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Protein synthesis is carried out on ribosomes based on the sequence of mRNA. It always starts from methionine, encoded by the codon AUG. However, a polycistronic mRNA should contain multiple initiating codons (see the figure below). On the other hand, a peptide may also contain several non-initiating methionine residues, also encoded by AUG. How could the system distinguish them? The answer lies in the initiation signals.
Figure 5-C-1. Expression of a bacterial operon. In this example, an mRNA encodes more than one peptide. Such mRNA is called polycistronic. Some bacterial mRNAs are polycistronic, but nearly all eukaryotic mRNAs are monocistronic (encodes a single peptide).
The synthesized peptide sequence is a translation of mRNA sequence according to the genetic code. It starts from the initiation codon, and then follows the mRNA sequence in a strictly "three nucleotides for one amino acid" manner. Therefore, a minor change in the mRNA sequence could produce a very different peptide. For instance, if a codon which codes for an amino acid is changed to a "stop" codon, the subsequent sequence will not be translated (e.g., Figure 5-A-9). Another example is the "frameshift". The translation is carried out by tRNA (more info). The entire procedure of protein synthesis includes initiation, elongation and termination (more info).
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