test

1. rRNA and tRNA
PTM
Sidra

2. 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

4. 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

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

6. • 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.

8. 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.

9. • 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.

10. Transfer RNAs Are Processed by
Nucleotide Removal, Addition,
and Modification
• cloverleaf-shaped secondary structure
• tRNA processing therefore includes nucleolytic
removal 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