12.1: Cutting and Trimming RNA
- Page ID
- 9982
pre‑rRNA
In E. coli, the rrn operon is transcribed into a 30S precursor RNA, containing 3 rRNAs and 2 tRNAs.
The segment containing 16S rRNA (small ribosomal subunit) and the one containing 23S rRNA (large ribosomal subunit) are flanked by inverted repeats that form stem structure in the RNA. The stems are cleaved by RNase III. There is no apparent single sequence at which RNase III cleaves ‑ perhaps it recognizes a particular stem structure. This plus subsequent cleavage events (by an activity called M16) generates the mature 16S and 23S rRNAs. The rRNAs are also methylated.
tRNA is liberated by RNases P and F and 5S rRNA is liberated by RNases E and M5
In eukaryotes
The initial precursor is 47S and contains ETS1, 18S rRNA, ITS1, 5.8S rRNA, ITS2, and 28S rRNA, where ETS = extragenic transcribed spacer and ITS = intragenic transcribed spacer. Specific cleavage events followed by methylations generate the mature products. Also, some rRNA genes in some species have introns that must be spliced out.
pre-tRNA in E. coli
Sequence specific cleavage by RNases P, F, D
- RNase P is an endonuclease that cleaves the precursor to generate the 5' end of the mature tRNA.
- RNase F is an endonuclease that cleaves the precursor 3 nucleotides past the 3' end of the mature tRNA.
- RNase D is an exonuclease that trims in a 3' to 5' direction to generate the 3' end or the mature tRNA.
The catalytic activity of RNase P is in the RNA component
- RNAse P is composed of a 375 nt RNA and a 20 kDa protein.
- The catalytic activity is in the RNA. The protein is thought to aid in the reaction, but is not required for catalysis. All enzymes are not proteins!
- This was one of the first instances discovered of catalytic RNA, and Sidney Altman shared the Nobel Prize for this.
The enzyme tRNA nucleotidyl transferaseadds CCA to the 3' ends of pre-tRNAs.
- Virtually all tRNAs end in CCA, forms the amino acceptor stem.
- For most prokaryotic tRNA genes, the CCA is encoded at the 3' end of the gene.
- No known eukaryotic tRNA gene encodes the CCA, but rather it is added posttranscriptionally by the enzyme tRNA nucleotidyl transferase. This enzyme is present in a wide variety of organisms, including bacteria, in the latter case presumably to add CCA to damaged tRNAs.