Inhibition of eukaryotic translation by nucleoside 5'-monophosphate analogues of mRNA 5'-cap: changes in N7 substituent affect analogue activity

Nucleotide cap analogues of 7-methylguanosine 5'-monophosphate (m7GMP) were synthesized in which the 7-methyl moiety was replaced with 7-ethyl (e7), 7-propyl (p7), 7-isopropyl (ip7), 7-butyl (b7), 7-isobutyl (ib7), 7-cyclopentyl (cp7), 7-(carboxymethyl) (cm7), 7-benzyl (bn7), 7-(2-phenylethyl) [7-(2-PhEt)], and 7-(1-phenylethyl) [7-(1-PhEt)]. These derivatives were assayed as competitive inhibitors of capped mRNA translation in reticulocyte lysate. We observed that N7 alkyl and alicyclic substituents larger than ethyl significantly decreased the inhibitory activity of these cap analogues presumably by decreasing their affinity for cap binding proteins, which participate in the initiation of translation. This result defined a maximum size for this class of N7 substituents in the nucleotide binding domain of cap binding proteins. Like m7GMP, the N7-substituted GMP derivatives synthesized in this study were found to be predominantly in the anti conformation as determined by proton NMR analyses. However, bn7GMP and 7-(2-PhEt)GMP, which have aromatic N7 substituents, were more effective than m7GMP as competitive inhibitors of translation. The increased affinity of bn7GMP for cap binding proteins was further examined by synthesis of beta-globin mRNA containing 5'-bn7G, 5'-m7G, or 5'-e7G cap structures. These modified mRNAs were tested as translation templates. Messenger RNA capped with bn7G was observed to increase the translation activity of the template 1.8-fold relative to that of its m7G-capped mRNA counterpart. By contrast, e7G-capped mRNA was 25% less active than m7G-capped mRNA.2+V photo-cross-linking of m7G-capped mRNA to cap binding proteins     

Influence of potassium salt concentration and temperature on inhibition of mRNA translation by 7-methylguanosine5'-monophosphate.

The effect of 7-methylguanosine 5'-monophosphate (pm7G) on mRNA translation was examined in the wheat germ and rabbit reticulocyte cell-free systems. Differences between the two cell extracts with respect to inhibition of translation by pm7G can be attributed to different conditions commonly used for in vitro protein synthesis. Inhibition of globin mRNA translation by pm7G is strongly influenced by the concentration of potassium salt and to a lesser extent by incubation temperature. The effectiveness of the inhibitor increases with potassium salt concentration and diminishes with increasing temperature. Translation is inhibited by pm7G at physiological K+ concentration in both cell-free systems in that only the rate of binding of mRNA to ribosomes is affected by the inhibitor, not the extent of binding. Translation of different capped mRNAs is affected differently by pm7G, but this appears to be property of the mRNA rather than the translation system. These results indicate that while the 5'-terminal cap structure may be more important for translation of some mRNA's than others, this structure functions in translation of capped mRNAs in all types of cells.     

Inhibition of noncapped mRNA translation by the cap analogue, 7-methylguanosine-5'-phosphate.

The cap analogue, 7-methylguanosine-5′-phosphate (pm⁷G), inhibits the translation of the noncapped STNV (satellite tobacco necrosis virus) RNA and CPMV (cowpea mosaic virus) RNA in the $\text{in vitro}$ wheat germ protein synthesizing system. While the translation of some capped mRNAs is inhibited more strongly by the analogue, other capped mRNAs have a level of sensitivity similar to that of the noncapped RNAs. Evidence is presented demonstrating that the effect of the analogue is exerted at a cap binding site even when it is inhibiting noncapped mRNAs. These results therefore indicate that the cap binding site of the translational system is either part of or is closely linked to another mRNA binding component, this component being specific for a site on the mRNA other than the 5′ cap. The observations also suggest caution in the use of pm⁷G inhibition to indicate the presence of a 5′ cap on a particular mRNA.     

