TC binds to the small (40S) ribosomal subunit with the help of eIFs 1, 1A and 3, forming a 43S pre-initiation complex (PIC). Aminoacylated initiator tRNA is delivered to the P site of the ribosome as part of a ternary complex (TC) with GTP-bound eukaryotic initiation factor 2 (eIF2). Translation initiation in eukaryotes requires at least twelve initiation factors. There are significant differences between how this process is accomplished in eukaryotes and in bacteria. The initiation of protein synthesis is clearly a crucial process for the cell (reviewed in ). (In animal mitochondria, a single methionine-bearing tRNA species appears to serve as both initiator and elongator.

In addition, the initiator tRNA must be distinguished from the other methionine-bearing tRNA present in the cytoplasm, the elongator methionyl tRNA that contributes methionine residues during peptide chain elongation. It is the only tRNA that binds directly to the P site of the ribosome during the translational cycle it is also one of the only tRNAs that must avoid binding to elongation factor Tu (EF-Tu eEF1A in eukaryotes). The initiator tRNA must perform functions different from those of any other tRNA. In the course of its duties, the initiator tRNA must accomplish a number of tasks involving interactions with several binding partners: aminoacylation with methionine by methionyl-tRNA synthetase in bacteria and eukaryotic organelles, formylation of the methionine moiety by methionyl-tRNA transformylase interaction with one or more initiation factors and binding to the ribosome. Thus, a detailed understanding of the mechanism of initiator tRNA function in translation may have important medical relevance. In addition, the availability of the initiator tRNA may be limiting for translation a roughly twofold decrease in initiator tRNA expression in yeast increased the doubling time approximately threefold, and a “modest” overexpression of the initiator tRNA resulted in greatly elevated rates of translation and allowed mouse embryonic fibroblasts to induce tumors in mice. A properly located translational start is crucial, since by beginning translation in the wrong location the cell not only fails to produce the desired protein, but also creates an aberrant, possibly toxic one. It reads the start codon, allowing the initiating ribosome to begin translation in the correct location. Translation is terminated once the 30S and 50S ribosomal subunits as well as mRNA dissociate.The initiator tRNA plays a critical role in the cell. When release factor binds to the complex, the polypeptide is cleaved and released. As there is no tRNA corresponding to the stop codon, the A-site is occupied by a release factor instead. The process in translation elongation repeats until a stop codon enters the A-site.

The uncharged tRNA moves into the E site and it is then released. Ribosome translocates to the next codon with the help of elongation factor G and hydrolysis of GTP. The growing peptide chain is then transferred to the A-site. Elongation of amino acids begin when tRNA enters the A-site of the ribosome Peptidyl transferase in ribosome facilitates the bond formation between the amino acid on tRNA in P site and that on the newly entered tRNA. The initiator tRNA is positioned at the P-site. With the aid of some initiation factors, initiator tRNA N-formylmethionine binds to the start codon on mRNA and large 50S subunit also associates with the complex to form an mRNA-ribosome complex. During the initiation stage of translation, the Shine-Dalgarno sequence near the 5’ end of mRNA binds to 16S rRNA in 30S small subunit. In addition, there are three positions namely aminoacyl site (A-site), peptidyl site (P-site) and exit site (E-site) for holding amino acids. 16S rRNA and 21 proteins assemble to form 50S large subunit whereas 5S rRNA, 23S rRNA and 34 proteins combine to form 30S small subunit. The two subunits combine to made up a total density of 70S. In prokaryotes, each ribosome has a large 50S and small 30S ribosomal subunits. It has a complex structure which a large proportion is made up of ribosomal RNA (rRNA) and some proteins. Ribosome is important for protein synthesis in which mRNA is translated into polypeptide.

Ribosome initiater t rna