Caps, Splices, Edits, and Poly-A Tails on Eukaryotic RNAs
Eukaryotic RNAs are processed at their beginning, middle, and terminal sections. Although the structures resulting from these posttranscrip-tional modifications are known, why the modifications occur is not clear.
Figure 5.11 The structure of the cap found on eukaryotic messenger RNAs.The first base is 7-methylguanylate connected by a 5’-5’ triphosphate linkage to the next base. The 2’ positions on bases 1 and 2 may or may not be methylated.
The 5’ ends of nearly all cellular messenger RNAs contain a “cap,” a guanine in a reversed orientation plus several other modifications (Fig. 5.11). More precisely, the cap is a guanine methylated on its 7 position joined through 5’-5’ pyrophosphate linkage to a base derived from transcription. Both the first and second bases after the capping nucleo-tide usually are methylated. These modifications were discovered by tracking down why and where viral RNA synthesized in vitro could be methylated. The trail led to the 5’ end whose structure was then deter-mined by Shatkin. The cap helps stabilize the RNA, is sometimes involved in export to the cytoplasm, may be involved with splicing, and also assists translation. Ribosomes translate cap-containing messengers more efficiently than messengers lacking a cap.
A sizable fraction of the RNA that is synthesized in the nucleus of eukaryotic cells is not transported to the cytoplasm. This RNA is of a variety of sizes and sequences and is called heterogeneous nuclear RNA. Within this class of RNA are messenger RNA sequences that ultimately are translated into protein, but, as synthesized, the nuclear RNAs are not directly translatable (Fig. 5.12). They contain extraneous sequences that must be removed by cutting and splicing to form the continuous
5.12 Maturation of aprimary transcript
to yield a translatable message.
coding sequence that is found in the mature messenger in the cytoplasm. The intervening sequences, which are removed from mature RNA, are called introns, and the other parts of the messenger sequence are often referred to as exons.
Another type of modification in RNA after its synthesis is called editing. Although most RNAs are spliced, editing occurs very rarely. One prominent example of editing is the apolipoprotein. Humans contain a single copy of this gene. In the liver its gene product is 512,000 daltons molecular weight, but in intestinal cells its gene product is only 242,000 daltons molecular weight. Examination of the mRNA shows that a specific cytosine of the RNA is converted to a uracil or uracil-like nucleotide in intestinal cells, but not in hepatic cells. This conversion creates a translation stop codon, hence the shorter gene product in intestinal cells. The conversion process involved is called RNA editing. It is also found in some animal viruses. In a few extreme cases, hundreds of nucleotides in an RNA are edited. In these cases, special short guide RNA molecules direct the choice of inserted or deleted nucleotides.
The final type of RNA modification found in eukaryotic cells is the posttranscriptional addition to the 3’ end of 30 to 500 nucleotides of polyadenylic acid. This begins about 15 nucleotides beyond the poly-A signal sequence AAUAAA. Transcription itself appears to terminate somewhat beyond the poly-A signal and processing quickly removes the extra nucleotide before the poly-A addition.
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