An improved method for the purification of tRNA by chromatography on dihydroxyboryl substituted cellulose

Structure and mechanism of the T-box riboswitches

In most Gram-positive bacteria, including many clinically devastating pathogens from genera such as Bacillus, Clostridium, Listeria, and Staphylococcus, T-box riboswitches sense and regulate intracellular availability of amino acids through a multipartite messenger RNA (mRNA)-transfer RNA (tRNA) interaction. The T-box mRNA leaders respond to nutrient starvation by specifically binding cognate tRNAs and sensing whether the bound tRNA is aminoacylated, as a proxy for amino acid availability. Based on this readout, T-boxes direct a transcriptional or translational switch to control the expression of downstream genes involved in various aspects of amino acid metabolism: biosynthesis, transport, aminoacylation, transamidation, and so forth. Two decades after its discovery, the structural and mechanistic underpinnings of the T-box riboswitch were recently elucidated, producing a wealth of insights into how two structured RNAs can recognize each other with robust affinity and exquisite selectivity. The T-box paradigm exemplifies how natural noncoding RNAs can interact not just through sequence complementarity but can add molecular specificity by precisely juxtaposing RNA structural motifs, exploiting inherently flexible elements and the biophysical properties of post-transcriptional modifications, ultimately achieving a high degree of shape complementarity through mutually induced fit. The T-box also provides a proof-of-principle that compact RNA domains can recognize minute chemical changes (such as tRNA aminoacylation) on another RNA. The unveiling of the structure and mechanism of the T-box system thus expands our appreciation of the range of capabilities and modes of action of structured noncoding RNAs, and hints at the existence of networks of noncoding RNAs that communicate through both, structural and sequence specificity.

Efficiency of a self-aminoacylating ribozyme: effect of the length and base-composition of its 3' extension

Variants of a previously described small self-aminoacylating ribozyme are tested in order to uncover the potentialities of a 3' extension responsible for the esterification. The base-composition and the length of this specific part of the ribozyme are investigated. Very short extensions can still reach the active site, reflecting the small persistence length of RNA. The yield of aminoacylation is particularly high for ribozymes with extensions made up of a poly-U, for which a maximum of efficiency is observed for a total length of about 10 nucleotides. A simple model describing the behavior of this region of the ribozyme can account for the data.

An antifungal agent inhibits an aminoacyl-tRNA synthetase by trapping tRNA in the editing site

Aminoacyl-transfer RNA (tRNA) synthetases, which catalyze the attachment of the correct amino acid to its corresponding tRNA during translation of the genetic code, are proven antimicrobial drug targets. We show that the broad-spectrum antifungal 5-fluoro-1,3-dihydro-1-hydroxy-2,1-benzoxaborole (AN2690), in development for the treatment of onychomycosis, inhibits yeast cytoplasmic leucyl-tRNA synthetase by formation of a stable tRNA(Leu)-AN2690 adduct in the editing site of the enzyme. Adduct formation is mediated through the boron atom of AN2690 and the 2'- and 3'-oxygen atoms of tRNA's3'-terminal adenosine. The trapping of enzyme-bound tRNA(Leu) in the editing site prevents catalytic turnover, thus inhibiting synthesis of leucyl-tRNA(Leu) and consequentially blocking protein synthesis. This result establishes the editing site as a bona fide target for aminoacyl-tRNA synthetase inhibitors.

Boronate affinity adsorption of RNA: possible role of conformational changes

Batch equilibrium adsorption isotherm determination is used to characterize the adsorption of mixed yeast RNA on agarose-immobilized m-aminophenylboronic acid. It is shown that the affinity-enhancing influence of divalent cations depends strongly on the precise nature of the cation used, with barium being far more effective than the conventionally-used magnesium. This adsorption-promoting influence of barium is suggested to arise primarily from ionic influences on the structure and rigidity of the RNA molecule, as the adsorption of ribose-based small molecules is not similarly affected. The substitution of barium for the standard magnesium counterion does not greatly promote the adsorption of DNA, implying that the effect is specific to RNA and may be useful in boronate-based RNA separations. RNA adsorption isotherms exhibit sharp transitions as functions of temperature, and these transitions occur at different temperatures with Mg2+ and with Ba2+. Adsorption affinity and capacity were found to increase markedly at lower temperatures, suggestive of an enthalpically favored interaction process. The stoichiometric displacement parameter, Z, in Ba2+ buffer is three times the value in Mg2+ buffer, and is close to unity.

