Guanosine caps

Addition of a 3 poly-A tail and a 5 -7-methyl guanosine cap to mRNA. 

Prokaryotic mRNA is generally identical to its primary transcript, whereas eukaryotic mRNA is extensively modified posttranscriptionally. For example, a 7-methyl-guanosine "cap" is attached to the 5-terminal end of the mRNA through a triphosphate linkage by guanylyl-transferase. A long poly-Atail—not transcribed from the DNA—is attached to the 3 -end of most mRNAs. Many eukaryotic mRNAs also contain introns that must be removed to make the mRNA functional. Their removal requires small nuclear RNAs. 

Figure 23-7 Structure of 7-methyl-guanosine capped eukaryotic mRNA. (From Stryer, L. (1988) Biochemistry,...

In vitro, the fidelity of translation is strongly influenced by the concentration of Mg2 + ions in the reaction. In the range from 1 to 4 mM Mg2 +, the ribosome will require a Shine-Delgarno sequence (prokaryotic) or a 7-methyl-guanosine cap (eukaryotic) on the mRNA before translation will initiate. For this reason, in vitro translation experiments performed with naturally occurring mRNAs are most... 

Overview of RNA processing showing 7-methyl guanosine cap addition to the 50 end, processing of hnRNA into mRNA by the process of RNA splicing,... 

Processed mRNA Contains a 50-Methyl Guanosine Cap and A 30-Polyadenylated Tail... 

Figure 30.7 shows the transcription pattern of early and late adenoviral transcription units which are transcribed from both strands. Adenoviral mRNAs are processed by the cellular machinery and contain 50 methyl guanosine caps and poly(A) tails. The adenovirus major late promoter is responsible for directing the transcription of a single precursor transcript, which gives rise to different mRNA classes that terminate at one of five polyadenylation signals. These mRNAs are then differentially spliced to remove internal portions of the coding sequence. 

Hypermethylated guanosine-capped mRNA molecules are important in cellular transport and RNA splicing. The chemical synthesis of a 5 -terminal 2,2,7-trimethylguanosine-capped tri ribobonucleotide has been described by condensation of (185) with (186) in the presence of CDI in 40% yield. The synthesis includes a novel three step synthesis of 2N,2N-dimethylguanosine from guano-sine. The product was characterised by proton and phosphorus NMR. 

Guanosine cap. GTP and SAM make methylated guanosine triphosphate (methylated GTP) to form the 7-methylgnanosine cap (7-MG cap) that protects the 5 end. 

Figure 11.4 Analysis of in vitro synthesized RNAs. 32P-Radiolabeled RNAs (48 nucleotides) capped with m7Gp3G (A and C) or m27,3 °Gp3G (B and D) were digested with either RNase T2 (A and C) or RNase T2 plus tobacco acid pyrophosphatase (TAP) (B and D) followed by anion-exchange HPLC on a Partisil 10SAX/25 column as described in the text. Fractions of 1 ml were collected, and the Cerenkov radiation was determined. The elution times of the following standard compounds, detected by ultraviolet (UV) absorption, are indicated with arrows 3,-CMP (Cp), S UMP (Up), 37-AMP (Ap), 3 -GMP (Gp), 3, 5 -m7GDP (pm7Gp), 3, 5 -GDP (pGp), 5 -GDP (p2G), 5 -GTP (p3G), and guanosine-SCtetraphosphate (P4G).

Kadokura, M., Wada, T., Urashima, C., and Sekine, M. (1997). Efficient synthesis of g-methyl-capped guanosine. V-mphosphate as a SLtcrminal unique structure of U6 RNA via a new triphosphate bond formation involving activation of methyl phosphor-imidazolidate using ZnCl2 as a catalyst in DMF under anhydrous conditions. Tetrahedron Lett. 38, 8359-8362. 

The answer is e. (Katzung, p 842.) Ribavirin most likely interferes with guanosine triphosphate synthesis, resulting in inhibition of capping of viral messenger RNA and viral RN A-dependent RNA polymerase. It is effective in moderating infections with respiratory syncytial virus. 

It is a guanosine analog which probably interferes with the synthesis of guanosine triphosphate, inhibiting capping of viral mRNA and to inhibit the viral RNA-dependent RNA polymerase. 

In a further experiment we assayed for the presence of a cap structure on the mRNAs for both Inhibitors I and II by competitive inhibition by 7-methyl-guanosine 5 -monophosphate (m G p) of the in vitro translation of these messengers. Concentrations of 40 pM m G p inhibited by 50% the in vitro translation of total tomato leaf poly(A)" " mRNA (Fig. 7A). This level is 40-fold lower than that required to similarly inhibit rabbit globin mRNA translated in a rabbit reticulocyte lysate (17) and 4-fold lower than that required to inhibit the same mRNA in a wheat germ system (18). It was of interest that the translation of Inhibitor I is inhibited to 50% by 20 pM m G p while 50% inhibition of Inhibitor II requires less than 10 pM (Fig. 7B). The basis of this difference is not understood but... 

Here, we shall describe the methylation reactions at the 7-position of guanosine residue of the cap structure. 

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