Prokaryotic Translation

Translation, or protein synthesis, is quite similar in prokaryotes and eukaryotes. We will look at the details of the process in prokaryotes, and consider the differences in eukaryotes afterwards.

The process of translation can be divided into three basic steps: initiation, elongation, and termination. Each of these steps will be considered in turn.

Initiation

Initiation requires a large and small ribosomal subunit (the subunits exist separately when not actively engaged in protein synthesis), a molecule of mRNA, a set of proteins known as initiation factors, GTP (for energy), and an initiator tRNA. The initiator tRNA has UAC for its anticodon, to allow it to base pair with the AUG start codon. It is charged with a special amino acid, formyl methionine, or fmet for short.

1. The small ribosomal subunit binds to Initiation Factor 3 (IF3).

2. The small subunit/IF3 complex binds to the mRNA. Specifically, it binds to the sequence AGGAGG, known as the Shine-Delgarno sequence, which is found in all prokaryotic mRNAs.

3. Meanwhile, the fmet tRNA binds to Initiation Factor 2 (IF2), which promotes binding of the tRNA to the start codon.

4. The small subunit/IF3 complex scans along the mRNA until it encounters the start codon. The tRNA/IF2 complex also binds to the start codon. This complex of the small ribosomal subunit, IF3, initiator tRNA, and IF2 is called the initiation complex.

At this point, the large ribosomal subunit joins in. A molecule of GTP is hydrolyzed, and the initiation factors are released. The ribosomal complex is now ready for protein synthesis.

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