Quadruplet codons: implications for code expansion and the specification of translation step size.

Area of Science:

• Molecular Biology
• Synthetic Biology
• Genetics

Background:

• Engineering the genetic code requires unique codons for novel amino acids.
• Quadruplet codons (four nucleotides) offer potential for expanding the genetic code beyond the standard triplet codons.

Purpose of the Study:

• To investigate the potential of the UAGA quadruplet codon to specify a single amino acid.
• To examine the role of a mutant transfer RNA (tRNA) with an altered anticodon loop in decoding the UAGA quadruplet.
• To understand how flanking sequences and tRNA structure influence quadruplet codon translation.

Main Methods:

• Utilized a mutant transfer RNA (tRNA(Leu)) with an additional nucleotide in its anticodon loop.
• Employed the UAGA quadruplet as a potential codon in Escherichia coli.
• Partially inactivated release factor 1 to facilitate quadruplet codon translation.
• Analyzed the influence of the 5' flanking codon on UAGA translation efficiency.

Main Results:

• The UAGA quadruplet, in conjunction with the mutant tRNA(Leu), specified leucine with 13-26% efficiency.
• The anticodon loop bases of the mutant tRNA(Leu) likely adopt alternative conformations.
• The efficiency of UAGA translation was influenced by the preceding codon and the potential for tRNA anticodon rearrangement.
• A purine base at position 32 of the 5' flanking tRNA appeared to affect UAGA decoding.

Conclusions:

• The UAGA quadruplet codon can be engineered to specify an amino acid (leucine) using a modified tRNA.
• Anticodon loop structure and flanking sequence context play critical roles in quadruplet codon decoding.
• This study provides a foundation for expanding the genetic code using non-standard codon structures.