Initiation of translation in prokaryotes

Initiation of Translation in Prokaryotes

The first stage in protein synthesis in prokaryotes is Initiation. In this, a complex is formed which further do elongation. Ribosomes of prokaryotes have two ribosomal subunits, one is the 30s and another is 50s. Both subunits has 3 sites each that are;

• E-site
• P-site
• A-site

E-site is Exit site where uncharged tRNA fall out of the ribosome.

P-site has referred P due to peptidyl transferase activity here which transfer peptide bond.

A-site is attachment site where newly charged tRNA attaches itself due to complementarity between its anti-codon sequence against mRNA codon.

               There are few factors involved with the initiation, they are termed as IF1(initiation factor 1), IF2 (initiation factor 2), IF3 (initiation factor 3). IF1 attaches to A-site and prevents tRNA to attach here. IF3 occupy E-site so that first tRNA bound to correct position that is AUG codon at P-site.

The first step is to attach mRNA to the 30s subunit. It takes place with the help of Shine-Dalgarno sequence present on mRNA, 5-10 nucleotides upstream from the start codon. 16s part of 30s subunit has complementarity sequence of S-D sequence, so mRNA attaches here. It contains AUG that is start codon. After formation of this 30s complex, IF3 releases and large subunit attach with small subunit at E site. IF1 and IF2 also releases. As IF2 is GTP enzyme so GTP is hydrolyzed to GDP + Pi. Here initiation ends and proceeds to elongation.

Translation in prokaryotes

Translation is a term referred to the synthesis of protein within the living body. Livings are of two types i-e Prokaryotes & Eukaryotes. Prokaryotes have the simplest structure, so have the simple protein synthesis machinery. In prokaryotes, translation needs

• mRNA
• Ribosomal subunits ( 30s+ 50s = 70s )
• tRNA (charged with amino acid)
• Some other proteins (such as factors involved)
• GTP

In prokaryotes, transcription and translation occur simultaneously as there is no definite nuclear membrane. There are 3 main stages involved

1.  Initiation
2. Elongation
3. Termination

Initiation: It is the phase where mRNA bound to 30s subunit of a ribosome and charged tRNA find its complementary sequence on mRNA and binds to it. Then 50s subunit binds with small subunit. It results in the formation of initiation complex as several other factors are also involved.
Elongation:  In this stage formation of a peptide bond between amino acid on tRNA and translocation occurs.
Termination: In this phase elongation stop with the help of STOP codon which does not specify for any amino acid and stops protein formation with the release of the polypeptide chain and all other factors.