Spliceosome

[citation needed] In 1977, work by the Sharp and Roberts labs revealed that genes of higher organisms are "split" or present in several distinct segments along the DNA molecule.

The split gene structure was found when adenoviral mRNAs were hybridized to endonuclease cleavage fragments of single stranded viral DNA.

It was realised that the looped out regions, the introns, are excised from the precursor mRNAs in a process Sharp named "splicing".

Phillip Sharp and Richard J. Roberts were awarded the Nobel Prize in Medicine 1993 for the discovery of introns and the splicing process.

The snRNAs that make up the major spliceosome are named U1, U2, U4, U5, and U6, so-called because they are rich in uridine, and participate in several RNA-RNA and RNA-protein interactions.

[1] The assembly of the spliceosome occurs on each pre-mRNA (also known as heterogeneous nuclear RNA, hn-RNA) at each exon:intron junction.

[10] The molecular framework of spliceosome at near-atomic-resolution demonstrates Spp42 component of U5 snRNP forms a central scaffold and anchors the catalytic center in yeast.

Splice variants have been used to account for the relatively small number of protein coding genes in the human genome, currently estimated at around 20,000.

By varying the concentration of spliceosomes and pre-mRNAs based on their proximity to nuclear speckles, cells could potentially regulate the efficiency of splicing.

[13] The model for formation of the spliceosome active site involves an ordered, stepwise assembly of discrete snRNP particles on the pre-mRNA substrate.

[17] The presence of a pseudouridine residue in U2 snRNA, nearly opposite of the branch site, results in an altered conformation of the RNA-RNA duplex upon the U2 snRNP binding.

Specifically, the altered structure of the duplex induced by the pseudouridine places the 2' OH of the bulged adenosine in a favorable position for the first step of splicing.

[25] A group of less abundant snRNAs, U11, U12, U4atac, and U6atac, together with U5, are subunits of the minor spliceosome that splices a rare class of pre-mRNA introns, denoted U12-type.

Spliceosomal splicing cycle
Figure 1. Above are electron microscopy [ 8 ] fields of negatively stained yeast ( Saccharomyces cerevisiae ) tri-snRNPs. Below left is a schematic illustration of the interaction of tri-snRNP proteins with the U4/U6 snRNA duplex. Below right is a cartoon model of the yeast tri-snRNP with shaded areas corresponding to U5 (gray), U4/U6 (orange) and the linker region (yellow).