PreRNA

PreRNA, or precursor RNA, refers to primary transcripts that are not yet functional RNA molecules and require processing to become mature RNA. In biology, the term often encompasses precursor forms of various RNA classes, including pre-mRNA, pre-rRNA, pre-tRNA, and precursors to small RNAs. These molecules serve as intermediate products of transcription that must undergo enzymatic processing before they participate in cellular processes.

In eukaryotes, pre-mRNA is transcribed by RNA polymerase II as a long heterogeneous nuclear RNA that contains both exons and introns. It undergoes 5' capping, 3' polyadenylation, and removal of introns by splicing. The resulting mature mRNA is exported to the cytoplasm for translation. This maturation also influences mRNA stability and translational efficiency, integrating regulatory signals with gene expression.

Pre-rRNA is generated as a large precursor transcript (in humans, a 47S or 45S precursor) by RNA polymerase I and is processed in the nucleolus by small nucleolar ribonucleoproteins into mature 18S, 5.8S, and 28S rRNA; 5S rRNA is produced separately by RNA polymerase III. Pre-tRNA transcripts are processed by RNase P and other enzymes, trimming leaders and trailers and adding appropriate cytosine residues andCCA ends to yield functional tRNA molecules essential for protein synthesis.

Other preRNA species include precursor microRNAs and other small RNAs, which require cleavage and maturation steps to become active. The concept of preRNA highlights the central role of RNA processing and quality control in gene expression, as maturation steps determine stability, localization, and function.