Do you know that the rRNA and tRNA also undergo post-transcriptional modifications!!!


Post-transcriptional Modifications:

Ribosomal RNAs and Transfer RNAs processing

Posttranscriptional processing is not limited to mRNA. Ribosomal RNAs of bacterial, archaeal, and eukaryotic cells are made from longer precursors called preribosomal RNAs, or pre-rRNAs. Transfer RNAs are similarly derived from longer precursors. These RNAs may also contain a variety of modified nucleosides; some examples are shown Pseudo-uridine, Dihydro-uridine and Thio-uridine.

Pseudo-uridine
Pseudo Uridine

                   
Dihydro Uridine

                  
Thio Uridine

  

rRNA Processing

Ribosomal RNAs in bacteria, 16S, 23S, and 5S rRNAs (and some tRNAs, although most tRNAs are encoded elsewhere) arise from a single 30S RNA precursor of about 6,500 nucleotides. RNA at both ends of the 30S precursor and segments between the rRNAs are removed during processing. The 16S and 23S rRNAs contain modified nucleosides. In E. coli, the 11 modifications in the 16S rRNA include a pseudouridine and 10 nucleosides methylated on the base or the 29-hydroxyl group or both. The 23S rRNA has 10 pseudouridines, 1 dihydrouridine, and 12 methylated nucleosides. In bacteria, each modification is generally catalyzed by a distinct enzyme. Methylation reactions use S-adenosylmethionine as a cofactor. No cofactor is required for pseudouridine formation.




tRNA Processing

Cleavage at 5’ and 3’ ends of Pre-tRNA, the addition of CCA sequence to 3’ end of pre-tRNA after trimming done mostly in all eukaryotes but some bacterial and archeal pre-tRNAs (because some of the bacteria already have CCA sequence upstream to 3’ of pre-tRNA so need of addition). This 3’ CCA sequence will be the binding site for an amino acid in a mature tRNA molecule. The next step in the processing of tRNA is multiple nucleotide modification in which mostly adenine is converted into pseudouridine, inosine, and dihydrouridine. Over a 100 such modifications or conversions take place in this processing. The final step is the splicing of introns which is a noncoding part of DNA. A greater part of the eukaryotic transcript and a few parts of the bacterial transcript consist of introns that have to be removed for further moving on.













Publisher Name: Muhammad Hannan Sharif

In case of any query, contact: mhsbios.hannan@gmail.com

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