Base stacking of simple mRNA cap analogues. Association of 7,9-dimethylguanine, 7-methylguanosine and 7-methylguanosine 5'-monophosphate with indole and purine derivatives in aqueous solution

Equilibrium constants for the association of different ionic forms of 7,9-dimethylguanine, 7-methylguanosine and 7-methylguanosine 5'-monophosphate with indole, caffeine and various methylated adenines have been determined by distributing the latter compounds between an organic solvent and aqueous solutions of the 7-methylguanine derivatives. The data are compared to those obtained for the association of unsubstituted purine with the same cosolutes. The stacking affinity of both cationic and zwitterionic forms of the 7-methylguanine ring correlates with the ring polarizability rather than the polarizing power of the cosolute. The cationic species stacks usually more efficiently. The chemical nature of the N9-substituent has only a moderate influence on the base-stacking properties.     

Antigody-nucleic acid complexes. Inhibition of translation of silkmoth chorion messenger ribonucleic acid with antibodies specific for 7-methylguanosine.

Antibodies specific for 7-methylguanosine (m7G) were evaluated for their ability to inhibit the translation of chorion mRNA in a wheat germ, cell-free amino acid incorporating system. Results obtained with antibody concentrations of 0.5--1.5 microM revealed dose-dependent inhibition of [3H]-labeled amino acid incorporation into acid-insoluble radioactivity. Inhibition of translation was attributed to the interaction of anti-m7G antibodies with the 5' termini of chorion mRNAs on the basis that (a) anti-m7G antibodies coupled to Sepharose (anti-m7G-Sepharose) immunospecifically retained 5'-terminal cap structures of chorion mRNAs, i.e., m7G (5')ppp(5')Nm, (b) significant inhibition of translation required a 2-h preincubation of anti-m7G antibodies with mRNA, and (c) similar preincubation periods with anti-m7G antibodies in the presence of the competing nucleoside hapten (m7G) obviated the inhibitory effect of the antibody. The nature of the anti-m7G antibody-mRNA complex was examined by digesting chorion mRNA with nuclease P1 before (predigested) and after (postdigested) immunospecific adsorption to anti-m7G-Sepharose adsorbent. Whereas predigested preparations yielded a single cap structure of the type m7G(5')ppp(5')N, the predominating cap in the postdigested sample was m7G(5')ppp(5')NpNpN. These latter data revealed that the nucleotide sequence adjacent to the cap was not significantly masked by the antibody and suggest the utility of anti-m7G antibody as a site-specific probe.     

Differential inhibition with partially purified and endogenous rabbit reticulocyte globin mRNA by 7-methylguanosine 5'-monophosphate.

The effect of 7-methylguanosine 5′-monophosphate (m⁷G^5′ p) on translation of partially purified globin mRNA and of polysome-associated endogenous globin mRNA has been studied. Under identical experimental conditions, with 0.4 mM m⁷G^5′ p, translation with partially purified globin mRNA is inhibited 50%; translation with endogenous globin mRNA is inhibited 10%. The inhibition of protein synthesis by m⁷G^5′ p occurs at a step before the first peptide bond formation as evidenced by studies with pactamycin; 0.4 mM m⁷G^5′ p inhibited the first dipeptide synthesis 43% when the partially purified globin mRNA was used whereas 15% inhibition was observed with the endogenous mRNA. The inhibition of m⁷G^5′ p appears to be related to the structural integrity of globin mRNA.     

Translational recognition of the 5'-terminal 7-methylguanosine of globin messenger RNA as a function of ionic strength.

The translation of rabbit globin mRNA in cell-free systems derived from either wheat germ or rabbit reticulocyte was studied in the presence of various analogues of the methylated 5' terminus (cap) as a function of ionic strength. Inhibition by these analogues was strongly enhanced by increasing concentrations of KCl, K(OAc), Na(OAc), or NH4(OAc). At appropriate concentrations of K(OAc), both cell-free systems were equally sensitive to inhibition by m7GTP. At 50 mM K(OAc), the reticulocyte system was not sensitive to m7GMP or m7GTP, but at higher concentrations up to 200 mM K(OAc), both nucleotides caused strong inhibition. The compound in m7G5'ppp5'Am was inhibitory at all concentrations of K(OAc) ranging from 50 to 200 mM, although more strongly so at the higher concentrations. Over the same range of nucleotide concentrations, the compounds GMP, GTP, and G5'ppp5'Am were not inhibitors. The mobility on sodium dodecyl sulfate-polyacrylamide electrophoresis of the translation product was that of globin at all K(OAc) concentrations in the presence of m7GTP. Globin mRNA from which the terminal m7GTP group had been removed by chemical treatment (periodate-cyclohexylamine-alkaline phosphatase) or enzymatic treatment (tobacco acid pyrophosphatase-alkaline phosphatase) was translated less efficiently than untreated globin mRNA at higher K(OAc) concentrations, but retained appreciable activity at low K(OAc) concentrations.     

