‐Y (33–37) of the anticodon loop. Two other regions were accessible to oligonucleotide binding: (a) part of the extra arm; (b) the (3′‐5′)T‐Ψ‐C sequence of the ribosylthymine loop. However, the binding constants of trimers and tetramers complementary to these parts of tRNAPhe are very low.The data indicate that the anticodon loop of tRNAPhe has an asymmetric structure, which is not compatible with the Fuller‐Hodgson model. The low binding constants for oligonucleotides complementary to the extra arm or to the TpΨpC sequence are discussed. They seem to be due either to sterical hindrance or to different conformations of tRNA. In the latter case, one conformation would be accessible to complementary oligonucleotides for helix formation, the other would not. Present available oligonucleotide binding data of four tRNAs suggest that each tRNA has an individual pairing scheme, according to which the loops of the cloverleaf contribute to the three‐dimensional structure.

Investigation of exchangeable protons and the extent of base pairings in yeast phenylalanine transfer RNA by high resolution nuclear magnetic resonance

High resolution nuclear magnetic resonance spectra were obtained for yeast transfer RNA Phe and unfractionated yeast tRNA in aqueous solutions. Resonances due to the free amino protons, hydrogen-bonded amino protons and hydrogen-bonded ring N—H protons were observed in three non-overlapping spectral regions. The assignment of the exchangeable proton resonances in yeast tRNA Phe is based on their pH and temperature dependence, spectral position, and their disappearance in deuterium oxide solvent. The number of each kind of exchangeable proton was determined and compared with the number expected on the basis of the cloverleaf model. The total number of base pairs observed by nuclear magnetic resonance (20±2) agrees well with the number computed from the cloverleaf model (20±1) depending upon whether or not one 2MeG.C and one G.U base pair are included. Furthermore, the separate numbers of A.U and G.C base pairs, 7 to 8 and 11 to 12, respectively, determined by nuclear magnetic resonance, are in good agreement with the 8 and 12 respective pairs expected from the cloverleaf model. The nuclear magnetic resonance spectra provided little evidence for additional strong Watson-Crick type base pairs resulting from tertiary structure such as are proposed in several models of tRNA. Resonances observed in the −9 to −11 parts per million region appear to be related to tertiary structure, but their assignment remains open at this tune. The results obtained with unfractionated yeast tRNA parallel those obtained with yeast tRNA Phe and therefore indicate that the interpretation of these latter data may be applicable to other tRNA species. The high resolution nuclear magnetic resonance data on the behavior of the exchangeable protons in tRNA are compared with the results of recent tritium exchange experiments. The anomalous behavior of hydrogen-bonded amino protons in both the nuclear magnetic resonance and tritium exchange experiments is discussed.

Raman studies of nucleic acids VI. Conformational structures of tRNA fMet, tRNA Val and tRNA Phe2

Laser-excited Raman spectra of tRNA fMet, tRNA Val and tRNA Phe 2 from Escherichia coli indicate that each tRNA has a similar conformation in aqueous solution. The spectra of tRNA fMet and tRNA Val show further that their specific base-paired and base-stacked secondary structures are consistent with the proposed clover-leaf models. The onset of melting in tRNA fMet and tRNA Val occurs at about 40°C. At 70°C all base pairs are broken. Above 70°C and up to 90°C, significant order is retained in the tRNA backbones and appreciable stacking of G residues persists. In the presence of a 3-fold molar excess of Mg 2+, the onset of melting occurs at about 60°C. Comparison of Raman spectra of tRNA and rRNA reveals that the latter molecules have a more ordered ribose-phosphate backbone and contain more A but less G residues in stacked configurations at 32°C.

On the conformation of transfer RNA in solution: Dependence of denaturation temperature and structural parameters of mixed and formylmethionyl Escherichia coli transfer RNA on sodium ion concentration

There is at present considerable evidence that transfer RNA in the presence of Mg 2+ exists in a more highly ordered state than predicted by simple cloverleaf models (Levitt, 1969; Cramer, 1970, Abstr. 8th Internat. Cong. Biochem. p. 214; Cramer, 1971). This highly ordered structure is shown here to correspond closely to that found in the presence of high concentrations of Na + (2 m) at pH 6.5. As the Na + counterion concentration is decreased, progressive denaturation occurs to a state finally approximating a coil. We have determined the dependence of the mid-point of the thermal denaturation profiles ( T m), ultraviolet absorbance, viscosity, circular dichroism spectra, and tritium exchange-out rates and number on counterion concentration for both unfractionated (mixed) Escherichia coli tRNA as well as the purified E. coli formylmethionine specific tRNA. The transition and physical data are consistent with a simple interpretation of the states of tRNA present as a function of ionic strength. Specifically, at pH 6.5 in the region of 0.1 to 2 m-NaCl, below 25 °C, tRNA exists as a cloverleaf with varying degrees of superstructure converging to the Mg 2+ state. Below 0.1 m-Na + denaturation of the cloverleaf structure itself takes place, representing the electrostatically induced transition to some type of coil configuration. In the vicinity of 0.1 m-Na + we suggest that the average structure in solution approximates a cloverleaf conformation, although we have no information on the distribution of molecules among closely related states. This picture provides a useful reference for physical and functional studies, which may define more completely the tertiary conformation and specific biological interactions of this molecule.

