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Abstract
In the yeast Saccharomyces cerevisiae, starvation for a single amino acid elicits the increase in transcription of genes encoding enzymes in multiple amino acid biosynthetic pathways, a phenomenon termed "general control". Temporal characterization of the mRNA elevation response established that cells deprived of amino acids rapidly effect increased transciption in a biphasic process. Message levels continuously increase during the initiation phase of the response and persist at elevated levels during the maintenance phase.
Coregulation of the genes subject to general control regulation is mediated by the products of five positive regulatory genes: GCN1, GCN2, GCN3, GCN4, and GCN5. The requirement for these gene products is manifested at distinct times during the response. GCN1, GCN3, and GCN4 function in the activation of derepression and GCN2 serves to maintain elevated mRNA levels during prolonged starvation.
The GCN1, GCN2, and GCN4 genes were isolated. Characterization of the RNA expression of these regulatory genes demonstrated that the GCN1 and GCN4 genes are transcribed constitutively at the transcriptional level. Transcription of the GCN2 gene increases during amino acid limitation.
Genetic studies suggest that the GCN4 gene product is essential for the increase in transcription of general control-regulated genes, but the requirement for the GCN1, GCN2, and GCN3 gene products can be bypassed under certain conditions. Detailed characterizaton of the GCN4 gene demonstrated that (i) the expression of the GCN4 gene is subject to translational control; (ii) sequences in the unusually long 5' untranslated region of the corresponding mRNA are required for increased translation under starvation conditions; and (iii) the GCN1, GCN2, and GCN3 gene products are required for the increased translation of the GCN4 message. Molecular constructions that increase the cellular concentration of the GCN4 gene product render cells constitutively derepressed for genes subject to general control. Thus, high levels of the GCN4 protein are sufficient for transcriptional stimulation of genes subject to general control and the other positive effectors of this regulatory circuit serve to increase intracellular levels of GCN4 protein under starvation conditions.
