SnRNP, small nuclear ribonucleoprotein

In addition to the three major types of RNA described above, other RNAs are present in cells. These RNAs include the oligonucleotides that serve as primers for DNA replication and the RNAs in the small nuclear ribonucleoproteins (snRNPs or snurps) that are involved in the splicing and modification reactions that occur during the maturation of RNA precursors (see Chapter 14). 

Splicing takes place in a 50-60S small nuclear ribonucleoprotein (snRNP) particle called spliceosome consisting of a pre-mRNA, small nuclear RNAs (snRNAs) and a variety of pre-mRNA binding proteins, which ensures the proper excision of all introns and precise splicing of exons. 

Urlaub, H., Hartmuth, K., Kostka, S., Grelle, G., and Luhrmann, R. (2000) A general approach for identification of RNA-protein cross-linking sites within native human spliceosomal small nuclear ribonucleoproteins (snRNPs). Analysis of RNA-protein contads in native U1 and U4/U6.U5 snRNPs./. 

Splicing occurs in a large protein-nucleic acid complex, termed the spUceosome. Components of the spliceosome are, apart from the pre-mRNA, a number of proteins and small RNAs, termed the Ul, U2, U4, U5 and U6. The RNAs found in the spUceo-some are bound to specific proteins. The complexes are termed snRNPs (small nuclear ribonucleoprotein). Depending upon the type of RNA bound, there are Ul, U2, U5 and U4/U6 snRNPs. 

Removal of introns. snRNP = small nuclear ribonucleoprotein particle. 

To assist in the modification and processing of mRNAs, eukaryotic cells contain in their nuclei small nuclear RNAs (snRNAs), which are complexed with specific proteins to form small nuclear ribonucleoprotein particles (snRNPs). These RNAs have been named U1, U2, U3,.. . , U13 and range in size from 100 to 220 bases. One, U3, is... 

NADH nicotinamide adenine dinucleotide snRNP small nuclear ribonucleoprotein... 

The small nuclear ribonucleoprotein particles (snRNPs or snurps ) that carry out the splicing reaction use RNA-RNA basepairing to select the splice sites. Almost all intron-exon junctions contain the sequence AG-GU with the GU beginning the intron sequence. Furthermore, the consensus sequence for the beginning of the intron has a longer sequence complementary to the U1 RNA. Thus, assembly of the splicing complex, called the spliceosome, starts when the RNA component of the U1 snRNP base pairs with the junction between the 3 end of the exon and the 5 end of the intron. See Figure 12-13. 

Table 11.4 Small Nuclear Ribonucleoprotein Particles (snRNP) in Splicing of mRNA Precursors...

D, Keller W. Functions of the abundant U-snRNPs. In Small Nuclear Ribonucleoprotein Particles. Birnstiel ML, ed. 1988. Springer-Verlag, Berlin. 53. 

Table 28.3. Small nuclear ribonucleoprotein particles (snRNPs) in the splicing of mRNA precursors...

The splicing of mRNA precursors is carried out by spliceosomes, which consist of small nuclear ribonucleoprotein particles (snRNPs). Splice sites in mRNA precursors are specified by sequences at ends of introns and by branch sites... 

Most striking is the splicing of mRNA precursors, which is catalyzed by spliceosomes consisting of small nuclear ribonucleoprotein particles (snRNPs). The small nuclear RNA (snRNA) molecules in these complexes play a key role in directing the alignment of splice sites and in mediating catalysis. Indeed, some RNA molecules can splice themselves in the absence of protein. This landmark discovery by Thomas Cech and Sidney Altman revealed that RNA molecules can serve as catalysts and greatly influenced our view of molecular evolution. 

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