IDENTIFICATION OF GCD14 AND GCD15, NOVEL GENES REQUIRED FOR TRANSLATIONAL REPRESSION OF GCN4 MESSENGER-RNA IN SACCHAROMYCES-CEREVISIAE

In Saccharomyces cerevisiae, expression of the transcriptional activat or GCN4 increases at the translational level in response to starvation for an amino acid. The products of multiple GCD, genes are required f or efficient repression of GCN4 mRNA translation under nonstarvation c onditions. The majority of the known GCD genes encode subunits of the general translation initiation factor eIF-2 or eIF-2B. To identify add itional initiation factors in yeast, we characterized 65 spontaneously arising Gcd(-) mutants. In addition to the mutations that were comple mented by known GCD genes or by GCN3, we isolated mutant alleles of tw o new genes named GCD14 and GCD15. Recessive mutations in these two ge nes led to highly unregulated GCN4 expression and to derepressed trans cription of genes in the histidine biosynthetic pathway under GCN4 con trol. The derepression of GCN4 expression in gcd14 and gcd15 mutants o ccurred with little or no increase in GCN4 mRNA levels, and it was dep endent on upstream open reading frames (uORFs) in GCN4 mRNA that regul ate its translation. We conclude that GCD14 and GCD15 are required for repression of GCN4 mRNA translation by the uORFs under conditions of amino acid sufficiency. The gcd14 and gcd15 mutations confer a slow-gr owth phenotype on nutrient-rich medium, and gcd15 mutations are lethal when combined with a mutation in gcd13. Like other known GCD genes, G CD14 and GCD15 are therefore probably required for general translation initiation in addition to their roles in GCN4-specific translational control.