Amino acid replacements resulting from super-suppression of nonsense mutants of iso-1-cytochrome c from yeast

The eight class I, set 1 super-suppressor genes, SUP2, SUP3, SUP4, SUP5, SUP6, SUP7, SUP8 and SUP11 are not closely linked and map at distinct loci throughout the genome of yeast. Each of these suppressors causes the production of 5 to 10% of the normal amount of iso-1-cytochrome c when it is individually coupled to the ochre (UAA) mutant cy1-2. All eight iso-1-cytochromes c contain a residue of tyrosine at position 20 which corresponds to the site of the ochre codon. Several of these super-suppressors also were shown to act on cy1-9, but at a much lower efficiency. It was shown that iso-1-cytochrome c from one of the suppressed cy1-9 strains contains a tyrosine at position 2, which corresponds to the site of the ochre codon in this mutant. It is suggested that the gene product of the eight super-suppressors is tyrosine transfer RNA.