A hierarchy of trans-acting factors modulates translation of an activator of amino acid biosynthetic genes in Saccharomyces cerevisiae.

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Abstract
The GCN4 gene encodes a positive effector of amino acid biosynthetic genes in Saccharomyces cerevisiae. Genetic analysis has suggested that GCN4 is regulated by a hierarchy of interacting positive and negative effectors in response to amino acid starvation. Results presented here for a GCN4-lacZ gene fusion support this regulatory model and suggest that the regulators of GCN4 exert their effects primarily at the level of translation of GCN4 mRNA. Both the GCN2 and GCN3 products appear to stimulate translation of GCN4 mRNA in response to amino acid starvation, because a recessive mutation in either gene blocked derepression of GCN4-lacZ fusion enzyme levels but did not reduce the fusion transcript level relative to that in wild-type cells grown in the same conditions. The GCD1 product appears to inhibit translation of GCN4 mRNA because under certain growth conditions, the gcd1-101 mutation led to derepression of the GCN4-lacZ fusion enzyme level in the absence of any increase in the fusion transcript level. In addition, the gcd1-101 mutation suppressed the low translational efficiency of GCN4-lacZ mRNA observed in gcn2- and gcn3- cells. A deletion of four small open reading frames in the 5' leader of GCN4-lacZ mRNA mimicked the effect of a gcd1 mutation and derepressed translation of the fusion transcript in the absence of either starvation conditions or the GCN2 and GCN3 products. By contrast, in a gcd1- strain, the deletion resulted in little additional increase in the translational efficiency of the fusion transcript. These results suggest that GCD1 mediates the translational repression normally exerted by the GCN4 leader sequences and that GCN2 and GCN3 antagonize these negative elements in response to amino acid starvation. The effects of the trans-acting mutations on the translation of GCN4-lacZ mRNA remained intact even when transcription of the fusion gene was placed under the control of the S. cerevisiae GAL1 transcriptional control element.