Spliceosome, snRNAs

Villa, T.. Pleiss, J. A., and Guthrie, C. 2002. Spliceosomal snRNAs Mg dependent chemistry at the catalytic core Cell 109 149—152. 

Hudson AJ, Moore A, Elniski D, Joseph J, Yee J, Russell AG. Evolutionarily divergent spliceosomal snRNA and a conserved non-coding RNA processing motif in Giardia lamblia. Nucleic Acids Res. 2012 40 10996-1006. doi 10.1093/nar/gks887. 

Simoes-Barbosa A, Melon D, Wohlschlegel JA, Konarska MM, Johnson PJ. Spliceosomal snRNAs in the unicellular eukaryote Trichomonas vaginalis are stnicturally conserved but lack a 5 -cap structure. RNA. 2008 14 1617-31. 

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. 

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). 

A small fraction of eukaryotic mRNA introns are characterized by AU and AC (rather than GU and AG) ends. The spliceosomes that act on these introns contain modified snRNAs U4 and U6, which are designated U4 ., and U6.,f.,.. They also require Ull and U12 snRNPs.534 625 626... 

It has long been snspected that the two rran -esterification reactions to remove the introns from a pre-mRNA are catalyzed by the RNA constitnents in the spliceosome (128). This idea is bolstered by the fact that U snRNAs in the spliceosome do form functional strnctures (the extended U6 intramolecnlar stem in complex B2 and the U6 intramolecular stem plus U2-U6 Helix Ib in complex Cl see Fig. 1) resembling domain... 

Remarkably, Ull, U12, U4atac, and Ubatac form structures that are almost identical to their counterparts in the major spliceosome, namely Ul, U2, U4, and U6, respectively, despite the fact that the two sets of snRNAs are quite different in primary sequences (136, 137). Equally strikingly, the network of RNA-RNA interactions detected in the major spliceosome also exists in the Ul 2-dependent spliceosome, further validating the importance of these dynamic interactions during spliceosome assembly and splicing (136, 137). 

Madhani HD, Guthrie C. A novel base-pairing interaction between U2 and U6 snRNAs suggests a mechanism for the catalytic activation of the spliceosome. Cell 1992 71 803-817. 

Gornemann J, Kotovic KM, Hujer K, Neugebauer KM. Cotran-scriptional spliceosome assembly occurs in a stepwise fashion and requires the cap binding complex. Mol. Cell 2005 19 53-63. Lacadie SA, Rosbash M. Cotranscriptional spliceosome assembly dynamics and the role of U1 snRNA 5 ss base pairing in yeast. Mol. Cell 2005 19 65-75. 

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