rRNA and tRNAPTM

Sidra

Ribosomal RNA Precursors May Be Cleaved, Modified, and Spliced

• E. coli has three types of rRNAs, the 5S, 16S, and 23S rRNAs

• The polycistronic primary transcripts of these operons are 5500 nt long

• The initial processing, which yields products known as pre-rRNAs

• It consists of specific endonucleolytic cleavages by RNase III, RNase P, RNase E, and RNase F

• The 5’ and 3’ ends of the pre-rRNAs are trimmed away in secondary processing steps through the action of RNases D, M16, M23, and M5 to produce the mature rRNAs

snoRNAs Direct the Methylation of Eukaryotic rRNAs

• has several hundred tandemly repeated copies of rRNA genes

• These genes are transcribed and processed in the nucleolus

• The primary eukaryotic rRNA transcript is an 7500-nt 45S RNA that contains the 18S, 5.8S, and 28S rRNAs separated by spacer sequences

•In the first stage of its processing, 45S RNA is specifically methylated at numerous sites (106 in humans) that occur mostly in its rRNAsequences.•About 80% of these modifications yield O2’-methylribose residues, and the remainder yield methylated bases such as N6,N6-dimethyladenine and 2- methylguanine.

• Mammals have 200 snoRNAs, most of which are encoded by the introns of structural genes

• whose lengths vary from 70 to 100 nt

• RNA can act as an enzyme (a ribozyme

• isolated pre-rRNA of the ciliated protozoan Tetrahymena thermophila is incubated with guanosine or a free guanine nucleotide (GMP, GDP, or GTP), but in the absence of protein, its single 413-nt intron excises itself and splices together its flanking exons; that is, the pre-rRNA is self-splicing.

1. The 3’-OH group of the guanosine attacks the intron’s 5’ end, displacing the 3’-OH group of the 5’ exon and thereby forming a new phosphodiester linkage with the 5’ end of the intron.

2. The 3’-terminal OH group of the newly liberated 5’ exon attacks the 5’-phosphate of the 3’ exon to form a new phosphodiester bond, thereby splicing together the two exons and displacing the intron.

3. The 3’-terminal OH group of the intron attacks a phosphate of the nucleotide 15 residues from the intron’s end, displacing the 5’ terminal fragment and yielding the 3’ terminal fragment in cyclic form.

• group I introns occur in the nuclei, mitochondria, and chloroplasts of diverse eukaryotes (but not vertebrates), and in some bacteria.

• Group II introns, which occur in the mitochondria and chloroplasts of fungi and plants, employ an internal A residue (instead of an external G) to form a lariat intermediate.

Transfer RNAs Are Processed by Nucleotide Removal, Addition,

and Modification• cloverleaf-shaped secondary structure

• tRNA processing therefore includes nucleolyticremoval of the extra nucleotides.

• The three nucleotides, CCA, at the 3¿ termini of all tRNAs, the sites at which amino acids are attached

• trinucleotide is appended by the enzyme CCA-adding polymerase, which sequentially adds two C’s and an A to tRNA using CTP and ATP as substrates