Isolation of tRNA isoacceptors by affinity chromatography with immobilized elongation factor Tu from Escherichia coli

Elongation factor Tu (EF-Tu) from E. coli was coupled to activated CH Sepharose 4B. The immobilized EF-Tu maintained most of the guanosine nucleotide binding activity, but its ability to bind aminoacyl-tRNA depended on the type of complex used in the coupling reaction. The EF-Tu.GTP.aminoacyl-tRNA.kirromycin complex yielded an immobilized factor that was much more active in binding aminoacyl-tRNA than that obtained by coupling EF-Tu.GDP or EF-Tu.GDP.kirromycin to CH Sepharose 4B. Like the free factor, the immobilized EF-Tu.GTP did bind aminoacyl-tRNAs, but not unacylated tRNAs or N-acylated-aminoacyl-tRNAs. The antibiotic kirromycin was used to obtain the rapid conversion of EF-Tu.GDP to EF-Tu.GTP and the release of aminoacyl-tRNA from the matrix-bound EF-Tu by GDP. When total tRNA was aminoacylated by one amino acid only, a column of immobilized EF-Tu-GTP.kirromycin allowed the isolation of the aminoacylated tRNA from bulk tRNA. A rapid and nearly quantitative recovery of highly purified tRNA isoacceptors for various amino acids was obtained in one chromatographic step.

Alterations at the 3'-CCA end of Escherichia coli and Thermus thermophilus tRNA(Phe) do not abolish their acceptor activity

The 3'-CCA end of tRNA(Phe) from Escherichia coli and Thermus thermophilus was changed to AAA, CCC, UUU and UUA by the stepwise degradation procedure of the 3'-CCA end of tRNA(Phe) followed by the ligation with oligoribonucleotides. Substrate activity of tRNA(UUAPhe) and tRNA(CCCPhe) in tRNA aminoacylation was shown. tRNA(AAAPhe) is a bad substrate for E. coli and Th. thermophilus phenylalanyl-tRNA synthetases. tRNA(UUUPhe) has no detectable activity in tRNA aminoacylation. Therefore the nature of the 3'-end of tRNA(Phe) plays an important role in tRNA binding and its substrate efficiency. Nevertheless the CCA sequence at the 3'-end of tRNA(Phe) does not seem to be an absolute requirement for tRNA aminoacylation.

Modification of nuclear matrix proteins by ADP-ribosylation. Association of nuclear ADP-ribosyltransferase with the nuclear matrix

Nuclear matrices were isolated by treatment of isolated HeLa cell nuclei with high DNase I, pancreatic RNase and salt concentrations. ADP-ribosylated nuclear matrix proteins were identified by electrophoresis, blotting and autoradiography. In one experimental approach nuclear matrix proteins were labeled by exposure of permeabilized cells to the labeled precursor [32P]NAD. Alternatively, the cellular proteins were prelabeled with [35S]methionine and the ADP-ribosylated nuclear matrix proteins separated by aminophenyl boronate column chromatography. By both methods bands of modified proteins, though with differing intensities, were detected at 41, 43, 46, 51, 60, 64, 69, 73, 116, 140, 220 and 300 kDa. Approximately 2% of the total nuclear ADP-ribosyltransferase activity, but only 0.07% of the nuclear DNA, was tightly associated with the isolated nuclear matrix. The matrix-associated enzyme catalyzes the incorporation of [32P]ADP-ribose into acid-insoluble products of molecular mass 116 kDa and above, in a 3-aminobenzamide-inhibited, time-dependent reaction. The possible function of ADP-ribosylation of nuclear matrix proteins and of the attachment of ADP-ribosyltransferase to the nuclear matrix in the regulation of matrix-associated biochemical processes is discussed.

