The naturally occurring silent invertase structural gene suc2 zero contains an amber stop codon that is occasionally read through.

The yeast invertase structural gene SUC2 has two naturally occurring alleles, the active one and a silent allele called suc2 zero. Strains carrying suc2 zero are unable to ferment sucrose and do not show detectable invertase activity. We have isolated suc2 zero and found an amber codon at position 232 of 532 amino acids. However, transformants carrying suc2 zero on a multicopy plasmid were able to ferment sucrose and showed detectable invertase activity. Full-length invertase was found in gels stained for active invertase and in immunoblots. Therefore we concluded that the amber codon is occasionally read as an amino acid. The calculated frequency of read-through is about 4% of all translation events.