<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> <P> Well - known coding regions that do not have AUG initiation codons are those of lacI (GUG) and lacA (UUG) in the E. coli lac operon . Two more recent studies have independently shown that 17 or more non-AUG start codons may initiate translation in E. coli . </P> <P> Elongation of the polypeptide chain involves addition of amino acids to the carboxyl end of the growing chain . The growing protein exits the ribosome through the polypeptide exit tunnel in the large subunit . </P>

An amino acid is synthesized by the prokaryote e coli