Affinity electrophoresis for monitoring terminal phosphorylation and the presence of queuosine in RNA. Application of polyacrylamide containing a covalently bound boronic acid

An affinity electrophoretic method has been developed to study the state of terminal phosphorylation of RNAs and the presence of the hypermodified base Q in tRNA. It is based on the copolymerization of acryloylaminophenylboronic acid into standard polyacrylamide gels and the interaction of this derivative with free cis-diol groups present in the RNA. In the case of terminal phosphorylation, free ribose groups are present either as such, or may be introduced by enzymatic reactions specific for a particular phosphorylation pattern (e.g. using T4 RNA ligase or guanylyltransferase). Additionally, tRNA species containing the Q base may be resolved from Q-lacking tRNAs by boronate affinity electrophoresis. The introduction of a non-destructive, one-step electrophoretic procedure not only offers an alternative to classical analytical methods, but also provides a means of isolating such populations of RNAs for which other methods are unavailable or are less convenient.

Assay of diadenosine tetraphosphate hydrolytic enzymes by boronate chromatography

A new procedure was described for assay of diadenosine tetraphosphate (Ap4A) hydrolases based on boronate chromatography. Potential reaction products, AMP, ADP, and ATP, of the hydrolysis of Ap4A were separated from residual substrate by chromatography on a boronate-derivatized cation-exchange resin, Bio-Rex 70. Separation was achieved by changing the concentrations of ethanol and ammonium acetate in the elution buffers. Picomole masses of products were detectable, blank dpm values were less than 0.5% of the total dpm, and auxiliary enzymes were not required. The procedure was specifically described for Ap4A pyrophosphohydrolase from Physarum polycephalum. The assay is generally applicable for dinucleoside polyphosphate hydrolases which hydrolyze other substrates such as Ap3A, Ap5A, Ap6A, and Gp4G. Dinucleotide polyphosphates are readily purified by chromatography on this boronate resin in a volatile buffer. Tes, Tricine, and Tris buffers significantly interfered with the chromatography of ATP.

Incorporation of 1,N6-ethenoadenosine into the 3' terminus of tRNA using T4 RNA ligase. 2. Preparation and ribosome interaction of fluorescent Escherichia coli tRNAMetf

A fluorescent derivative of tRNAMetf from Escherichia coli has been prepared which contains 1,N6-etheno-adenosine (epsilon A) in the place of adenosine 73, the fourth residue from the 3' end. The labeled tRNA, tRNAMetf epsilon A73, is fully active with respect to aminoacylation, formylation and formylmethionyl transfer to puromycin. The preparation procedure entails the chemical removal of four nucleotides from the 3' end of tRNAMetf, ligation of the truncated molecule with epsilon A 3',5'-bisphosphate by use of T4 RNA ligase and repair of the C-C-A end with nucleotidyl transferase. The fluorescence of fMet-tRNAMetf epsilon A73 has been exploited for studying tRNA-ribosome complexes. Upon binding the tRNA into the ribosomal P site, the fluorophor experiences a change of its molecular environment as indicated by an increased fluorescence intensity. On the other hand, iodide quenching experiments indicate that, in the complex, the fluorophor is not shielded against solvent access. The results suggest that (a) adenosine 73 is not involved in direct contacts with the ribosome and (b) the stacking of the 3'-terminal A-C-C-A sequence is changed upon binding to the ribosome.

Isolation of tRNA isoacceptors by affinity chromatography on immobilized bacterial elongation factor Tu

Elongation factor Tu from Escherichia coli or Thermus thermophilus was immobilized on cyanogen bromide-activated Sepharose. Immobilized elongation factor Tu X GDP could be converted to Tu X GTP, which is able to bind aminoacyl-tRNA. When bulk tRNA from E. coli, baker's yeast, or bovine liver was aminoacylated by one amino acid only, the resulting aminoacyl-tRNA could be separated in one step from the rest of the tRNA using an affinity column of immobilized elongation factor Tu X GTP. Specific tRNA isoacceptors can be isolated in amounts sufficient for gel electrophorectic analysis, sequence determinations, and hybridization experiments.

