it is the slide about the structure and function of tRNA

1. PRESENTED BY
SUCHITRA sahoo
HARSHITA BHAWSAR

2. Outline
< FEATURES
< STRUCTURE
-- PRIMARY STRUCTUTRE
-- SECONDARY STRUCTURE
-- TERTIARY STRUCTURE
< FUNCTIONS

3. •A transfer RNA (abbreviated tRNA and referred to as sRNA, for
soluble RNA) is an adaptor molecule composed of RNA.
* typically 70 to 80 nucleotides in length, that serves as
the physical link between the mRNA and the amino acid
sequence of proteins.

4. t
PRIMARY STRUCTURE
SECONDARY STRUCTURE
TERTIARY STRUCTURE

5. The primary structure is short.
It contains about 70 to 80 nucleotides.
Three of the nucleotides make up the anticodon

6. 5’ 3’
Amino acid acceptor armT y C armD arm Anticodon loop

8.  Robert Holley proposed clover leaf model for the first time in 1968.
 It is a two dimensional description of the t-RNA.

9. Recognizes
specific enzyme
Amino acid
accepter arm
Ribosome
binding site

10. Amino acid acceptor
arm
• Double helical (both 5’ and 3’ ends
of tRNA)
• At 3’ end, 5’CCA3’ protrudes with –
OH at the tip
• Site for attachment of amino acid
• -COOH of specific amino acid joins
with –OH of A in CCA to form
amino acyl tRNA
Ester bond

11. D arm
• DHU or D arm – This arm consists of
stem and loop with unusual pyrimidine
nucleotide dihydrouracil.
• 4 bp stem with a loop contain
dihydrouridine
• Recognition site for the specific enzyme
aminoacyl-tRNA synthetase that
activate the amino acid
• Play a important role in the stabilization
of the tRNA's tertiary structure.

12. T y C arm
• TψC arm is named for the presence
of sequence TψC (thymine –
pseudouridine (ψ) – cytosine), where
pseudouridine is unusual base.
• This arm also consists of stem and
loop.
• Stem contains 5 base pairs;
outermost of these pairs is C-G.
Loop contains 7 unpaired
nucleotides
• This loop contains a ribosome
recognition site.

13. • This arm also contains stem and
loop.
• Stem consists of 5 base pairs and
loop (called as anticodon loop or
loop II) contains 7 unpaired
nucleotides.
• Out of these 7 unpaired
nucleotides the middle three form
anticodon.
• Anticodon recognizes and codon
of mRNA and binds to it.
Anticodon loop

14. Variable arm
 The variable arm has between 3 and 21 nucleotides, depending
on which amino acid the tRNA encodes.
 Between anticodon loop and TΨU loop
 This tRNA's variable arm is very short so it looks quite
different from the other arms of the molecule.
 May present or absent, it depends on species.
 The length of the variable arm is important in the recognition of
the aminoacyl tRNA synthetase for the tRNA.
 Variable arm helps is stability of tRNA
 tRNAs are called class 1 if they lack it, and class 2 if they have
it.

15. L-Shaped structure
(i) Acceptor stem and ΨU stem form extended helix
(ii) Anticodon stem and D-loop stem form extended helix
(iii) D-loop and T loop align together
(iv) Both extended helices align at right angle

16. Stability of L-structure
 Tertiary structure of t-RNA is
produced by hydrogen
bonding –
 Between N-bases
 Between N-bases and ribose-
phosphate backbone
 Between ribose-phosphate
backbone

17. TRNA FUNCTION
• tRNA as a primer
• tRNA as an Enzyme Inhibitor
• Aminoacyl-tRNATransferases
• Channeled tRNA Cycle during Protein Synthesis
• tRNA and the Regulation of Enzyme Synthesis

18. tRNA used as a primer
 A reverse transcriptase (RT) is an enzyme used to
generate complementary DNA (cDNA) from an RNA
template, a process termed reverse transcription. It
is mainly associated with retroviruses.
 It has been found that a particular species of tRNA is
used as a primer in this process.
 Avian myeloblastosis virues reverse transcriptase
(AMV-RT) uses tRNA-Trp, whereas the murine
leukemia virus enzyme uses tRNA-Pro as a primer.
 Recent studies shown that the reverse transcriptase
has a strong affinity for the tRNA primer.
 many plant viral as well as animal viral RNAs possess
a "tRNA-like" structure at the 3’-end of the RNA
which found to act as substrates for aminoacylation
by aa-tRNA synthetases.

19. tRNA as an Enzyme Inhibitor
 tRNA is a potent inhibitor of E. coli endonuclease I.
 The work of Goebel & Helsinki
 They suggests that tRNA alters the mode of action
of endonuclease I from that double strand scission of
DNA to a nicking activity.
 A specific isoacceptor species of tRNA-Tyr in
Drosophila has been found to act as an inhibitor to
the enzyme tryptophan pyrrolase , which is involved
in the conversion of tryptophan to an intermediate
in brown-pigment synthesis.
 In this case, an uncharged tRNA appears to act in a
regulatory capacity by directly interfering with an
individual enzymatic activity,

20. Aminoacyl-tRNA Transferases
 Aminoacyl-tRNA transferases are a group of enzymes that
catalyze the transfer of an amino acid from aa-tRNA to
specific acceptor molecules without the participation of
ribosomes or other kinds of nucleic acid.
 The acceptor molecules can be divided into three classes:-
1. The acceptor can be an intact protein, in which the amino
acid is added to the N-terminus of the protein.
2. The acceptor can be a phosphatidyl glycerol molecule, in
which the enzyme catalyzes the formation of aminoacyl
esters of phosphatidyl glycerol that are components of cell
membranes.
3. The acceptor can be an N-acetyl muramyl peptide, an
intermediate of the synthesis of inter-peptide bridges in
bacterial cell walls.
 These are important links in cell wall biosynthesis, and for
this specialized tRNAs are used.
 The aa-tRNA transferases have recently been reviewed by
Softer.

21. tRNA and the Regulation of
Enzyme Synthesis
 One of the remarkable features of aa-tRNA is
the fact that it has been shown to play a role
in regulating the transcription of messenger
RNA for enzymes associated with
biosynthesis of its amino acid.
 This was first discovered in the operon for
histidine biosynthesis.
 The regulatory role of tRNA has been
reviewed recently.

22. Channeled tRNA cycle
during Protein Synthesis
 Translation system is highly organized in vivo and the
intermediates in the process, aminoacyl-tRNAs (aa-tRNA), are
channeled i.e., they are directly transferred.
 As we know tRNA plays a central role in translation, acting as the
carrier of both the monomeric units of proteins i.e the amino
acids, and the growing polypeptide chains
 During this, the tRNAs interact, with most of the components of
the protein-synthesizing machinery-including the aminoacyl-
tRNA synthetases, the elongation factors, and the ribosomes
without dissociating into the cellular fluid.
 Proof:- Using a permeabilized CHO (Chinese hamster ovary) cell
system that closely mimics living cells, they found that there is no
leakage of endogenous tRNA during many cycles of translation.
 tRNAs, upon leaving the ribosome, must reassociate with their
cognate aminoacyl-tRNA synthetases to regenerate the
aminoacyl-tRNAs