Figure 30.5 :Difference between initiator and elongation t RNA

 

Steps involved in initiation process: In the first step, small subunit of ribosome binds to mRNA such that initiation codon lies in partial P site. This gets possible due to activity of IF-3. It basically prevents untimely re-association of large and small subunit of ribosome. Moreover it promises accuracy of initiation site selection. In mRNA there is ribosomal binding site which consists of Shine -dalgarno sequence and initiation codon. This Shine-dalgarno sequence which is 5’-AGGAGGU-3’ and is located 10bp upstream of initiation codon is complementary to region near 3’ end of 16s rRNA, a component of small subunit of RNA as discussed in given below Figure 30.6.

Figure 30.6 :Ribosome binding site

In the next step, initiator tRNA carrying N-formyl methionine enters partial P site and binds to mRNA via its anti-codon loop. IF-2 is responsible for this activity. It directs initiator tRNA to its correct position in the initiation complex. It also exhibits ribosome dependant GTPase activity. Once GTP is hydrolysed then 50S subunit joins to form complete ribosome, the different steps of initiation are given in Figure 30.7. Finally, when larger subunit also joins the complex it forms complete P site and A-site. Second charged tRNA (amino acid laden) enters A site. This tRNA as per the rule has anticodon corresponding to codon in mRNA.