- Recoding events occur when the meaning of a codon or series of codons is changed from that predicted by the genetic code. It may involve altered interactions between aminoacyl-tRNA and mRNA that are influenced by the ribosome.
- Changes in codon meaning can be caused by mutant tRNAs or by tRNAs with special properties.
- The reading frame can be changed by frameshifting or bypassing, both of which depend on properties of the mRNA.
The reading frame of a messenger usually is invariant. Translation starts at an AUG codon and continues in triplets to a termination codon. Reading takes no notice of sense: insertion or deletion of a base causes a frameshift mutation, in which the reading frame is changed beyond the site of mutation. Ribosomes and tRNAs continue ineluctably in triplets, synthesizing an entirely different series of amino acids.
There are some exceptions to the usual pattern of translation that enable a reading frame with an interruption of some sort—such as a nonsense codon or frameshift—to be translated into a full-length protein. Recoding events are responsible for making exceptions to the usual rules, and can involve several types of events.
Changing the meaning of a single codon allows one amino acid to be substituted in place of another, or for an amino acid to be inserted at a termination codon. Figure 7.28 shows that these changes rely on the properties of an individual tRNA that responds to the codon:
- Suppression involves recognition of a codon by a (mutant) tRNA that usually would respond to a different codon (see 7.12 Suppressor tRNAs have mutated anticodons that read new codons).
- Redefinition of the meaning of a codon occurs when an aminoacyl-tRNA is modified (see 7.8 Novel amino acids can be inserted at certain stop codons).
Changing the reading frame occurs in two types of situation:
- Frameshifting typically involves changing the reading frame when aminoacyl-tRNA slips by one base (+1 forward or –1 backward) (see 7.17 Frameshifting occurs at slippery sequences). The result shown in Figure 7.29 is that translation continues past a termination codon.
- Bypassing involves a movement of the ribosome to change the codon that is paired with the peptidyl-tRNA in the P site. The sequence between the two codons fails to be represented in protein. As shown in Figure 7.30, this allows translation to continue past any termination codons in the intervening region.