Introns removal

The proce.ss of intron removal and exon Joining is called splicing. 

Small nuclear RNAs (snRNAs), a subset of these RNAs, are significantly involved in mRNA processing and gene regulation. Of the several snRNAs, Ul, U2, U4, U5, and U6 are involved in intron removal and the processing of hnRNA into mRNA (Chapter 37). The U7 snRNA may be involved in production of the correct 3 ends of histone mRNA—which lacks a poly(A) tail. The U4 and U6 snRNAs may also be required for poly(A) processing. 

Mutations in splice sites affect the accuracy of intron removal from hnRNA during posttran-scriptionai processing. As illustrated in Figure 1-4-4, if a splice site is lost through mutation, spliceosomes may ... 

Prokaryotic and eukaryotic tRNAs are also made from longer precursor molecules. These must have an intervening sequence (intron) removed, and the 5 - and 3 -ends of the molecule are trimmed by ribonuclease. A 3 -CCA sequence is added and bases at specific positions are modified, producing "unusual" bases. 

In at least one eukaryote, Tetmhymem, the pre-rRNA molecule contains an intron. Removal of the intron during processing of the pre-rRNA does not require the assistance of any protein Instead, in the presence of guanosine, GMP, GDP or GTP, the intron excises itself, a phenomenon known as selfsplicing. This was the first demonstration of ribozymes, that is, catalytic RNA molecules that catalyze specific reactions. The list of ribozymes is growing. For example, self-splicing introns have been discovered in some eukaryotic mRNAs and even peptidyl transferase, a key enzyme activity in protein synthesis, is now known to be a ribozyme (see Topic H2). 

This intron must be removed in order to create a functional tRNA molecule. Its removal occurs by cleavage by an endonuclease at each end of the intron and then ligation together of the tRNA ends. This RNA splicing pathway for intron removal is totally different from that used to remove introns from pre-mRNA molecules in eukaryotes (Topic G8) and must have evolved independently. 

Both capping and poly A formation precede intron removal. Thus, the first steps of processing result in a pre-mRNA that has a 5 cap and a 3 polyA tail, but with all its introns present. 

In summary, mRNA processing requires an interconnected set of reactions Cap and polyA synthesis, followed by intron removal. Soluble enzymes catalyze the former two reactions, while the latter set of reactions involves both RNA and protein components. 

Thomas Cech and his coworkers discovered RNA catalysis in the early 1980 s while studying the removal of an intron from the riboso-mal RNA of a small ciliated protozoan, Tetrahymena. They established the reaction in vitro and then purified the components of the reaction mixture to isolate what they thought would be the enzyme responsible. To their surprise, they could remove all the detectable proteins from the mixture and still get intron removal to occur efficiently. 

Takahara K, Schwatze U, Imamura Y, Hoffman GG, Toriello H et al (2(X)2) Order of intron removal influences multiple splice outcomes, including a two-exon skip, in a COL5A1 acceptor-site mutation that results in abnormtil pro-tilphal(V) N-propeptides tind Ehlers-Danlos syndrome type I. Am J Hum Genet 71 451 65... 

Component of the spliceosome, the intron-removing apparatus in eukaryotic nuclei. See Graveley, B.R., Sorting out the complexity of SR protein fnnctions, RNA 6, 1197-1211, 2000 Will, C.L. and Luhrmann, R., Spliceosomal UsnRNP biogenesis, strnctnre, and function, Curr. Opin. Cell Biol. 13, 290-301, 2001 Turner, LA., Norman, C.R., Chnrcher, M.J., and Newman, A.J., Roles of the U5 snRNP in spliceosome dyanamics and catalysis, Biochem. Soc. Trans. 32, 928-931, 2004. 

Class I introns were originally discovered in ciliated protozoa and subsequently were found in fungi, bacteriophages, and other organisms. The RNA itself in a class I in-tron has catalytic activity and class I introns remove themselves from primary RNA transcripts by a self-splicing reaction. Class I introns are not true enzymes in that they function only once. The nucleotides in the intron that is spliced out are recycled in the cell. 

Exons are protein-coding regions that must be joined by removing introns, the noncoding intervening sequences. The process of intron removal and exon Joining is called splicing. 

EXAMPLE 9.17 What modifications other than intron removal and exon splicing occur in the primary mRNA transcript ... 

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