Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 207

Two or more polypeptides encoded by the same polycistronic mRNA can be translationally coupled. Two genes are translationally coupled if translation of the upstream gene affects the efficiency of the translation of the gene immediately downstream.

Figure 2.35 shows an example of how two genes could be translationally coupled. The TIR including the AUG initiation codon of the second gene is sequestered in a hairpin on the mRNA, so it cannot be recognized by an initiating ribosome. However, a ribosome arriving at the UAA stop codon for the first gene can open up this secondary structure, allowing another ribosome to bind to the downstream TIR and initiate translation of the second gene. Thus, translation of the second gene in the mRNA depends on the translation of the first gene. Mutations that disrupt translation of the upstream coding sequence (e.g., nonsense mutations or frameshift mutations that result in premature termination because the ribosome encounters nonsense codons in the new reading frame) therefore affect not only the gene in which they are located, but also the translationally coupled downstream gene.