High-performance liquid chromatography of transfer RNAs. Separation of transfer RNAs from mammalian sources

A survey of recent advances in high-performance liquid chromatography (HPLC) of tRNA is presented here. The polystyrene and reversed-phase anion exchangers are discussed for their ability to resolve tRNAs without loss of the aminoacyl-tRNA bond. The HPLC of a tRNA of choice, based on the affinity principle, is studied. Both chemical (boronate) and biological (plant lectins) affinity groups for the tRNA interaction are described. A comprehensive scheme is presented for the separation of four mammalian tRNAs. Scope of future research in this area is also discussed.

Separation of glycosylated haemoglobins using immobilized phenylboronic acid. Effect of ligand concentration, column operating conditions, and comparison with ion-exchange and isoelectric-focusing

Haemoglobins from diabetic and non-diabetic individuals have been separated by affinity chromatography using immobilized phenylboronate, which interacts specifically with diol-containing compounds such as glycosylated haemoglobin. The effects of ligand concentration, flow rate, column geometry, preincubation of sample, buffer composition and temperature have been investigated. Significant correlation was found between results from affinity-chromatography and ion-exchange and isoelectric-focusing methods. Isoelectric-focusing of the haemoglobin fractions obtained from affinity chromatography indicate that, in addition to haemoglobin A1c, some haemoglobin A is also bound to immobilized phenylboronic acid. Assays of haemolysates obtained from red blood cells incubated in glucose solutions suggest that unstable pre-(haemoglobin A1c) does not interfere. The assay is not affected by the presence of haemoglobin F.

m-Aminophenylboronate agarose specifically binds capped snRNA and mRNA

m-Aminophenylboronate-substituted agarose binds specifically RNA chains carrying a mature 5' cap. The binding occurs most effectively at pH greater than 8 and involves diester formation between the negatively charged tetrahedric boronate group and the cisdiol of the ribose of the cap. The positive charge introduced by the m7G methylation is necessary for efficient binding although two closely spaced cis-diol groups alone (as in the cap analog NADH) are sufficient for binding. Non-capped RNA (like poly (U) and rRNA) or decapped RNA are not bound. It is shown that the matrix can be used for the isolation of capped small nuclear RNA and mRNA.

The complement of cytoplasmic tRNAs, including queuosine-containing tRNAs, in adult and senescent Wistar rat liver and their levels of aminoacylation

Our previous studies showed that both total cytoplasmic tRNAs and aminoacyl-tRNA synthetases isolated from senescent (24-30 month) female Wistar rat liver were less capable of supporting cell-free protein synthesis than were the same fractions isolated from adult (10-13 month) rat liver. The present study investigates the molecular basis for this age-related result. No significant age-related differences were found in the extent of aminoacylation of the liver cytoplasmic tRNA population, the total tRNA synthetase activity, the rate of aminoacylation of individual tRNAs, or in the overall complement of tRNA species as detected by two-dimensional gel electrophoresis. In homologous senescent aminoacylation assays, consisting of tRNAs and tRNA synthetases from senescent animals, alanine, arginine and aspartic acid were charged to a greater extent and methionine to a lesser extent compared to homologous adult assays. In heterologous assays, adult synthetases were significantly more active than senescent synthetases when charging isoleucine, methionine, phenylalanine, proline and glutamic acid, and less active when charging alanine, aspartic acid and serine. Also, senescent synthetases charged both adult and senescent tRNAs with methionine to a lesser extent than did adult synthetases. In homologous senescent assays with queuosine-containing tRNAs, asparagine, aspartic acid and histidine were charged to a greater extent and tyrosine to a lesser extent compared to homologous adult assays. Results with queuosine-tRNAs are discussed in terms of their potential ability to lower the efficiency of translation in senescent liver.

