Guanine cap

Both ends of mRNA molecules are chemically modified. The 5 end is capped by 7-methylguanosine triphosphate. This modified guanine cap helps to protect the mRNA from attack by hydrolytic enzymes. Beyond that, the 7-methylguanosine triphosphate cap acts as a recognition element for ribosomes. The 3 end is modified by the addition of a poly A tail. This may consist of as few as 30 or as many as 200 adenine nucleotides. 

During the processing of primary transcripts destined for mRNA formation in eukaryotes, a methylated guanine cap is attached to the 5 end. This is essential for efficient initiation of translation of the mature eukaryotic mRNA and does not occur in prokaryotes. 

During the processing of primary transcripts that are destined for mRNA formation in eukaryotes, a methylated guanine cap is attached to the 5 end. This is essential for efficient initiation of translation of the mature eukaryotic mRNA, and it does not occur in prokaryotes. In prokaryotes, rRNA in the small ribosomal subunit binds first to the Shine-Dalgarno sequence of the mRNA, before binding tRNA. In eukaryotes, the ribosome binds first to the 5 cap and then scans along the mRNA, using the tRNA to detect the AUG codon. 

Hausmann, S., and Shuman, S. (2005). Giardia lamblia RNA cap guanine-N2 methyltrans-ferase (Tgs2). /. Biol. Ghent. 280, 32101—32106. 

Cap formation This process is the addition of a single guanine base to the 5 end of the RNA molecule. The guanine is attached to the terminal nucleotide via a triphosphate link. The guanine is methylated in position 7 of the base, catalysed by a methyltransferase. The cap plays a role in translation by facilitation of the binding of mRNA to the ribosome (see below). 

The 5 cap is formed by condensation of a molecule of GTP with the triphosphate at the 5 end of the transcript. The guanine is subsequently methylated at N-7, and additional methyl groups are often added at the 2 hydroxyls of the first and second nucleotides adjacent to the cap (Fig. 26-12). The methyl groups are derived from S-adenosylmethionine. All these reactions occur... 

The first processing event (Eq. 28-6) for most of the pre-mRNA and snRNA transcripts made by RNA polymerase II is addition to the 5 end of a "cap," a terminal structure containing 7-methylguanosine from which a proton has dissociated to form a dipolar ion.563 565 The cap structure may be abbreviated 5 -m7G(5 )pppNm —. The 5 terminal ribose is often methylated on 02 , as shown below. More complex caps are methylated at additional sites, e.g., the guanine may be dimethylated on the 2-NH2 group.551 Most snRNAs, including the U1-U5 and U7-U13 snRNAs, have such 2,2,7-trimethylguanosine... 

Capping. Covalent modification involving the addition of a modified guanine group in a 5 -5" linkage. It occurs only in eukaryotes, primarily on mRNA molecules. 

X Mao, S Shuman. Vaccinia virus mRNA (guanine-7-)methyltransferase mutational effects on cap methylation and AdoHcy-dependent photo-cross-linking of the cap to the methyl acceptor site. Biochemistry 35 6900-6910, 1996. 

Figure 6 illustrates some separations of a 20-mer antisense oligonucleotide with a neutral backbone. With the sample diluted in water, a symmetrical peak is obtained for a small 100-mbs injection (data not shown). The buffer is 100 mM CAPS, pH 11.7, with 6 M urea. The solute is separated as an anion, with thymine and guanine being ionized at the high pH. Figure 6 shows the separations at expanded scale. Some subtle but surprising observations are made. In 1 M salt diluent, two impurity peaks on the tail of the main component show improved resolution, while the impurity peak that elutes just before the main... 

While transcription is in process, the 5 end of the mRNA is capped with a methyl guanine nucleotide (m Gppp). 

One subunit of the capping enzyme removes the y phosphate from the 5 end of the nascent RNA emerging from the surface of an RNA polymerase II. Another domain of this subunit transfers the GMP moiety from GTP to the 5 -diphosphate of the nascent transcript, creating the unusual guanosine 5 -5 -triphosphate structure. In the final steps, separate enzymes transfer methyl groups from 5-adenosyl-methionine to the N/ position of the guanine and the 2 oxygens of riboses at the 5 end of the nascent RNA. 

Capping - The first modification occurs at the 5 end of the pre-mRNA. A GTP residue is added in reverse orientation and forms, together with the first two nucleotides of the chain, a structure known as a cap (Figure 28.30). The cap is "decorated" by the addition of methyl groups to the N-7 position of the guanine and to one or two sugar hydroxyl groups of the cap nucleotides. The cap structure serves to position the mRNA on the ribosome for translation. 

---