prämRNA

Prä-mRNA, or pre-mRNA, is the primary transcript produced by RNA polymerase II from a gene. It is synthesized in the nucleus and initially contains both exons and introns; it must be processed to generate mature messenger RNA (mRNA) that can be translated.

Processing includes 5' capping, splicing, and 3' end formation. The 5' cap is added co-transcriptionally and consists of a 7-methylguanosine linked to the first nucleoside. The cap protects the transcript from degradation and assists in splicing and translation initiation. Splicing removes introns and joins exons via the spliceosome, guided by conserved sequences at intron borders (5' GU and 3' AG) and the branch point, with alternative splicing producing multiple mRNA isoforms from a single gene.

3' end formation involves cleavage and polyadenylation, producing a poly(A) tail that increases mRNA stability, nuclear export, and translation efficiency. Most of these processing steps are coupled to transcription by RNA polymerase II, so pre-mRNA processing often occurs cotranscriptionally. The resulting mature mRNA is exported to the cytoplasm, where it can be translated; cellular quality-control mechanisms can degrade aberrant transcripts.

Alternative splicing and polyadenylation, in combination with transcriptional control, enable regulation of gene expression and proteome diversity. Errors in pre-mRNA processing can contribute to diseases, highlighting the importance of RNA maturation in cellular function.