intronexcision

Intronexcision is the cellular process by which introns are removed from precursor messenger RNA (pre-mRNA) transcripts to produce mature, translatable messenger RNA. This process is a central step in eukaryotic gene expression and is typically carried out by a complex ribonucleoprotein machine called the spliceosome. The spliceosome comprises small nuclear ribonucleoproteins (snRNPs) designated U1, U2, U4, U5, and U6, together with many associated proteins.

During intronexcision, the spliceosome recognizes conserved sequence elements at the 5' splice site, the branch point, and the 3' splice site. It catalyzes two transesterification reactions: first, the 2'-hydroxyl of a branch-point adenosine attacks the 5' splice site to form a lariat intermediate; second, the freed 3'-OH of the upstream exon attacks the 3' splice site, releasing the intron as a lariat and joining the two exons. In many organisms, intronexcision occurs co-transcriptionally, as RNA is synthesized by RNA polymerase II.

In addition to constitutive splicing, many genes undergo alternative splicing, generating multiple mature mRNA isoforms from a single transcript. This regulation involves a network of splicing factors, including serine/arginine-rich proteins and hnRNPs, and regulatory elements such as enhancers and silencers near splice sites.

Defects in intronexcision can lead to disease by producing abnormal proteins or altering protein abundance. Examples include spinal muscular atrophy and certain hemoglobinopathies. Understanding intronexcision informs basic biology and has guided therapeutic strategies that modulate splicing patterns.