RNA Splicing

RNA splicing is the process in which introns, or intervening sequences within a gene, are removed from ribonucleic acid (RNA) transcribed from deoxyribonucleic acid (DNA), prior to translation of RNA into protein.

Prior to the early 1970s, the structure of genes had been elucidated and it was understood that genes were located with linear DNA sequences. The central dogma of molecular biology had been established, which described the flow of information to be from DNA to RNA to protein. Since many experiments investigating gene regulation were performed in bacteria, the molecular biology of vertebrate cells were later found to be more complex. An indication that eukaryotic cells utilized a more complicated pathway of gene expression than bacteria was suggested and prompted further investigation. It soon became clear that a subpopulation of RNA in the nucleus called heterogeneous nuclear RNA (hnRNA) was found to be approximately 4–5 fold longer than the cytoplasmic mRNA, necessitating the establishment of a molecular relationship between the two related RNA molecules.

American chemist Phillip A. Sharp used an adenovirus (a human virus that can cause the common cold) as a model for gene regulation and began by identifying the critical gene regions in the virus's genome. Next, he characterized the individual transcripts (mRNA) and compared it to the genomic DNA sequence of the virus by hybridizing (based on complementary base pairing) single stranded DNA to the mRNA. In these experiments, it was observed that the adenoviral DNA did not hybrize completely to the mRNA. There were loops on the DNA representing missing sequences on the mRNA strand. Soon after, other researcher reported this split gene structure in higher organisms. It was later determined that these extra regions on the DNA were removed shortly after the RNA strand was produced. The sequences removed were later called introns, while the sequences that remained in the processed RNA, which represented mRNA, were called exons. This process of removing introns is called RNA splicing. Sharp was later awarded the Nobel Prize for his scientific discoveries.

RNA splicing occurs in the nucleus of the cell where DNA transcription takes place. There are several types of known splicing mechanisms. One of which involves the spliceosome, an array of proteins that function to splice out introns. The human spliceosome has been found to contain 44 different components. Another mechanism involves excision of introns by the RNA itself. Introns have also been shown to be removed by tRNA.

The spliceosome system is one of the most widely understood splicing mechanisms. Five small nuclear RNAs (snRNAs) and more than 50 different proteins comprise the splicing machinery. snRNAs are essential splicing factors. Each snRNA aggregates with various proteins to achieve five distinct small nuclear ribonucleoprotein (snRNPs) complexes (U1, U2, U3, U4, U5). These snRNP complexes and other protein splicing factors, collectively called the spliceosome, determine the exon–intron borders of the pre–processed mRNA. It is believed that the RNA (not the protein) are the active sites for the reaction. The proteins serve to initiate, stabilize, and break the RNA–RNA interactions that form during this process. A set of enzymes cuts the intron from the RNA and joins the two ends or exons.

In comparing different tissues or developmental stages, the mRNA produced from the same gene may be different depending on how the RNA gets processed. Thus, for an identical gene, many different proteins can be produced. The process is called alternative splicing and represents an important principle in how the genetic message is determined. It is not definitely determined at the stage when the RNA is first synthesized. Instead, the splicing pattern determines how the genetic information will be delivered and the nature of the final protein product.