Conversion of Exposed Cytidine Residues to Uridine Residues in Escherichia coli Formylmethionine Transfer Ribonucleic Acid

Abstract Treatment of Escherichia coli formylmethionine tRNA with sodium bisulfite results in conversion of 6 cytidine residues to uridine residues under conditions where other cytidine residues in the tRNA are unreactive. Comparison of double and single stranded polymers shows that the bisulfitemediated deamination reaction is strongly inhibited by ordered structure. Under conditions where over 90% of the cytidine residues in poly(C) are deaminated, there is no detectable reaction of poly(I) poly(C). The 6 reactive cytidine residues in formylemethionine tRNA are modified at the same rate as cytidine residues in poly(C) and all are located in single stranded regions of the clover-leaf model. The modified residues include the cytidine at the 5'-phosphate terminus, the cytidine in the anticodon, the two cytidines in the dihydrouridine loop, and the two cytidines in the 3'-terminal CpCpA sequence. The O-methylcytidine in the anticodon loop shows only about 20% reaction under conditions where reactive cytidines are completely modified. No reaction of the cytidine in the small loop or of the cytidine in the TψC loop is observed. Aside from the unusual unpaired base at the 5' terminus, the initiator tRNA resembles several other tRNAs with respect to the locations of exposed and protected cytidine residues in the ordered structure of the molecule.

Tertiary structure and function of transfer-RNA

In analogy to the globular protein structures the predominant stabilization factor of which is surface energy a compact model with the smallest possible surface of the tertiary structure of t-RNA is suggested. The principle of folding is — according to the clover leaf model — the superposition of the three leaves. The three loops form a ring (loop-stack). The stem can be placed on this molecular body bringing the CCA end into the vicinity of the loop-stack, where the CCA end can be bound. The structures of the RNA chain are subordinated to this globular structural principle. Under the influence of the sequence the CCA end is able to undergo structural changes, thus allowing to explain the role of t-RNA in the aminoacylation process and the structural differences between t-RNA, aminoacyl-t-RNA, and peptidyl-t-RNA. Furthermore it is possible to comment on the existence of the characteristic and modified bases, the role of magnesium ions and the chemical reactions of these compounds.

Hydrogen exchange study of some polynucleotides and transfer RNA

The apparent disagreement between published transfer RNA hydrogen exchange results and the tRNA cloverleaf model, prompted a re-investigation of the relationship between hydrogen exchange data and nucleic acid structure. Hydrogen-tritium exchange experiments were carried out with samples of pure and mixed tRNA and with the synthetic polynucleotide bihelices: poly(rA) · poly(rU), poly(rI) · poly(rC), poly(rG) · poly(rC) and poly(dG) · poly (dC). Studies With the synthetic polynucleotides show that, to interpret nucleic acid hydrogen exchange data in terms of quantity of base-paired structure, one must count 5 H for each G · C pair and 2 or 3 for A · U. Both poly(rG) · poly(rC) and poly(dG) · poly(dC) clearly show 5 slowly exchanging H per base pair. For A · U and I · C only 2 were detected, though other workers have found 3 for some A-T systems. These are all base-pair-bound H. The ribose OH is too fast to measure. The reasons for the surprisingly slow exchange of the exposed NH 2 protons are unknown. The hydrogen exchange-rate behavior found for the polynucleotides suggests that some local structural distortion is necessary for any of the exchangeable H to react, including the exposed NH 2 protons, and that the distortion important for hydrogen exchange is different from that occurring in thermal denaturation. All the tRNA samples show very similar hydrogen exchange profiles. The pure samples (formylmethionine and tyrosine tRNA from Escherichia coli) have ~120 slowly exchanging protons, far more than the ~55 Watson-Crick hydrogen bonds in the simple cloverleaf models. With the above numeration, however, the cloverleaf models for the two pure tRNA samples account for all but approx-imately 20 of their slowly exchanging H. The excess of 20 H is very close to the number required by models having extra tertiary structure. The tRNA's were found to exchange more slowly even than poly(rG) · poly(rC) and to have a unique salt and pH dependence. These anomalies could also be explained by the presence of some tertiary folding. Unacylated and 70% aminoacylated E. coli tRNA f Met were found to have identical hydrogen exchange behavior, suggesting absence of structure change upon aminoacylation. A method was developed for isolating authentic poly(rG) from normally heterodisperse mixtures by Sephadex gel filtration in 90% dimethyl sulfoxide. Formation of the 1:1 poly(rG) · poly(rC) complex was achieved by mixing-experiments in concentrated urea solutions at high temperature.