Molecular design of a eukaryotic messenger RNA and its chemical synthesis.

A designed mRNA consisting of 42 ribonucleotides having the cap structure was synthesized. The capped leader sequence of the brome mosaic virus (BMV) mRNA 4, m7G5'pppGUAUUAAUA (F-1), was synthesized by the phosphotriester method and followed by the capping reaction. A 32-mer consisting of an initiation codon (AUG), the coding region corresponding to a bacterial pheromone cAD1 and two stop codons, was constructed by the 18-mer (F-2) and 14-mer (F-3), which were synthesized by the phosphoramidite method. 2'-,3'-O-Methoxymethylene-guanosine 5'-phosphate was condensed with F-3 using P1-2',3'-O-methoxymethyleneguanosine-5'-yl P2-adenosine-5'-yl pyrophosphate (9) with T4 RNA ligase. The chemically synthesized RNA fragments were ligated successively with T4 RNa ligase to afford the whole RNA molecule.     

RNA 3'-phosphate cyclase (RtcA) catalyzes ligase-like adenylylation of DNA and RNA 5'-monophosphate ends

RNA 3'-phosphate cyclase (Rtc) enzymes are a widely distributed family that catalyze the synthesis of RNA 2',3'-cyclic phosphate ends via an ATP-dependent pathway comprising three nucleotidyl transfer steps: reaction of Rtc with ATP to form a covalent Rtc-(histidinyl-N)-AMP intermediate and release PP(i); transfer of AMP from Rtc to an RNA 3'-phosphate to form an RNA(3')pp(5')A intermediate; and attack by the terminal nucleoside O2' on the 3'-phosphate to form an RNA 2',3'-cyclic phosphate product and release AMP. The chemical transformations of the cyclase pathway resemble those of RNA and DNA ligases, with the key distinction being that ligases covalently adenylylate 5'-phosphate ends en route to phosphodiester synthesis. Here we show that the catalytic repertoire of RNA cyclase overlaps that of ligases. We report that Escherichia coli RtcA catalyzes adenylylation of 5'-phosphate ends of DNA or RNA strands to form AppDNA and AppRNA products. The polynucleotide 5' modification reaction requires the His(309) nucleophile, signifying that it proceeds through a covalent RtcA-AMP intermediate. We established this point directly by demonstrating transfer of [(32)P]AMP from RtcA to a pDNA strand. RtcA readily adenylylated the 5'-phosphate at a 5'-PO(4)/3'-OH nick in duplex DNA but was unable to covert the nicked DNA-adenylate to a sealed phosphodiester. Our findings raise the prospect that cyclization of RNA 3'-ends might not be the only biochemical pathway in which Rtc enzymes participate; we discuss scenarios in which the 5'-adenylyltransferase of RtcA might play a role.     

Messenger RNA affinity column fractionation of eukaryotic initiation factor and the translation of myosin messenger RNA.