Functional assay of tRNA molecules transcribed from a purified gene

Purified tRNA genes are expressed when microinjected into the nucleus of X.laevis oocytes. In this paper we describe a method to assay the capacity to be aminoacylated of the tRNA transcribed in the frog oocytes. The method exploits the radiochemical purity of the transcript and relies on the binding of aminoacyl-tRNA but not of uncharged tRNA to purified elongation factor EF-Tu. We also present some preliminary results on several single point mutants of tRNAPro from Caenorhabditis elegans. We show that nucleotide 73 of tRNAPro can be substituted by any other nucleotide without loss of acceptor activity. A double mutant, causing transition from G45G46 to A45A46 has lost acceptor activity. Also inactive is a mutant carrying an insertion of a single base in the anticodon loop.

Synthesis of a nitrobenzeneboronic acid substituted polyacrylamide and its use in purifying isoaccepting transfer ribonucleic acids

Highly purified isoaccepting species of transfer ribonucleic acid (tRNA) were prepared by use of a polyacrylamide substituted with nitrobenzeneboronic acid functional groups. This method exploits the well-known ability of boronic acids to complex with RNA cis-diols. tRNA isoacceptors were obtained by enzymatically acylating a mixture of tRNA species with a single amino acid and passing the mixture over a solid-state nitrobenzeneboronic acid at pH 6.5 or 7.0. Pure aminoacyl-tRNA eluted at the column liquid volume, and unacylated tRNA species were bound. The bound species were recovered by lowering the pH of the eluant to 4.5. This procedure is uncomplicated, rapid, and applicable to nearly all tRNA isoacceptors. It does not chemically modify the tRNA(s) of interest or adversely affect their ability to be aminoacylated. Since boronic acids must be ionized to complex with cis-1,2-diols, boronic acid derivatives were prepared which ionize at a pH compatible with the stability of the aminoacyl bond. Two isomeric benzeneboronic acids with pKas of 6.8 and 7.4 were synthesized by introducing electron-withdrawing nitro groups into the aromatic ring. The addition of succinyl side chains permitted the nitrobenzeneboronic acids to be coupled to aminoethylpolyacrylamide. The properties of the nitrobenzeneboronic acid substituted acrylamide were illustrated by enriching phenylalanyl-tRNA at pH 7.0 to greater than 95% purity (1.63 nmol of phenylalanine accepted per A260 unit of tRNA) and seryl-tRNA isoacceptors at pH 6.5 to essentially theoretical purity (1.58 nmol of serine accepted per A260 unit of tRNA.

Decreased protein synthesis by polysomes, tRNA and aminoacyl-tRNA synthetases isolated from senescent rat liver

A cell-free in vitro protein-synthesizing system isolated from adult(10-13-month) and senescent(24-30-month) female Wistar rat liver is described. Optimal concentrations of polysomes, aminoacyl-tRNA synthetases, tRNA, Mg 2+, K+, and labeled amino acids are reported for this in vitro system. Both are rate and extent of protein synthesis are lower with the system from senescent liver compared to that from adult liver. Senescent polysomes, tRNA synthetases, and tRNA all contribute to this decrease. Free adult or senescent polysomes support protein synthesis to the same extent. However, compared to adult polysomes, senescent membrane-bound polysomes support less protein synthesis and account for an increased proportion of the total polysome fraction. Ribonuclease and protease activities are low in the adult and the senescent in vitro systems and do not account for the decreased protein synthesis with the latter.

Protein-bound polymeric and monomeric ADP-ribose residues in hepatic tissues. Comparative analyses using a new procedure for the quantification of poly(ADP-ribose)

Determination of poly(ADP-ribose) levels was performed by a new procedure involving covalent chromatography of released polymer, degradation to phosphoribosyl AMP and quantification of this specific derivative by a radioimmunoassay. In adult rat liver, about 85 pmol polymeric ADP-ribose residues/g tissue was found. Similar values were obtained when the determination was carried out by an independent procedure not involving boronate chromatography or sedimentation of DNA. In adult rat liver, polymeric ADP-ribose residues amounted to about 1/200 of total monomeric ADP-ribose residues. Most of the polymeric ADP-ribose residues were linked to proteins by NH2OH-sensitive bonds, while mono(ADP-ribose)-protein conjugates consisted of about equal amounts of NH2OH-sensitive and NH2OH-resistant subfractions. Poly(ADP-ribose) levels immediately after birth were similar to the adult status. They decreased, however, by a factor of three at the time of most rapid post-natal liver growth (day 17). A comparison with the protein-bound monomeric ADP-ribose residues indicated independent changes, and therefore presumably independent functions of these monomeric ADP-ribose residues.

