Transcriptional editing process

Figure 20.20 Summary of transcription, RNA processing and polypeptide synthesis. Polymerisation of the DNA template by RNA polymerase produces pre-mRNA (the primary transcript) this is transcription. The pre-mRNA is now processed, which involves capping, polyadenylation, editing and splicing (see text). The resultant mRNA transfers from the nucleus to the cytosol, where amino acids are polymerised to produce a polypeptide using the instructions present in the codons of the mRNA.

FIGURE 2 RNA editing of the transcript of the cytochrome oxidase subunit II gene from Trypanosoma brucei mitochondria, (a) Insertion of four U residues (pink) produces a revised reading frame, (b) A special class of guide RNAs, complementary to the edited product, may act as templates for the editing process. 

Figure 2.2 Transcription, mRNA processing, and translation. DNA sense strand is designated by bold lines, hnRNA and mRNA by thinner lines. Exons are shown as rectangles and introns as the intervening spaces between exons. (From An Introduction to Biochemical Toxicology, 3rd edition, E. Hodgson and R. C. Smart, eds., Wiley, 2001.)...

The sequence content of some mRNAs is altered after transcription. RNA editing is the term for a change in the base sequence of RNA after transcription by processes other than RNA splicing. RNA editing is prominent in some systems... 

The 5 ends of mRNA precursors become capped and methylated in the course of transcription. A 3 poly(A) tail is added to most mRNA precursors after the nascent chain has been cleaved by an endonuclease. RNA editing processes alter the nucleotide sequence of some mRNAs, such as the one for apolipoprotein B. 

The primary structure of a protein is its sequence of amino acids, translated from the sequence of three-letter nucleotide codons in mRNA that itself is copied (transcribed) from DNA. The transcription of RNA, its posttranscriptional editing, processing, and transport from the nucleus to the site of translation is a huge subject, beyond the scope of this article. Interested readers are advised to consult other review articles, for example Bag (1991), Kozak (1991), and Ross (1989). 

These processes are summarized in Figure 28-1. We have examined several of these mechanisms in previous chapters. Posttranscriptional modification of mRNA, by processes such as alternative splicing patterns (see Fig. 26-19b) or RNA editing (see Box 27-1), can affect which proteins are produced from an mRNA transcript and in what amounts. A variety of nucleotide sequences in an mRNA can affect the rate of its degradation (p. 1020). Many factors affect the rate at which an mRNA is translated into a protein, as well as the posttranslational modification, targeting, and eventual degradation of that protein (Chapter 27). 

Editing seems like a roundabout way to get a functional message. Why not just make the transcript so that it can be translated in the first place. Possibly, editing serves a regulatory function. Alternatively it has been proposed that editing may be a device used to accelerate the evolutionary process for a gene. 

Stuart, K., RNA editing in mitochondrial mRNA of trypanoso-matids. Trends Biochem. Sci. 16 68-72, 1991. RNA editing is processing that involves the removal, addition, and modification of nucleotides in the coding regions of nascent transcripts. 

Processing the primary transcript (this includes capping, splicing out introns, addition of the polyA tail, and RNA editing in which specific nucleotides are changed),... 

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