RNAsprocessing

RNA processing refers to the series of molecular events that convert primary RNA transcripts into mature, functional RNA molecules. In eukaryotes, many transcripts produced by RNA polymerase II are modified in the nucleus through capping of the 5' end, splicing to remove introns, and 3' end processing that includes cleavage and polyadenylation. These steps often occur co-transcriptionally and help regulate RNA stability, export, and translation.

For mRNA, the 5' cap protects the nascent transcript and aids translation initiation, while the poly(A) tail influences stability and export from the nucleus. Splicing is carried out by the spliceosome, a complex of small nuclear RNAs and proteins that removes introns and joins exons. Alternative splicing enhances protein diversity by producing multiple mRNA isoforms from a single gene.

Ribosomal RNA and transfer RNA undergo extensive processing in the nucleolus and cytoplasm. rRNA transcripts are cleaved and chemically modified by small nucleolar ribonucleoproteins to form mature 18S, 5.8S, and 28S rRNAs. tRNA precursors are trimmed by RNases P and Z and often receive post-transcriptional base modifications; the 3' CCA sequence is added for amino acid attachment.

Small RNA pathways involve dedicated processing steps. MicroRNAs are transcribed as pri-miRNAs and processed by Drosha in the nucleus to pre-miRNAs, exported to the cytoplasm, and further processed by Dicer into mature miRNA duplexes that guide Argonaute-containing complexes. Small interfering RNAs and piRNAs follow related, but distinct, maturation routes and participate in gene silencing and transposon control.

Quality control mechanisms, including nonsense-mediated decay and surveillance pathways, monitor RNA integrity and regulate transcript lifetimes. RNA processing thereby integrates transcription, maturation, export, and degradation to shape gene expression.