Transfer RNA genes of Drosophila melanogaster

Three recombinant plasmids containing randomly sheared genomic D. melanogaster tRNAs have been identified and characterized in detail. One of these, the plasmid 14C4, has a D. melanogaster (Dm) DNA segment of 18 kb, and has three tRNA2Arg and two tRNAAsN genes. The second plasmid, 38B10, has tRNAHis genes, while the third plasmid, 63H5, contains coding sequences for tRNA2Asp. The Dm DNA segments in each recombinant plasmid are derived from unique cytogenetic loci. 14C4 is from 84 F, 38B10 is from 48 F and 63H5 is from 70 A.

Purification and properties of the major tRNAHis from sheep liver

The major isoacceptor of tRNAHis from sheep liver was purified. Two different methods were described which both took advantage of the presence of the hypermodified Q nucleotide in this tRNA. In the first procedure, CNBr treatment of tRNA which was previously enriched in tRNAHis greatly increased the efficiency of the subsequent chromatographic steps. The tRNAHis obtained by this technique showed a specific activity of 1,690 picomoles of histidine per A260 unit. In the second one, a twenty-fold enrichment in tRNAHis was obtained by fractionation of crude tRNA on acetylated DBAE-cellulose columns. The nucleotide composition of the tRNA obtained by the CNBr procedure was determined. No thymine ribotide could be detected. When this tRNA was submitted to periodate oxidation and tritium borohydride reduction, a radioactive label was introduced in this molecule which was assumed to be located in the Q nucleotide.

Structural changes in the glutamine-chargeable Escherichia coli transfer RNA-Trp produced by chemical modification with sodium bisulfite

Glutamine-mischargeable tRNA produced by sodium bisulfite-treated Escherichia coli tRNA-Trp was isolated by dihydroxyboryl-cellulose affinity column chromatography. This tRNA was shown to have dual specificity tryptophan and glutamine, and, when charged with either amino acid, bound to ribosomes in response to the non-sense codon UAG but not in response to the tryptophan codon UGG. The results were consistent with the reported properties of Su+7 tRNA. The bisulfite-treated tRNA-Trp migrated as two bands during polyacrylamide gel electrophoresis. The faster moving band (band I) coincided with that of untreated tRNA-Trp. The slower moving band (band II) coincided with the glutamine-chargeable tRNA-Trp. Su+7 tRNA behaved like band II tRNA upon gel electrophoresis. Nucleotide sequence analysis showed that a cytidine-uridine transition occurred at the 1st or the 2n position of the anitcodon of band II tRNA. Band I and band II tRNAs differed from each other in their thermal melting profiles. It is suggested that the single base change in the anticodon is responsible for the altered conformation of band II tRNA.

