Microbiology, 2021

426 11 • Mechanisms of Microbial Genetics
the ribosome. This interaction anchors the 30S ribosomal subunit at the correct location on the mRNA
template. At this point, the 50S ribosomal subunit then binds to the initiation complex, forming an intact
ribosome.
In eukaryotes, initiation complex formation is similar, with the following differences:
• The initiator tRNA is a different specialized tRNA carrying methionine, called Met-tRNAi
• Instead of binding to the mRNA at the Shine-Dalgarno sequence, the eukaryotic initiation complex
recognizes the 5’ cap of the eukaryotic mRNA, then tracks along the mRNA in the 5’ to 3’ direction until the
AUG start codon is recognized. At this point, the 60S subunit binds to the complex of Met-tRNAi, mRNA,
and the 40S subunit.
Figure 11.15
Translation in bacteria begins with the formation of the initiation complex, which includes the small ribosomal subunit, the
mRNA, the initiator tRNA carrying N-formyl-methionine, and initiation factors. Then the 50S subunit binds, forming an intact ribosome.
Elongation
In prokaryotes and eukaryotes, the basics of elongation of translation are the same. In E. coli, the binding of
the 50S ribosomal subunit to produce the intact ribosome forms three functionally important ribosomal sites:
The A (aminoacyl) site binds incoming charged aminoacyl tRNAs. The P (peptidyl) site binds charged tRNAs
carrying amino acids that have formed peptide bonds with the growing polypeptide chain but have not yet
dissociated from their corresponding tRNA. The E (exit) site releases dissociated tRNAs so that they can be
recharged with free amino acids. There is one notable exception to this assembly line of tRNAs: During
initiation complex formation, bacterial fMet−tRNA fMet or eukaryotic Met-tRNAi enters the P site directly
without first entering the A site, providing a free A site ready to accept the tRNA corresponding to the first
codon after the AUG.
Elongation proceeds with single-codon movements of the ribosome each called a translocation event. During
each translocation event, the charged tRNAs enter at the A site, then shift to the P site, and then finally to the E
site for removal. Ribosomal movements, or steps, are induced by conformational changes that advance the
ribosome by three bases in the 3’ direction. Peptide bonds form between the amino group of the amino acid
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