Splicing

Richard J. Roberts and Phillip A.Sharp received the Nobel Prize in Physiology and Medicine in 1993 for the discovery of the splicing mechanism.

The term "splicing" comes from the English splice 'to join together', 'to stick together and refers to the joining of the exons to form the mature mRNA.' Many genes in higher organisms are present "fragmented", i.e. the gene sequence is interrupted by DNA segments that do not contain "building instructions" for proteins. Those regions that contain the biological information are called exons (expressed sequences). Those DNA segments that lie in between and do not contain biological information are referred to as introns (intervening sequences) (Matera AG et al. 2014).

During the conversion of genetic information into proteins, an RNA copy of the entire gene sequence is first created(transcription), which is referred to as the primary transcript (pre-mRNA). The introns are then removed via a complicated regulatory mechanism.

The DNA segments containing the biological information, the exon sequences, are joined together. This process is called "splicing". The now completed mRNA then travels to the ribosomes to be recopied (translated) into an amino acid chain (protein).

In many cases, the exons can be combined in different ways during splicing ("alternative", "differential" or "tissue-specific splicing").

In this way, a single gene segment can give rise to a large number of different proteins. Splicing occurs along with polyadenylation (tailing) of the 3' end after transcription, so it is a post-transcriptional process. In contrast, capping of the 5' end is a cotranscriptional process.

The process of splicing is catalyzed by the spliceosome, a large protein-RNA complex in which RNA performs the catalytic function (Matera AG et al. 2014). Two different types of spliceosome are distinguished(Patel AA et al. 2003):

• the main spliceosome, consisting of five different uridine-rich small nuclear ribonucleoprotein particles (U-snRNP) named after the respective snRNA component (U1,U2, U4/6 and U5).

and

• the minor spliceosome, also consisting of five different snRNPs (U11, U12, U4atac/U6atac and U5), where the U5 snRNP is identical to that of the major spliceosome. The U snRNAs are associated with additional uridine-specific proteins and seven protein co-factors: the Sm proteins B/B', D1, D2, D3, E, F, G (or LSm proteins 2-8) (Will CL et al. 2011).

Together, they thus form the mature U-snRNP . More than 99% of splicing reactions in eukaryotes are catalyzed by the major spliceosome, which is why these reactions are also referred to as canonical splicing (Burset M et al. (2000). The minor spliceosome is responsible for less than 0.5% of splicing reactions. The importance and function of this mechanism is poorly understood.

1. Burset M et al. (2000) Analysis of canonical and non-canonical splice bibliography 98 sites in mammalian genomes. Nucleic Acids Res 28: 4364-4375.
2. Johnson JM et al.(2003) Genome-wide survey of human alternative pre-mRNA splicing with exon junction microarrays. Science 302: 2141-2144.
3. Matera AG et al. (2014) A day in the life of the spliceosome. Nat Rev Mol Cell Biol 15: 108-121.
4. Patel AA et al (2003) Splicing double: insights from the second spliceosome. Nat Rev Mol Cell Biol 4: 960-970.
5. Tseng CK et al. (2008) Both catalytic steps of nuclear pre-mRNA splicing are reversible. Science 320: 1782-1784.
6. Valadkhan S et al.(2009) Protein-free small nuclear RNAs catalyze a two-step splicing reaction. Proc Natl Acad Sci U S A 106: 11901-11906.
7. Valadkhan S. et al. (2007) Protein-free spliceosomal snRNAs catalyze a reaction that resembles the first step of splicing. RNA 13: 2300-2311.
8. Wang Y et al.(2015) Mechanism of alternative splicing and its regulation. Biomed Rep 3: 152-158.
9. Will CL et al. (2011) Spliceosome structure and function. Cold Spring Harb Perspect Biol 3 (7).