Ribosome binding site analysis of ovalbumin messenger ribonucleic acid

The region of the ovalbumin messenger ribonucleic acid (mRNAov) molecule bound to the 40S ribosomal subunit and its associated initiation factors in the wheat germ cell-free translation system were isolated and characterized. Two mRNAov fragments, 87 and 92 nucleotides in length, were protected from T1 ribonuclease digestion by binding of guanosine 5',beta,gamma-methylenetriphosphate and were shown by hybridization and fingerprint mapping to be derived from the 5' end of mRNAov. Both these mRNAov fragments were of sufficient length to contain both the cap structure and the AUG initiation codon. Four T1-resistant oligonucleotides, prepared by direct digestion of mRNAov with T1 ribonuclease were also found to bind to the wheat germ 40S ribosomal subunit. Nucleotide sequence analysis of these oligonucleotides revealed (1) that they were not a subset of the ribosome binding fragments described above, (2) that they were derived from within the mRNAov molecule (one from within the coding region and three from the noncoding region at the 3' end of the mRNAov molecule), and (3) that three of the four mRNAov nucleotides contained 3'-terminal AUG trinucleotides. These data suggested that features of the mRNAov molecule in addition to the nucleotide sequence might be important in specifying the correct ribosome binding site for the initiation of protein synthesis. The amount of mRNAov bound to the wheat germ 40S ribosomal subunit in a preinitiation complex was found to vary inversely with the potassium ion concentration. Lowering the potassium concentration to levels suboptimal for translation also resulted in the protection of larger fragments of the mRNAov molecule derived from the same 5'-end region as the ribosome binding fragments described above. The ability of the cap analogue 7-methylguanosine 5'-phosphate (m7G5'p) to reduce the amount of mRNAov bound to the wheat germ 40S ribosomal subunit was found to depend directly on thepotassium concentration. Interestingly, the effects of potassium on the amount of mRNAov bound in a preinitiation complex and the inhibition of this binding by m7G5'p could be observed by changing the potassium concentration after binding had occurred. These data suggested that the interaction between the wheat germ 40S ribosomal subunit and mRNAov was very sensitive to the ionic environment.

Correlation between the presence of tRNA His GUG and the erythropoietic function in foetal sheep liver

Histidyl-tRNAs from foetal and adult sheep liver were compared to their reticulocyte counterparts. The combination of various techniques revealed the existence of two histidyl-tRNA species in reticulocytes, one of which was not retained on acetylated DBAE-cellulose columns and was guanylatable. Three histidyl-tRNA isoacceptors were identified in foetal liver. Two of these species were not adsorbed on acetylated DBAE-cellulose but only one was found to be guanylatable. An identical chromatographic behaviour on RPC-5 columns was observed for guanylated histidyl-tRNAs from both origins. These results suggest the occurrence of a GUG anticodon in these guanine-accepting tRNAs. In foetal liver the amount of guanylatable histidyl-tRNA was estimated to be 7% of the total tRNA population. This observation is in agreement with the erythropoietic function of liver during the foetal life.

Regulation of the biosynthesis of aminoacyl-transfer ribonucleic acid synthetases and of transfer ribonucleic acid in Escherichia coli. VI. Mutants with increased levels of glutaminyl-transfer ribonucleic acid synthetase and of glutamine transfer ribonucleic acid

Spontaneous revertants of a temperature-sensitive Escherichia coli strain bearing a thermolabile glutaminyl-transfer ribonucleic acid (tRNA) synthetase have been selected for growth at 45 degrees C. Among 10 revertants still containing the thermolabile enzyme, 2 interesting strains were found. One strain has a fivefold elevated level of the thermolabile glutaminyl-tRNA synthetase; the genetic locus, glnR, responsible for this effect maps at min 24, far from glnS, the structural gene of the enzyme. In the other strain the levels of tRNA Gln and several other tRNAs are twice as high as in the parental strain; the locus responsible, glnU, maps at min 59.5 on the E. coli map.

Chemical modification study of aminoacyl-tRNA conformation

Chemical reactivity of cytosines in 32P-labeled E. coli tRNA1Leu, E. coli tRNAPhe and yeast tRNAPhe before and after aminoacylation was examined by use of a cytosine-specific reagent, semicarbazide-bisulfite mixture. In all the three tRNA species examined, the cytosine residues that were susceptible to the modification were the same in the aminoacylated tRNA and the unacylated tRNA. Only a limited number of the cytosine residues were modifiable: those that occur in the anticodon, the 3'-CCA terminus, the D-loop, and the extra loop. The sites accessible by the reagent are in good agreement with the general three-dimensional structure of tRNA proposed in literature. These results indicate that the gross conformation of these tRNAs does not change on aminoacylation, and consequently favor the view that the T psi C(G) sequence could become exposed in later steps of protein synthesis in order to achieve the binding of aminoacyl tRNA to ribosomes.