tRNA

1. vivek kumar
M.Sc
Microbiology

2.  In most organisms non-coding genes (ncRNA) are transcribed as
precursors which undergo further processing.
 In case of rRNA, they are often transcribed as a pre-rRNA which
contains one or more rRNAs.
 The pre-rRNA is cleaved and modified at specific sites by approximately
150 different small nucleolus restricted RNA species, called snoRNAs.
 SnoRNAs associate with proteins forming snoRNPs.
 While snoRNA part basepair with target RNA and thus position the
modification at a precise site, the protein part performs the catalytical
reaction.
 In eukaryotes, in particular a snoRNP called Rnase, MRP cleaves the
45S per-rRNA into the 28S, 5.8S and 18S rRNAs.
 The rRNA and RNA processing factors form large aggregates called the
nucleolus.

3. Processing of tRNA:
 tRNA undergoes extensive processing.
 The mature tRNAs consists of 80-90 nucleotides. But the precursor tRNA
is much longer.
 For example tRNATyr which is a tyrosine carrying tRNA contains 350
nucleotides. Processing discards useless sequences.
 This is done by enzymes RNase D, RNase E, RNase F and RNase P.
Nucleotides are removed from both 5′ and 3′ ends.
 Endonucleases also remove many sequences.
 Cleaving is done after the primary transcript has folded and formed
characteristic stems and loop structure by extensive complementary base
pairing.
 RNase P is a ribozyme.

4.  The 5′-CCA-3′ sequence at 3′-end of mature tRNA is added by the
enzyme tRNA nucleotidyl transferse.
 This generates the mature 3′-end the tRNA.
 Several unusual bases are formed by the modification of normal
existing bases A, G, C and U by the enzymatic action.
 These modified bases are pseudouridine (Ψ), 2- isopentenyladenosine
(2 ip A), 2-O-methylguanosine (2m G), 4-thiouridine (4 t μ),
Ribothymidine, dihyrouridine and inosine.

5.  tRNAs are commonly synthesized as precursor chains with
additional material at one or both ends.
 The extra sequences are removed by combinations of bacteria
endonucleolytic and exonucleolytic activities.
 One feature that is common to most tRNAs is that the three
nucleotides at the 3’ terminus, always the triplet sequence CCA, are
not coded in the genome, but are added as part of tRNA processing.

7.  Removal of leader sequence i.e., 5’ end of tRNA is generated by a
cleavage action catalyzed by the enzyme ribonuclease P (Rnase P).
 Replacement of nucleotide:
 RNase D cleaves and trims 3’ end of tRNA by degradation in
the 3’-5’ direction.
 RNase D also removes 2 nucleotides from 3’ residues.
 The reaction also involves several enzymes in eukaryotes.
 It generates a tRNA that needs the CCA trinucleotide sequence to be
added to the 3’ end.

8.  The 5′-CCA-3′ sequence at 3′-end of mature tRNA is added by the
enzyme tRNA nucleotidyl transferse.
 This generates the mature 3′-end of the tRNA.
 Addition of methyl and isopentyl groups to the heterocyclic rings of
purine bases.
 Methylation of the 2’-OH group in the ribose of any residue; and
conversion of specific uridines to dihydrouridine and pseudouridine.
 Excision of intron I.e, splicing is done to form mature tRNA.

9.  There are several models for the process, which may be different in
different organisms.
 In some organisms the process is catalysed by a single enzyme.
 In other organisms different enzymes are responsible for adding the
C and A residues, and they function sequentially.
 When a tRNA is not properly processed, it attracts the attention of
quality control system that degrades it.
 This ensures that the protein synthesis apparatus does not become
blocked by non functional tRNAs.