<P> Prokaryotic translation is the process by which messenger RNA is translated into proteins in prokaryotes . </P> <P> Initiation of translation in prokaryotes involves the assembly of the components of the translation system, which are: the two ribosomal subunits (50S and 30S subunits); the mature mRNA to be translated; the tRNA charged with N - formylmethionine (the first amino acid in the nascent peptide); guanosine triphosphate (GTP) as a source of energy, and the three prokaryotic initiation factors IF1, IF2, and IF3, which help the assembly of the initiation complex . Variations in the mechanism can be anticipated . </P> <P> The ribosome has three active 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, 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 . </P> <P> 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 of the 30S subunit . This sequence has been evolutionarily conserved and plays a major role in the microbial world we know today . 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 . </P>

What are the three sites on the ribosome