It is a key step to determine the reading frame.
Binding of a specific tRNA , the initiator tRNA, at the P-site on the small ribosomal subunit. The initiator Met-tRNA
The initiator tRNA binds to the AUG initiation codon of mRNA, which encodes the amino acid, methionine. Therefore, all proteins start with Met (at the NH2 end).
On the large ribosomal subunit, the initiator tRNA binds to the P-site. The ribosome is ready for the 1st cycle of elongation
Note: The initiator tRNA does not pass through site A (the only tRNA to not enter via site A)