Overdracht van RNA-synthese

Transfer RNAs, or tRNAs, are non-coding RNAs that play a major role in protein synthesis. Eukaryotic cells contain more than 50 distinct tRNAs, each carrying a specific amino acid.

The folded tRNA has three hairpin loops: an anticodon loop, a T loop, and a D loop. The 3’ end of the molecule has a conserved CCA sequence that covalently binds an amino acid. Additionally, tRNAs contain many modified bases at several positions.

A tRNA gene is transcribed by RNA Polymerase III as a long precursor tRNA, or pre-tRNA. The pre-tRNA contains a 5’ leader sequence, a 3’ trailer sequence comprising a polyuridine tract, a 14-nucleotide long intron, and unmodified bases. 

The precursor tRNA undergoes post-transcriptional processing and modifications before it yields a mature tRNA. The extent of processing varies significantly in order and kind for different tRNAs. 

The first step in the tRNA processing involves the removal of the 5’ leader sequence and is catalyzed by an RNA enzyme called Ribonuclease P, or RNase P. This enzyme contains a catalytically active RNA that removes the 5’ leader sequence.

In the second step, the trailer sequence at the 3′ end is trimmed by one or more nucleases, such as the exonuclease RNase D. In the third step of the series, the 3’ terminal trinucleotide CCA, which is missing in some bacterial and all eukaryotic tRNA precursors, is added. 

In all eukaryotic pre-tRNAs, an enzyme called tRNA nucleotidyltransferase adds the CCA sequence to the processed 3′ end. 

Next, multiple nucleotides in the pre-tRNA are chemically modified at specific positions. Common base modifications include methylation, deamination, reduction, and isomerization.

In the final step of tRNA processing, the intron sequence gets spliced from tRNA transcripts to produce a mature tRNA.