02-Methylguanosine

Figure 35-10. The cap structure attached to the 5 terminal of most eukaryotic messenger RNA molecules. A 7-methylguanosine triphosphate (black) is attached at the 5 terminal of the mRNA (shown in blue), which usually contains a 2 -0-methylpurine nucleotide.

The primary transcripts generated by RNA polymerase II—one of three distinct nuclear DNA-depen-dent RNA polymerases in eukaryotes—are promptly capped by 7-methylguanosine triphosphate caps (Figure 35-10) that persist and eventually appear on the 5 end of mature cytoplasmic mRNA. These caps are necessary for the subsequent processing of the primary transcript to mRNA, for the translation of the mRNA, and for protection of the mRNA against exonucleolytic attack. 

As mentioned above, mammahan mRNA molecules contain a 7-methylguanosine cap structure at their 5 terminal, and most have a poly(A) tail at the 3 terminal. The cap stmcmre is added to the 5 end of the newly transcribed mRNA precursor in the nucleus prior to transport of the mELNA molecule to the cytoplasm. The S cap of the RNA transcript is required both for efficient translation initiation and protection of the S end of mRNA from attack by S —> S exonucleases. The secondary methylations of mRNA molecules, those on the 2 -hydroxy and the N of adenylyl residues, occur after the mRNA molecule has appeared in the cytoplasm. 

Sequence analysis of the 5 -ends of viral and nuclear mRNA molecules reveals that these are frequently capped , with an unusual structure in which 7-methylguanosine is joined by a (5 - 5 ) triphosphate link to a 2 -0-methyl nucleoside moiety which is the first residue in the (3 ->5 )-linked polynucleotide chain.166-168 The presence of this cap structure is required for ribosomal binding and translation to take place,167- 168 and 7-methylguanosine-5 -phosphate inhibits translation by preventing formation of the ribosome-mRNA complex.169... 

A 7-methylguanosine cap is added to the 5 end while the RNA molecule is still being synthesized. The cap structure serves as a ribosome-binding site and also helps to protect the mRNA chain from degradation. 

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. 

FIGURE 26-12 The 5 cap of mRNA. (a) 7-Methylguanosine is joined to the 5 end of almost all eukaryotic mRNAs in an unusual 5, 5 -triphosphate linkage. Methyl groups (pink) are often found at the 2 position of the first and second nucleotides. RNAs in yeast cells lack the 2 -methyl groups. The 2 -methyl group on the second nucleotide... 

Most eukaryotic mRNAs have a 5 cap, a residue of 7-methylguanosine linked to the 5 -terminal residue of the mRNA through an unusual 5, 5 -triphosphate linkage (Fig. 26-12). The 5 cap helps protect mRNA from ribonucleases. The cap also binds to a specific capbinding complex of proteins and participates in binding of the mRNA to the ribosome to initiate translation (Chapter 27). 

Fukaryotic mRNAs are modified by addition of a 7-methylguanosine residue at the 5 end and by cleavage and polyadenylation at the 3 end to form a long poly(A) tail. 

Expression of genetic information by transcription. [Note RNAs shown are eukaryotic.] me-7Gppp = 7-methylguanosine triphosphate cap, described on p. 414. AAA = poly-A tail, described on p. 414. 

Posttranscriptional modification of mRNA showing the 7-methylguanosine cap and poly-A tail. 

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... 

Structure of the 5 methylated cap of eukaryotic mRNA. A 7-methylguanosine (in red) is attached through a triphosphate linkage formed between its 5 -OH and the 5 -OH of the terminal residue in... 

In contrast to prokaryotic mRNAs, mRNAs from various eukaryotic cells and viruses have been found to contain a terminal 7-methylguanosine (m7G) residue linked from its 5 -position through a triphosphate bridge which was presented commonly as shown in the following Scheme 1. ... 

We first describe the synthesis of unsymmetrical a.y-dinucleoside triphosphates involving 7-methylguanosine. For the construction of the terminal cap structure, there might be two possible reaction modes where an activatable protecting group (X) was introduced into a nucleotide by direct displacement with phosphate hydroxyl group (Method A) and by pyrophosphorylation between Xp and pN (Method B). 

Immediately after transcription, the 5 phosphate is removed, guanosyl transferase adds a G residue linked via a 5 -5 covalent bond, and this is methylated to form a 7-methylguanosine (m7G) cap (methylated in N-7 position of the base). The ribose residues of either the adjacent one or two nucleotides may also be methylated by methyl group addition to the 2 OH of the sugar. The cap protects the 5 end of the mRNA against ribonuclease degradation and also functions in the initiation of protein synthesis. 

Based on chemical considerations alone, ribozymes should be able to catalyze many different types of reactions. Ribozymes can maintain defined secondary and tertiary structures, just as protein enzymes do. Ribozymes can interact with substrates specifically via hydrogen bond networks, just as protein enzymes do. Finally, ribozymes have available to them a chemistry that, while more limited than of proteins, is substantial. RNA contains proton donors and acceptors with pK, values that cluster at 4 and 9.71 The critical lack of a good donor/acceptor with a pKt near 7 can be rectified by any of several simple expedients, such as modification of guanosine to 7-methylguanosine, protonation of triple base-paired cysto-sine,72 or inclusion of a proton donor/acceptor in an environment with a different polarity than water (in this respect, it is interesting to note that Dahm and Uhlenbeck have found that the cleavage reaction catalyzed by the hammerhead ribozyme is dependent on some dissociable proton with a pKa of 8.0).73... 

Attaches 7-methylguanosine to the penultimate nucleoside of RNA via a 5, 5 -triphosphate bridge. 

Both G, Furuichi Y, Muthukrishnan S, Shatkin A (1975) Ribosome binding to reovirus mRNA in protein synthesis requires 5 terminal 7-methylguanosine. Cell 6 185-195... 

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