Phosphorolysis of the tRNA. Conformations of specific tRNAs and effect of the localized regions on the stability of the structure.

Phosphorolysis studies of several specific tRNAs from Escherichia coli and yeast confirm the phenomenon observed with total tRNA: each tRNA can be differentiated into two classes of conformations, according to their ability (S class) or inability (R class) to be phosphorolyzed by polynucleotide phosphorylase. The proportion of these conformations at a given temperature varies from one specific tRNA to another. When the extent of phosphorolysis of different specific tRNAs are compared with the variation of hyperchromicity at different temperatures, we find that these two parameters are correlated. The phosphorolysis behavior of tRNA differs when it is phosphorolyzed in a mixture or alone. Studies with (E. coli) tRNATyr show that, at low temperature, the extent of phosphorolysis of this tRNA, assayed alone, is much higher than that of the total E. coli tRNA or other specific tRNAs tried; moreover, when this tRNA is mixed with (E. coli) tRNASer or included in the bulk of E. coli tRNA, it is scarcely attacked. In contrast, at high temperatures where all tRNA is completely degraded by the enzyme, there is no longer any difference in the rate of phosphorolysis between tRNATyr and tRNASer from E. coli. The differential phosphorolysis of these tRNAs at low temperatures is interpreted as a differential affinity of these substrates for the enzyme. One of the unique structural features of (E. coli) tRNATyr is the long nucleotide sequence which forms a long “extra” arm in the clover-leaf model. It is postulated that the presence of a long “extra” arm may be the reason for the observed low thermal transition at the first melting region of this tRNA and for the decreased affinity of the tRNA for the enzyme. Studies with yeast tRNASer, tRNAPhe, and bulk tRNA lead to similar results. The differences in the extent of phosphorolysis are discussed in connection with the topology of the tRNA structure, according to the length of the dihydrouridine loop and of its base-paired portion. It appears that tRNAs with a long dihydrouridine loop, a short base-paired portion and a long “extra” arm are best phosphorolyzed; those with a short dihydrouridine loop and a long base-paired region are more resistant.

Temperature-jump relaxation studies on transfer ribonucleic acids. Valine and tyrosine-specific tRNAs from Escherichia coli.

Equilibrium and kinetic measurements of the thermal denaturation of tRNA1Val and tRNA2Tyr from Escherichia coli have been obtained by absorption melting curves and temperature‐jump experiments. Besides a very fast effect attributed to changes in stacking equilibria, several relaxation times in the millisecond range are observed at 260 nm. The incomplete separation of the different relaxation times suggests a complex process, in agreement with the equilibrium data. At 335 nm, where the absorption is due only to 4‐thiouridine, the melting profile is monophasic and for both tRNAs only one relaxation time is observed in the millisecond range. The thermodynamic parameters derived from the results obtained at this wavelength indicate a cooperative melting of short double helical sequences. The results are discussed on the basis of the clover‐leaf model.

Conformation of the tRNA minor constituent dihydrouridine

FEBS LettersVolume 12, Issue 5 p. 257-259 Full-length articleFree Access Conformation of the tRNA minor constituent dihydrouridine Dietrich Suck, Dietrich Suck Max-Planck-Institut für experimentelle Medizin, Abteilung Chemie, Göttingen, GermanySearch for more papers by this authorWolfram Saenger, Wolfram Saenger Max-Planck-Institut für Eiweiss-und Lederforschung, Abteilung Röntgenstrukturforschung, München, GermanySearch for more papers by this author Dietrich Suck, Dietrich Suck Max-Planck-Institut für experimentelle Medizin, Abteilung Chemie, Göttingen, GermanySearch for more papers by this authorWolfram Saenger, Wolfram Saenger Max-Planck-Institut für Eiweiss-und Lederforschung, Abteilung Röntgenstrukturforschung, München, GermanySearch for more papers by this author First published: January 30, 1971 https://doi.org/10.1016/0014-5793(71)80191-6Citations: 5AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article.Citing Literature Volume12, Issue5January 30, 1971Pages 257-259 ReferencesRelatedInformation