Yeast mitochondrial tRNATrp can recognize the nonsense codon UGA

Area of Science:

• Molecular Biology
• Genetics
• Biochemistry

Background:

• Mitochondrial DNA (mtDNA) sequence analysis suggests UGA, a standard stop codon, functions as a sense codon in mitochondria.
• Homology between predicted and known protein sequences, like bovine mitochondrial cytochrome oxidase subunit II, indicates UGA may specify tryptophan.

Purpose of the Study:

• To investigate the function of the UGA codon in the mitochondrial genetic code.
• To confirm if UGA specifies tryptophan incorporation into proteins.

Main Methods:

• DNA sequence analysis of yeast and human mitochondrial genes (cytochrome oxidase subunit II).
• Isolation of yeast mitochondrial tRNATrp.
• Gene mapping of mitochondrial tRNATrp in pBR322-mitochondrial DNA recombinants.
• DNA sequencing of the mitochondrial tRNATrp gene.

Main Results:

• Yeast and human mitochondrial genes contain UGA codons in frame.
• A yeast mitochondrial tRNATrp was isolated and its gene sequenced.
• The mitochondrial tRNATrp anticodon was identified as 5'UCA3'.
• The U in the wobble position of the anticodon allows recognition of the UGA codon.

Conclusions:

• The UGA codon is used as a sense codon for tryptophan in yeast mitochondria.
• Mitochondrial tRNATrp with a 5'UCA3' anticodon directly translates the UGA codon.
• This finding clarifies a key aspect of the non-standard mitochondrial genetic code.