TRNA post-transcriptional processing

In prokaryotes, RNA transcribed from protein-coding genes (messenger RNA, mRNA), requires little or no modification prior to translation. In fact, many mRNA molecules begin to be translated even before RNA synthesis has finished. However, ribosomal RNA (rRNA) and transfer RNA (tRNA) are synthesized as precursor molecules that do require post-transcriptional processing (see Topics G9 and G10, respectively). 

FIGURE 3.31 The post-transcriptional processing of yeast tRNA . ... 

Transcription produces RNA molecules. In prokaryotes, rRNAs and tRNAs are synthesized as longer precursors which require post-transcriptional processing including methylation and cleavage by specific endonucleases before they are functional molecules. Polycistronic mRNAs (Section 7.4) do not require further enzymic modification. 

In eukaryotes, three RNA polymerases have been identified. RNA polymerase I, located in the nucleolus (Section 9.4), synthesizes 5.8S, 18S, and 28S rRNA. RNA polymerase II, found in the nucleoplasm, participates in mRNA synthesis. RNA polymerase III, also identified within the nucleoplasm, functions in tRNA and 5S rRNA synthesis. Post-transcriptional processing similar to that of prokaryotic transcripts produces rRNAs and tRNAs. 

Synthab md processia A precursor molecule of tRNA is synthesized by transcription from DNA then processed to the functional tRNA. For example, the precursor of tRNA consists of 126 nucleotides and is 41 nucleotides longer than the mature, active molecule. The extra sequence is removed by a specific endonuclease. Certain nucleotide residues also undergo further post-transcriptional modifications, resulting in several Rare nucleic acid compo-... 

The possibility of post-transcriptional control of enzyme synthesis has been raised by several workers, and it has been noted that methylation of purine residues of tRNA and heavy rRNA is enhanced in GA-treated aleurone tissue [18]. It is not clear how GA might influence this process, or why methylation of certain types of RNA should be essential for a-amylase synthesis. Other studies claiming to show post-transcriptional control by GA [16] have received no support [17]. 

The variety of modified nucleosides which have been discovered along the primary sequence of tRNA derives from a series of post-transcriptional enzymatic modifications. In fact, the biosynthesis of tRNA takes place through a very complex process, which as a first step includes the action of RNA polymerase, which forms a precursor tRNA. Such precursor molecule, which is longer than the mature size tRNA, is in most of the cases completely lacking modified nucleosides. A not yet completely unravelled complex process of maturation O,34 then occurs, which includes (i) nucleolytic tailoring of the longer precursor (ii) 3 -and 5 -end refinement (not always necessary) (Hi) formation of modified nucleosides, which in some instances - i.e., hypermodified nucleosides - derive... 

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