snRNAs

Small nuclear RNAs (snRNAs) are short non-coding RNAs that reside in the eukaryotic nucleus and play a central role in RNA processing. They are best known as core components of the spliceosome, the molecular machine that removes introns from pre-mRNA. In most organisms, snRNAs are about 100–200 nucleotides long and function by assembling with proteins to form small nuclear ribonucleoproteins (snRNPs).

snRNA genes are transcribed by RNA polymerase II (and, for some members such as U6, by RNA polymerase III). After transcription, snRNAs undergo maturation that includes 5' capping, 3' end processing, and site-specific chemical modifications such as 2'-O-methylation and pseudouridylation. In many pathways, mature snRNAs associate with Sm or LSm proteins in the cytoplasm to form snRNPs, which are then imported back into the nucleus for final maturation and assembly in specialized subnuclear domains.

Most snRNAs participate in the major spliceosome, which uses five snRNPs: U1, U2, U4, U5, and U6. These RNAs orient splice-site recognition and, with numerous proteins, drive the two transesterification reactions that remove introns and ligate exons. A minor splicing pathway uses the U11, U12, U4atac, and U6atac snRNPs together with U5 to recognize a distinct set of introns.

snRNAs are essential for accurate gene expression, and defects in their biogenesis or function can disrupt splicing and cell viability. They are widely studied in the context of spliceosome dynamics, splicing evolution, and diseases associated with splicing defects.