No abstract

In yeast, most amino acid biosynthetic pathways are coregulated: starvation for a single amino acid results in derepression of enzyme activities for many different biosynthetic pathways. This phenomenon is referred to as "general control of amino acid biosynthesis." In this paper we describe the isolation and characterization of 43 amino acid analog-sensitive (aasw) mutants that are perturbed in this general regulatory system. These 43 mutations define four unlinked complementation groups, AAS101, AAS102, AAS103, and AAS104, two of which identify previously unreported genes involved in general control. These aas mutants are unable to derepress a number of amino acid biosynthetic genes, resulting in increased sensitivity to amino acid analogs, reduced growth rates, and reduced enzyme activity levels under amino acid starvation conditions. Thus, the AAS' gene products function as positive regulatory elements for this system. We show that the AAS genes mediate these effects by regulating the mRNA levels of genes under their control.In prokaryotes, functionally related genes are organized into single transcriptional units called operons, the expression of which is controlled by positive or negative elements acting on a single adjacent regulatory region. In eukaryotes, however, functionally related genes are generally scattered on different chromosomes; neither operons nor polycistronic mRNAs have been identified (1-3). Still, coregulation of these unlinked genes occurs, suggesting that regulation is mediated by a mechanism fundamentally different from that used in prokaryotes.

In Saccharomyces cerevisiae, starvation for a single amino acid results in the derepression of enzyme activities in multiple amino acid biosynthetic pathways. Derepression is a consequence of increased transcription of the genes encoding these enzymes. Analysis of the kinetics of mRNA elevation established that derepression occurs within 5 min of a shift of the culture from rich medium to starvation medium. Any starvation condition was sufficient to trigger an initial high mRNA elevation; however, it was the severity of starvation which determined the steady-state mRNA levels that were subsequently established. The products of the positive regulatory genes AAS101, AAS103, and AAS2 were shown to be required in the initiation phase of this response, whereas the AAS102 gene product was required to maintain the new elevated steady-state mRNA levels. The AAS101 and AAS102 genes were cloned. Consistent with their respective roles in initiation and maintenance of derepression. AAS101 mRNA was found to be expressed at high levels in both rich and starvation media, whereas AAS102 mRNA was derepressed only under starvation conditions. The derepression of AAS102 mRNA is dependent on the AAS101 gene product.

In Saccharomyces cerevisiae, starvation for a single amino acid results in the derepression of enzyme activities in multiple amino acid biosynthetic pathways. Derepression is a consequence of increased transcription of the genes encoding these enzymes. Analysis of the kinetics of mRNA elevation established that derepression occurs within 5 min of a shift of the culture from rich medium to starvation medium. Any starvation condition was sufficient to trigger an initial high mRNA elevation; however, it was the severity of starvation which determined the steady-state mRNA levels that were subsequently established. The products of the positive regulatory genes AAS101, AAS103, and AAS2 were shown to be required in the initiation phase of this response, whereas the AAS102 gene product was required to maintain the new elevated steady-state mRNA levels. The AAS101 and AAS102 genes were cloned. Consistent with their respective roles in initiation and maintenance of derepression. AAS101 mRNA was found to be expressed at high levels in both rich and starvation media, whereas AAS102 mRNA was derepressed only under starvation conditions. The derepression of AAS102 mRNA is dependent on the AAS101 gene product.