5.12.1 Mechanism of Translation · Part 2

are usually dissociated from each other when not involved in translation (Fig. .13a) . Initiation of translation in E. coli begins with the formation of an initiation complex, consisting of the 30S subunits of the ribosome, a messenger RNA and the charged N-formyl methionine tRNA (f met – t RNA f met ), three proteinaceous initiation factors (IF1, IF2, IF3), GTP(Guanine Tri Phosphate) and Mg + . The components that form the initiation complex interact in a series of steps. IF3 binds to the 30S and allows the 30S subunit to bind to mRNA. Another initiation protein (IF2) then enhances the binding of charged formyl methionine tRNA to the small subunit in response to the AUG triplet. This step ‘sets’ the reading frame so that all subsequent groups of three ribonucleotides are translated accurately. The assembly of ribosomal subunits, mRNA and tRNA represent the initiation complex. Once initiation complex has been assembled, IF3 is released and allows the Fig. . b- Initiation Fig. . a-Translation components XII Std Zoology Chapter XII Std Zoology Chapter Molecular Genetics from the large subunit. The uncharged tRNA moves through the ‘ E’ site on the ribosome. The entire mRNA-tRNA-aa1-aa2 complex shifts in the direction of the ‘P’ site by a distance of three nucleotides (step ). This step requires several elongation factors (EFs) and the energy derived from hydrolysis of GTP. This results in the third triplet of mRNA to accept another charged tRNA into the A site (step ). The sequence of elongation is repeated over and over (step and initiation complex to combine with the 50S ribosomal subunit to form the complete ribosome (70S). In this process a molecule of GTP is hydrolyzed providing the required energy and the initiation factors (IF1 and IF2 and GDP) are released ( Fig. . b ). Elongation is the next phase of translation. Once both subunits of the ribosomes are assembled with the mRNA, binding sites for two charged tRNA molecules are formed. The sites in the ribosome are referred to as the