 For translation To be started successfully three events must occur and these three events include the ribosome must be recruited to the mRNA Second a charged tRNA must be placed into the P site of the Ribosome and thirdly the ribosome must be precisely Positioned over the start codon. So these are three requirements as soon as these three requirements are fulfilled Translation will be initiated The correct positioning of the ribosome over the start codon is Critical because this establishes the reading frame for the translation of the mRNA You will remember that the start codon is the one that makes the reading frame for the mRNA If this start codon is starting from the front or back, then entirely different proteins will be translated and synthesized So this correct positioning is very important In prokaryotes the assembly of the ribosome on an mRNA occurs One subunit at a time The small subunit associates with the mRNA first In prokaryotes the association of small subunit with the mRNA is mediated by base pairing interactions between the RBS ribosomal binding site and the 16s RNA of ribosome Or ideally positioned RBS S right ribosomal binding sites the small subunit is positioned on the mRNA Such that the start codon will be in the P site When the large subunit joins the complex so The start codon is A site, P site or E site It will come only on P site, not on A site or E site The large subunit joins its partner only at the very end of the initiation process Just before the formation of first peptide bond So when everything will happen or initiation is about to start At that time the larger subunit will be attached Thus many of the key events of translation initiation occur in the absence of full ribosome Translation initiation is the only time a tRNA binds to the P site Without previously occupying the A site Normally the tRNA comes first to the A site, then to the P site and then to the E site But in translation initiation This is the only time a tRNA will come directly to the P site without applying to the A site This event requires a special tRNA known as the initiator tRNA So this event is different It has to be attached to the P site so this tRNA should be different from the tRNA And this tRNA is called the initiator tRNA The initiator tRNA base pairs with the start codon and start codon AUG or GUG So it will come with them and base pair But when AUG or GUG will come in the open reading frame Then it will be coding different amino acids For example, AUG will do the methionine And GUG is coding valine Although the initiator tRNA is first charged with a methionine A 4-mile group is rapidly added to the methionine A minogroup by a separate enzyme which is called MET tRNA transformylase So the addition of this 4-mile group has been transformed into 4-mile methionine Thus rather than valine or methionine The initiator tRNA is coupled to N4-mile methionine The charged initiator tRNA is referred to as F-MET tRNA Because N4-mile methionine is the first amino acid to be incorporated into a polypeptide chain One might think that all prokaryotic proteins have a 4-mile group Add there a minoterminal Because the first codon will always be AUG And the first amino acid will always be a 4-mile methionine So when the first amino acid is a 4-mile methionine in the synthesis of all proteins Then easily one can expect that the first amino acid is a 4-mile methionine in all prokaryotes This is not the case However because an enzyme known as de-formylase Removes the 4-mile group from the amino terminus During or after the synthesis of polypeptide chain So when polypeptide chain is formed or synthesis is done Then an enzyme de-formylase comes Which cuts this 4-mile group In fact many mature prokaryotic proteins do not even start with a methionine Even if the 4-mile group is cut Then it comes to mind that the first amino acid will always be a methionine No, in many proteins the first amino acid will not be a methionine Because aminopeptidases often remove the aminoterminal methionine As well as one or two additional amino acids So from every protein synthesis Methionine or the first or two amino acids are removed Special enzymes from aminopeptidases So in this way every protein starts with different amino acids