Intrónar

Intrónar are noncoding sequences within genes that are transcribed into RNA but removed during RNA processing; the remaining exons are joined to form mature mRNA. They are present in most eukaryotic genes and in some organellar genomes. Their removal creates coding transcripts and allows regulation of gene expression.

Splicing removes intrónar from pre-mRNA and is mainly carried out by the spliceosome, a complex of small nuclear RNAs and proteins. Canonical splice sites typically begin with GU at the 5' end and end with AG at the 3' end; a branch point adenine forms a lariat intermediate. Alternative splicing can generate multiple protein isoforms from a single gene, contributing to proteome diversity.

Intrónar types and distribution: The major class consists of spliceosomal introns; a minor class known as U12-type introns uses a distinct splicing machinery. In some organisms, self-splicing introns (group I and II) occur in organellar genomes and, less commonly, in bacteria, capable of catalyzing their own removal. Intrónar sequences can harbor regulatory elements that influence transcription, mRNA export, stability, and translation.

Evolution and significance: The presence and sequence of intrónar vary widely. Hypotheses about their origin include introns-early and introns-late. Beyond splicing, intrónar contribute to gene expression regulation and can facilitate exon shuffling, increasing protein diversity.

Clinical and research relevance: Splicing defects caused by mutations at splice sites or regulatory elements can lead to disease. Therapeutic approaches targeting splicing are being developed to modulate gene expression.