Ribosomes contain two sites for binding tRNA molecules: the aminoacyl
site (A site), where each tRNA molecule first attaches, and the peptidyl
site (P site), where a tRNAholds the growing polypeptide chain. The bacterial
ribosome has two major subunits: a 30S subunit to which the
mRNA and tRNAs become bound, and a 50S subunit to which tRNAs
also bind. Each tRNA becomes charged by a tRNA aminoacyl synthetase
that attaches a specific amino acid to each species of tRNA. Each
tRNA molecule has a loop containing a triplet of ribonucleotides, called
the anticodon, that can base pair with a complementary triplet codon in
Translation in prokaryotes begins at a start codon. The bacterial
mRNA translation initation codon (AUG) encodes N-formylmethionine,
whereas internal AUG codons specify methionine. The 3'-UAC-5' anticodon
of the tRNApairs (in antiparallel fashion) with the complementary
5'-AUG-3' codon in the mRNA.
In the first step of translataion initiation, three protein initiation factors
(IF1, IF2, IF3) and GTP bind to the 30S ribosomal subunit (Figure
5-1a). The 30S subunit binds to a Shine-Dalgarno (S-D) recognition
sequence on the mRNA due to base pairing interactions with the 16S
rRNA component of the ribosome. Then, a tRNA loaded with the amino
acid N-formylmethionine (tRNAfMet
) binds to the 30S-mRNA complex
(Figure 5-1b). IF3 is released and then GTP hydrolysis releases IF1, IF2, GDP, and phosphate, allowing the 50S subunit to join the 30SmRNA-
complex to form a complete 70S initiation complex
(Figure 5-1c,d). The tRNAfMet
ends up in the P site of the ribosome.
This completes the initiation phase.
|Figure 5-1 Initiation of translation in bacteria.
The elongation phase proceeds with the help of a group of protein
elongation factors when a second activated tRNAenters the Asite (again
by specific codon-anticodon base pairing). This places N-formylmethionine
and the incoming amino acid next to one another so that a peptide
bond can be formed between them by action of an enzymatic, ribosomal
component called peptidyl transferase. The amino-acyl bond that held
the N-formylmethionine to the 3' end of its tRNA is broken simultaneously
with the formation of the peptide bond. The mRNA translocates
three nucleotides through the ribosome and a new codon is now positioned
in the A site. This process is repeated until a stop codon is encountered
at the A site (Figure 5-2).
|Figure 5-2 Diagram of protein synthesis.
At termination, the polypeptide is released from the tRNA with the
help of release factors. The mRNA and the last tRNA are released from
the ribosome as the ribosome dissociates into its 30S and 50S subunits.
Many antibiotics block protein synthesis
by binding to prokaryotic ribosomal