PretRNA

PretRNA, or pre-tRNA, is the primary RNA transcript produced from tRNA genes and is processed to yield mature transfer RNA that participates in protein synthesis. In most organisms, tRNA genes are transcribed as precursors that contain extra nucleotides at the 5' and 3' ends, and in many eukaryotes and some archaea, introns may be present within the tRNA sequence. The pretRNA therefore represents a processing intermediate rather than a functional tRNA on its own.

Biogenesis begins with transcription by RNA polymerase III (in eukaryotes and archaea) or the corresponding bacterial RNA polymerase. The 5' leader is removed by RNase P, and the 3' trailer is trimmed to create a mature 3' end. Many mature tRNAs require a CCA sequence at the 3' terminus; in bacteria, the gene may encode CCA or the enzyme may add it post-transcriptionally; in other organisms the CCA is added by a dedicated nucleotidyltransferase if not encoded in the gene. Intron-containing pretRNAs, found in many archaea and eukaryotes, undergo splicing: a tRNA splicing endonuclease removes the intron and a ligase joins the exons.

Subsequent modifications introduce diverse nucleotide changes that fine-tune structure and function. The resulting mature tRNA folds into the characteristic cloverleaf structure and, in three-dimensional form, an L-shaped molecule that delivers amino acids to ribosomes.

In eukaryotes, mature tRNA is exported from the nucleus to the cytoplasm by exportin-t; in organelles such as mitochondria, processing and function occur within the organelle. Quality-control mechanisms exist to degrade misprocessed pretRNA molecules and prevent faulty tRNAs from entering translation.