Small nuclear RNA

snRNA are always associated with a set of specific proteins, and the complexes are referred to as small nuclear ribonucleoproteins (snRNP, often pronounced "snurps").

Splicing, or the removal of introns, is a major aspect of post-transcriptional modification, and takes place only in the nucleus of eukaryotes.

Lsm-class snRNAs are transcribed by RNA polymerase III and never leave the nucleus, in contrast to Sm-class snRNA.

Lsm-class snRNAs contain a 5′-γ-monomethylphosphate cap[6] and a 3′ stem–loop, terminating in a stretch of uridines that form the binding site for a distinct heteroheptameric ring of Lsm proteins.

[7] Spliceosomes catalyse splicing, an integral step in eukaryotic precursor messenger RNA maturation.

The spliceosome is a large, protein-RNA complex that consists of five small nuclear RNAs (U1, U2, U4, U5, and U6) and over 150 proteins.

The snRNAs, along with their associated proteins, form ribonucleoprotein complexes (snRNPs), which bind to specific sequences on the pre-mRNA substrate.

These reactions will produce a free lariat intron and ligate two exons to form a mature mRNA.

The major class, which is far more abundant in eukaryotic cells, splices primarily U2-type introns.

[citation needed] U1 snRNP is the initiator of spliceosomal activity in the cell by base pairing with the 5′ splice site of the pre-mRNA.

When U1 snRNA genes were knocked out, genomic microarrays showed an increased accumulation of unspliced pre-mRNA.

[15] In addition, the knockout was shown to cause premature cleavage and polyadenylation primarily in introns located near the beginning of the transcript.

Thus, U1 snRNA–pre-mRNA base pairing was shown to protect pre-mRNA from polyadenylation as well as premature cleavage.

[citation needed] Medulloblastoma – The U1 snRNA is mutated in a subset of these brain tumors, and leads to altered RNA splicing.

[22] This mechanism regulating the abundance of snRNAs is in turn coupled to a widespread change of alternative RNA splicing.