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In eukaryotes, each mRNA normally encodes only a single polypeptide. In contrast, in bacteria and archaea, one mRNA can encode either one polypeptide (monocistronic mRNAs) or more than one polypeptide (polycistronic mRNAs). Polycistronic mRNAs must have a separate TIR for each coding sequence to allow them to be translated.

The name “polycistronic” is derived from “cistron,” which is the genetic definition of the coding region for each polypeptide, and “poly,” which means many. Similarly, “monocistronic” is derived from “mono,” which means one. Figure 2.34 shows a typical polycistronic mRNA in which the coding sequence for one polypeptide is followed by the coding sequence for another. The space between two coding regions can be very short, and the coding sequences may even overlap. For example, the coding region for one polypeptide may end with the termination codon UAA, but the last A may be the first nucleotide of the initiator codon AUG for the next coding region. Even if the two coding regions overlap, the two polypeptides on an mRNA can be translated independently by different ribosomes.

Polycistronic mRNAs do not exist in eukaryotes, in which, as described above, TIRs are much less well defined and translation usually initiates at the AUG codon closest to the 5′ end of the mRNA. In eukaryotes, the synthesis of more than one polypeptide from the same mRNA usually results from differential splicing of the mRNA or from high-level frameshifting during the translation of one of the coding sequences (see “Reading Frames” above); there are also specialized events in which an RNA element called an internal ribosome entry sequence directs binding of a ribosome to a site within the RNA. Polycistronic RNA leads to phenomena unique to bacteria, i.e., translational coupling and polarity, which are described below.

Figure 2.34 Structure of a polycistronic mRNA. (A) The coding sequence for each polypeptide is between the initiation codon and the stop codon. The region 5′ of the first initiation codon is called the leader sequence, and the untranslated region between a stop codon for one gene and the next initiation codon is known as the intercistronic spacer. (B) The association of the 30S and 50S ribosomes at a translational initiation region (TIR) and their dissociation at a stop codon. New 30S and 50S subunits associate at a downstream TIR.