Post translational modifications of tRNA

1. Post transcriptional modifications of
transfer RNA
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2.  Are adapter molecules that faithfully
translate the information in mRNA into a
specific sequence of amino acid.
 Most cells have 40-50 distinct tRNA.
 Eukaryotic cells have multiple copies of many
of the tRNA.
 Recognizes codon on mRNA
 Transcribed by RNA polymerase III
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3.  Relatively small
 Consists of a single stranded of RNA folded
into a precise 3-D structure
 tRNA of bacteria & eukaryotic cytosol have
~73-93 nucleotide residues & molecular
weight of 24000-31000
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4.  First nucleic acid to be completely sequenced
 Made by Robert Holley in 1965
 It contains 76 ncleotides
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5.  Clover leaf structure with 5 arms
Acceptor arm
T¥C arm
D arm
Anticodon loop
Variable loop
 Trinucleotide sequence 5’- CCA-3’ at the 3’ end
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Amino acid accceptor arm
It is the site of attachment of amino acid
5’-CCA-3’ sequence at the 3’ end
T¥C arm
contain a unusual base pseudouridine within the
sequencne
5’-T¥UCG-3’
Anticodon loop
contain anticodon
3 nucleotide long decoding element
D loop
contain dihydrouridines
Varible loop
size varies from 3-21 bases

7.  L-shaped 3-D structure
Interactions stabilising L shaped structure
1,hydrogen bond between bases
2,interactions between the bases & the sugar phosphate backbone
3,additional base stacking gained from formation of the two
extended regions of the base pairing
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9. STEPS INVOLVED :
1,Removal of leader & tail sequence
2,Replacement of nucleotide
3,Modification of certain bases
4,Excision of intron
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13.  Excision/splicing involves 2 steps
1,involves phosphodiester bond cleavage
catalysed by endonuclease
2,ligation reaction by RNA ligase
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14.  In bacteria,introns in pre t RNA are self spliced as group I or
group II autocatalytic introns
 In archaea & eukaryotes,pre t- RNA splicing involves the
action of 3 enzymes
1, endonuclease
that recognizes & cleaves the prescursors at both end
2, ligase
that joins the the t-RNA
3, 2’phosphotransferase
that removes the 2’phosphate group on spliced t-RNA
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15.  Is a heterodimeric protein
 Two catalytic subunit
Sen 34
Sen 2
 Two structural subunit
Sen 54
Sen 15
 Sen 34, sen 2 cleave 3’ & 5’splice sites
 Sen 54 determine the site of cleavage
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17.  Homodimers or homotetramers
 Each subunit has an active site
 Recognize a structural feature called
bulge-helix-bulge
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18.  The products of cleavage are a linear intron &
two half t-RNA molecules
 5’ terminus ends in a hydroxyl group
 3’ terminus ends in a 2’,3’ cyclic phosphate
group
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19.  Two halves base pairs to form a t-RNA
 Using ATP further reaction occurs
Catalysed by a single enzyme with multiple
enzymatic activity
1,cyclic phosphodiesterase activity
2,kinase activity
3,ligase activity
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