GCD12 encodes a translational repressor of the GCN4 protein, a transcriptional activator of amino acid biosynthetic genes in the yeast Saccharomyces cerevisiae. gcd12 mutations override the requirement for the GCN2 and GCN3 gene products for derepression of GCN4 expression, suggesting that GCN2 and GCN3 function indirectly as positive regulators by negative regulation of GCD12. In addition to their regulatory phenotype, gcd12 mutants are temperature-sensitive for growth (Tsm-) and, as shown here, deletion of the GCD12 gene is unconditionally lethal. Both the regulatory and the Tsm- phenotypes associated with gcd12 point mutations are completely overcome by wild-type GCN3, implying that GCN3 can promote or partially substitute for the functions of GCD12 in normal growth conditions even though it antagonizes GCD12 regulatory function in starvation conditions. The GCD12 gene has been cloned and mapped to the right arm of chromosome VII, very close to the map position reported for GCD2. We demonstrate that GCD12 and GCD2 are the same genes; however, unlike gcd12 mutations, the growth defect and constitutive derepression phenotypes associated with the gcd2-1 mutation are expressed in the presence of the wild-type GCN3 gene. These findings can be explained by either of two alternative hypotheses: (1) gcd12 mutations affect a domain of the GCD2 protein that directly interacts with GCN3, and complex formation stabilizes mutant gcd12 (but not gcd2-1) gene products; (2) gcd12 mutations selectively impair one function of GCD2 that is replaceable by GCN3, whereas gcd2-1 inactivates a different GCD2 function for which GCN3 cannot substitute. Both models imply a close interaction between these two positive and negative regulators in general amino acid control.
Alleles Amino Acids