Splicing snRNA

All eukaryotic cells have four major classes of RNA ri-bosomal RNA (rRNA), messenger RNA (mRNA), transfer RNA (tRNA), and small nuclear RNA (snRNA). The first three are involved in protein synthesis, and snRNA is involved in mRNA splicing. As shown in Table 37-1, these various classes of RNA are different in their diversity, stability, and abundance in cells. 

Spliceosome The macromolecular complex responsible for precursor mRNA splicing. The spliceosome consists of at least five small nuclear RNAs (snRNA Ul, U2, U4, U5, and U6) and many proteins. 

Small nuclear RNA (snRNA), vrfiich is also only found in the nucleus of eukaryotes. One of its major functions is to participate in splicing (removal of introns) mRNA. 

Introns are removed from hnRNA by splicing, accomplished by spliceosomes (also known as an snRNP, or snurp), which are complexes of snRNA and protein. The hnRNA molecule is cut at splice sites at the 5 (donor) and 3 (acceptor) ends of the intron. The intron is excised in the form of a lariat structuie and degraded. Neighboring exons are joined together to assemble the coding region of the mature mRNA. 

The removal of introns from pre-messenger RNAs in eukaryotes is catalyzed by the spliceosome, which is a large ribonucleoprotein consisting of at least 70 proteins and five small nuclear RNAs (snRNA) [144]. This splicing pathway involves two phosphotransfer reactions. In the first step, the 5 splice site is attacked by a 2 hydroxy group of an adenosine nucleotide within the intron [indicated by A in Fig. 12] that corresponds to the branch point in the lariat intermediate (Fig. 12,middle). In the second step, the 3 -OH group of the free 5 exon attacks the phosphodiester bond between the intron and... 

Of the five snRNAs, U2 and U6 interact with the reaction site (the 5 splice site and the branch point) in the first chemical step. These two snRNAs are known to anneal together to form a stable-based paired structure in the absence of proteins and in the presence of ions as shown in Fig. 13, with U2 acting as an inducer molecule that displaces the U4 (that is an antisense molecule that regulates the catalytic function of U6 RNA) from the initially formed U4-U6 duplex. The secondary (or higher ordered) structure of the U2-U6 complex consists of the active site of the spliceosome. Recent data suggests that these two snRNAs function as the catalytic domain of the spliceosome that catalyzes the first step of the splicing reaction [145]. 

Small nuclear RNAs (snRNAs) are involved in the splicing of mRNA precursors (see p.246). They associate with numerous proteins to form spliceosomes. ... 

Spliceosomal introns generally have the dinucleotide sequence GU and AG at the 5 and 3 ends, respectively, and these sequences mark the sites where splicing occurs. The Ul snRNA contains a sequence complementary to sequences near the 5 splice site of nuclear mRNA introns (Fig. 26-16a), and the Ul snRNP... 

The known catalytic repertoire of ribozymes continues to expand. Some virusoids, small RNAs associated with plant RNA viruses, include a structure that promotes a self-cleavage reaction the hammerhead ribozyme illustrated in Figure 26-25 is in this class, catalyzing the hydrolysis of an internal phosphodiester bond. The splicing reaction that occurs in a spliceosome seems to rely on a catalytic center formed by the U2, U5, and U6 snRNAs (Fig. 26-16). And perhaps most important, an RNA component of ribosomes catalyzes the synthesis of proteins (Chapter 27). 

One final class of eukaryotic RNA that deserves mention are the small nuclear RNAs (snRNAs). As their name suggests, this class of RNA is found in the nucleus and participates in the splicing of in-trons from certain types of eukaryotic mRNAs. Five snRNAs (Ul, U2, U4, U5, and U6) take part in these splicing reactions when combined with a... 

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