Five TGA “stop” codons occur within the translated sequence of the yeast mitochondrial gene for cytochrome c oxidase subunit II

Publisher Website
Google Scholar
Abstract
A mitochondrial mutation that genetically maps in the middle of the gene coding cytochrome c oxidase subunit II is a single-base-pair deletion. Three independently isolated spontaneous revertants of this mutant have different single-base-pair insertions within 15 nucleotides of the mutation. These findings clearly identify the location of the gene and suggest that the mutation causes a frame-shift. The sequence of about 900 base pairs surrounding the mutation has several chain termination codons in every possible reading frame. The sequence can be translated in 1 frame by assuming that the codon TGA does not cause chain termination in yeast mitochondria. If TGA codes for tryptophan residues, a polypeptide can be read from the yeast mtDNA that is identical to bovine cytochrome oxidase subunit II at 37.8% of its residues. The DNA sequences of the frame-shift revertants predict relative isolectric point differences between the wild-type and various revertant forms of the polypeptide. The detection of these isolectric point differences by 2-dimensional electrophoresis of subunit II from the various strains independently confirms the presumed reading frame of the gene. TGA is translated in yeast mitochondria, most probably as tryptophan.