Characterization of the adr1‐1 nonsense mutation identifies the translational start of the yeast transcriptional activator ADR1

We have characterized a nonsense mutation in the ADR1 gene that identifies the translational start of the ADR1 protein. The ADR1 gene of Saccharomyces cerevisiae is required for synthesis of the glucose‐repressible alcohol dehydrogenase (ADH2). The adr1‐1 mutation, which inhibits ADH2 expression, was identified as a C to G transversion at base pair +32. This alteration would result in a UGA nonsense codon in place of a serine codon that would lead to termination of the ADR1 polypeptide after the 10th amino acid. The effect of the adr1‐1 mutation was partially reversed by UGA‐tRNA suppressors, indicating that the adr1‐1 mutation affects ADR1 expression at the translational level. These observations establish that the first available AUG in the ADR1 sequence is used as the translational start site of ADR1. Tyrosine or leucine UGA‐tRNA‐suppressors resulted in levels of adr1‐1 activity similar to that found for a serine UGA‐tRNA‐suppressor, suggesting that serine residue‐11 is not essential to ADR1 function. Northern analyses showed that the 5·1 kb ADR1 mRNA was two‐ to three‐fold more abundant when isolated from a strain carrying the ADR1 allele than from an isogenic strain containing the adr1‐1 allele. These data confirm that the 5·1 kb mRNA is the ADR1 mRNA and suggest that inhibition of adr1‐1 mRNA translation results in more rapid degradation of the adr1‐1 mRNA.