Biology
The key to the bypass system is that there are identical (or synonymous) codons at either end of the sequence that is skipped. They are sometimes referred to as the "take-off" and "landing" sites. Before bypass, the ribosome is positioned with a peptidyl-tRNA paired with the take-off codon in the P site, with an empty A site waiting for an aminoacyl-tRNA to enter. Figure 7.32 shows that the ribosome slides along mRNA in this condition until the peptidyl-tRNA can become paired with the codon in the landing site. A remarkable feature of the system is its high efficiency, ~50%.
- Translation
Translation is the process by which a triplet base sequence of mRNA molecules are converted into a specific sequence of amino acids in a polypeptide chain in the cytoplasm of a cell. It consists of initiation, elongation, and termination. The mRNA, a...
- Recoding Changes Codon Meanings
KEY TERMS: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. KEY CONCEPTS:...
- Translocation Moves The Ribosome
KEY TERMS:Translocation is the movement of the ribosome one codon along mRNA after the addition of each amino acid to the polypeptide chain. KEY CONCEPTS: Ribosomal translocation moves the mRNA through the ribosome by 3 bases. Translocation moves deacylated...
- Use Of Fmet-trnaf Is Controlled By If-2 And The Ribosome
KEY TERMS:The context of a codon in mRNA refers to the fact that neighboring sequences may change the efficiency with which a codon is recognized by its aminoacyl-tRNA or is used to terminate protein synthesis. KEY CONCEPTS: IF-2 binds the initiator...
- Protein Synthesis Occurs By Initiation, Elongation, And Termination
KEY TERMS:The A site of the ribosome is the site that an aminoacyl-tRNA enters to base pair with the codon. The P site of the ribosome is the site that is occupied by peptidyl-tRNA, the tRNA carrying the nascent polypeptide chain, still paired with the...
Biology
Bypassing involves ribosome movement
Certain sequences trigger a bypass event, when a ribosome stops translation, slides along mRNA with peptidyl-tRNA remaining in the P site, and then resumes translation (see Figure 7.30). This is a rather rare phenomenon, with only ~3 authenticated examples (for review see Herr, Atkins, and Gesteland, 2000). The most dramatic example of bypassing is in gene 60 of phage T4, where the ribosome moves 60 nucleotides along the mRNA (Huang et al., 1988).
The key to the bypass system is that there are identical (or synonymous) codons at either end of the sequence that is skipped. They are sometimes referred to as the "take-off" and "landing" sites. Before bypass, the ribosome is positioned with a peptidyl-tRNA paired with the take-off codon in the P site, with an empty A site waiting for an aminoacyl-tRNA to enter. Figure 7.32 shows that the ribosome slides along mRNA in this condition until the peptidyl-tRNA can become paired with the codon in the landing site. A remarkable feature of the system is its high efficiency, ~50%.The sequence of the mRNA triggers the bypass. The important features are the two GGA codons for take-off and landing, the spacing between them, a stem-loop structure that includes the take-off codon, and the stop codon adjacent to the take-off codon. The protein under synthesis is also involved.
The take-off stage requires the peptidyl-tRNA to unpair from its codon. This is followed by a movement of the mRNA that prevents it from re-pairing. Then the ribosome scans the mRNA until the peptidyl-tRNA can repair with the codon in the landing reaction. This is followed by the resumption of protein synthesis when aminoacyl-tRNA enters the A site in the usual way.
Like frameshifting, the bypass reaction depends on a pause by the ribosome. The probability that peptidyl-tRNA will dissociate from its codon in the P site is increased by delays in the entry of aminoacyl-tRNA into the A site. Starvation for an amino acid can trigger bypassing in bacterial genes because of the delay that occurs when there is no aminoacyl-tRNA available to enter the A site (Gallant and Lindsley, 1998). In phage T4 gene 60, one role of mRNA structure may be to reduce the efficiency of termination, thus creating the delay that is needed for the take-off reaction.
- Translation
Translation is the process by which a triplet base sequence of mRNA molecules are converted into a specific sequence of amino acids in a polypeptide chain in the cytoplasm of a cell. It consists of initiation, elongation, and termination. The mRNA, a...
- Recoding Changes Codon Meanings
KEY TERMS: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. KEY CONCEPTS:...
- Translocation Moves The Ribosome
KEY TERMS:Translocation is the movement of the ribosome one codon along mRNA after the addition of each amino acid to the polypeptide chain. KEY CONCEPTS: Ribosomal translocation moves the mRNA through the ribosome by 3 bases. Translocation moves deacylated...
- Use Of Fmet-trnaf Is Controlled By If-2 And The Ribosome
KEY TERMS:The context of a codon in mRNA refers to the fact that neighboring sequences may change the efficiency with which a codon is recognized by its aminoacyl-tRNA or is used to terminate protein synthesis. KEY CONCEPTS: IF-2 binds the initiator...
- Protein Synthesis Occurs By Initiation, Elongation, And Termination
KEY TERMS:The A site of the ribosome is the site that an aminoacyl-tRNA enters to base pair with the codon. The P site of the ribosome is the site that is occupied by peptidyl-tRNA, the tRNA carrying the nascent polypeptide chain, still paired with the...