Initiation of protein synthesis from a termination codon

Area of Science:

• Molecular Biology
• Genetics
• Bacterial Protein Synthesis

Background:

• The standard initiation codon for protein synthesis is AUG.
• Termination codons (UAA, UAG, UGA) typically signal the end of translation.
• Investigating non-canonical initiation is crucial for understanding translation regulation.

Purpose of the Study:

• To determine if the amber termination codon (UAG) can initiate protein synthesis in Escherichia coli.
• To characterize the mechanism and efficiency of UAG-initiated translation.
• To explore the potential applications of UAG-initiated gene expression.

Main Methods:

• Mutagenesis of the chloramphenicol acetyltransferase (CAT) gene to create a UAG initiation codon (CATam1).
• In vitro translation using E. coli S-30 extracts and a mutant initiator tRNA (tRNA(fMetCUA)).
• In vivo studies involving bacterial transformation with mutant genes and assessment of CAT polypeptide synthesis and chloramphenicol resistance.

Main Results:

• The UAG codon successfully initiated the synthesis of a full-length CAT polypeptide in vitro and in vivo.
• Initiation with UAG likely incorporated glutamine, not methionine, facilitated by tRNA(fMetCUA) and glutaminyl-tRNA synthetase.
• UAG-initiated translation was efficient, achieving 60-70% of the efficiency of the standard AUG initiation.

Conclusions:

• The amber termination codon (UAG) can function as an initiation codon in Escherichia coli when paired with a specific mutant initiator tRNA.
• This UAG-initiated system allows for tightly regulated gene expression.
• The method holds promise for the overproduction of toxic proteins in E. coli and other bacteria.