Download Characteristics of tRNAs Translating the genetic code

Characteristics of tRNAs
• Each different tRNA
coded by a different
gene
• 73-93 nt
• All with same 3’
sequence (CCA)
where aa attaches
• Anticodon loop will
bp with mRNA
Attachment of aa to
tRNA (Charging tRNAs)
• Specific tRNA will bind
to tRNA synthetase
along with correct aa
• tRNA synthetase
attaches ATP to aa at
COOH
• tRNA synthetase
transfers aa from AMP
to 3’OH of tRNA
Fig. 7.28a
Fig. 7.27a
Translating the genetic code
mRNA sequence decoded in 3 base sequence called
the codon. 64 different codons.
Most codons are interpreted in same way in different
organisms (universal genetic code).
• 3 codons are not recognized by any tRNA: nonsense
(stop) codons: UAA, UAG, UGA
• 61 codons able to specify 1 of 20 aa’s, therefore most
amino acids with >1 codon.
• Some aa’s have several codons recognized by 1
tRNA: this possible via Wobble base-pairing where
the tRNA requires accurate base-pairing only at
bases 2 and 3 in anticodon
Wobble base-pairing and translation
Ala specified by
GCX codons
3’
Fig. 7.25
Translation: from mRNA
sequence to protein
Protein synthesis: Peptide bonds form between
a growing polypeptide chain and an amino
acid (both attached to a tRNA)
Also: Fig. 3.13
Translation
• Peptide bond forms
between COOH on
tRNA in P site
(peptidyl site) and
NH3 group of aa in
A (acceptor) site
• Transient
attachment of
peptide to tRNA in
A site, then
translocation of
ribosome
• P site tRNA to E
(exit) site
Translation and the ribosome
• tRNAs in 3 sites:
A, P, E
• Polypeptide
chain on tRNA in
P site (peptidyl
site)
• Charged tRNA
with aa in A site
(acceptor site)
Fig. 7.29
Fig. 7.29
Translation initiation,
Part I
• 30S subunit with IFs
binds to mRNA and
finds start site
• fMet-tRNA binds to
complex
• 50S subunit bindsfMet in the P site
• 2nd charged tRNA
binds to A site
• 1st peptide bond
forms
Interaction of
16S rRNA and mRNA
30S initiation complex defines start site by
identifying sequence on mRNA (= ShineDalgarno) with homology to 16S rRNA
Translation
termination
Stop codon in A site,
polypeptide on tRNA
in P site
• Release factors bind
at A site
• COOH is cleaved
from tRNA
• Complex dissociates
(70S!30S and 50S)
Variability of Shine-Dalgarno (or
ribosome binding) sequences
• Variety of purine
rich sequences,
often 5-10 bases
to the 5’ side of
start codon act
as RBS
• Most but not all
start codons are
AUG
Antibiotics that act against
bacteria
Antibiotic
Streptomycin
Chloramphenicol
Tetracycline
Site of action
Inhibits translation initiation at 30S
Inhibits translation elongation
Prevents aa-tRNA binding
Rifamycin
Ampicillin
Inhibits RNA polymerase
Inhibits cell wall synthesis
Novobiocin
Gyrase