Both myosin mRNA (26 S) and globin mRNA (9 S) have been bound to activated Sepharose 4B. The affinity of initiation factors derived from native 40 S ribosomal subunits from embryonic chick muscle for these messengers has been determined. Although both messengers bind the major components of the muscle factor preparation with the same affinity, some differences are noted in the minor components. There is an enrichment of components which bind myosin mRNA with a high affinity when the 15–18 S initiation factor complex is prepared from initiating 40 S ribosomal subunits found on myosin synthesizing polysomes rather than from total cellular factor preparations. The proteins which have a high binding affinity to myosin mRNA also have a discriminating effect when added to a wheat germ system containing myosin and globin mRNA. This is demonstrated by the fact that the synthesis of myosin heavy chain is specifically stimulated and the number of ribosomes found on myosin mRNA increase five to seven-fold; whereas neither the synthesis of globin nor the number of ribosomes associated with globin mRNA is increased. The components of an impure reticulocyte eukaryotic initiation factor 3 prepared in a similar manner as the muscle factor, do not bind myosin mRNA with the same high affinity, and these fractions separated on the myosin mRNA affinity column did not show a discriminatory effect. These results suggest that specific components of muscle 15–18 S initiation factor preparations have a higher binding affinity for myosin mRNA than globin mRNA and that these proteins may be those factors previously reported to be present which discriminate between mRNAs.     

Antagonistic action between antibodies directed against 7-methylguanosine and polyamines on translation in vitro of RNA induced by measles virus

Antibodies directed against N7-methylguanosine (m7Guo) were prepared and added to a wheat germ cell-free protein-synthesizing system programmed with RNA extracted from monkey cells persistently infected with measles virus. A dose-dependent inhibition of [35S]methionine incorporation was observed when RNA was preincubated with anti-m7Guo immunoglobulins. Antibodies preincubated with m7Guo did not show any inhibiting activity. The inhibitory effect of antibodies was abolished when RNA was preincubated with immunoglobulins in the presence of spermine and spermidine. When polyamines were added to the assay programmed with the IgG-RNA complex, no inhibition was observed.     

Comparison of eubacterial and eukaryotic 5S RNA structures: a chemical modification study

The 5S RNAs from Bacillus stearothermophilus and Saccharomyces cerevisiae were probed by nucleotide-specific reagents, with a view to compare and contrast their higher order structures. The progressive unfolding of the RNAs during heating, in the presence and absence of magnesium, was monitored. Evidence was provided for the double-helical segments which occur in the secondary structural models of both RNAs. The results also placed constraints on the possible structuring of the remainder of the RNA and yielded some insight into ways of folding up the molecule. Together with the data from our earlier studies, employing ribonucleases, these results provide a detailed picture of the structuring and topography of the 5S RNAs. The main structural differences between the eubacterial and eukaryotic RNAs occur throughout the loop D/helix IV/loop E/helix V arm; in particular strong evidence is provided for loop D of the eukaryotic RNA being involved in a tertiary interaction.     

Inhibition of transfer messenger RNA aminoacylation and trans-translation by aminoglycoside antibiotics

Transfer messenger RNA (tmRNA) directs the modification of proteins of which the biosynthesis has stalled or has been interrupted. Here, we report that aminoglycosides can interfere with this quality control system in bacteria, termed trans-translation. Neomycin B is the strongest inhibitor of tmRNA aminoacylation with alanine (K(i) value of approximately 35 micro m), an essential step during trans-translation. The binding sites of neomycin B do not overlap with the identity determinants for alanylation, but the aminoglycoside perturbs the conformation of the acceptor stem that contains the aminoacylation signals. Aminoglycosides reduce the conformational freedom of the transfer RNA-like domain of tmRNA. Additional contacts between aminoglycosides and tmRNA are within the tag reading frame, probably also disturbing reprogramming of the stalled ribosomes prior protein tagging. Aminoglycosides impair tmRNA aminoacylation in the presence of all of the transfer RNAs from Escherichia coli, small protein B, and elongation factor Tu, but when both proteins are present, the inhibition constant is 1 order of magnitude higher. SmpB and elongation factor Tu have RNA chaperone activities, ensuring that tmRNA adopts an optimal conformation during aminoacylation.     

Inhibition of prostaglandin biosynthesis by 7-oxa- and 5-oxa-prostaglandin analogues (Short Communication)

Some oxaprostaglandin derivatives have been shown to inhibit prostaglandin biosynthesis from arachidonate by a particulate prostaglandin synthetase preparation. The most potent inhibitor was 5-oxaprost-13-trans-enoate, and inhibition by this compound appeared to be competitive. Certain structure-activity relationships were ascertained.