Initiation
Initiation of translation in prokaryotes involves the assembly of the components of the translation system which are: the two ribosomal subunits (50S & 30S subunits), the mRNA to be translated, the first (formyl) aminoacyl tRNA (the tRNA charged with the first amino acid), GTP (as a source of energy), and three initiation factors (IF1, IF2, and IF3) which help the assembly of the initiation complex.
The ribosome has three sites: the A site, the P site, and the E site. The A site is the point of entry for the aminoacyl tRNA (except for the first aminoacyl tRNA, fMet-tRNAfMet, which enters at the P site). The P site is where the peptidyl tRNA is formed in the ribosome. And the E site which is the exit site of the now uncharged tRNA after it gives its amino acid to the growing peptide chain.
The selection of an initiation site (usually an AUG codon) depends on the interaction between the 30S subunit and the mRNA template. The 30S subunit binds to the mRNA template at a purine-rich region (the Shine Dalgarno sequence) upstream of the AUG initiation codon. The Shine Dalgarno sequence is complementary to a pyrimidine rich region on the 16S rRNA component on the 30S subunit. During the formation of the initiation complex, these complementary nucleotide sequences pair to form a double stranded RNA structure that binds the mRNA to the ribosome in such a way that the initiation codon is placed at the P site.
Read more about this topic: Prokaryotic Translation
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