REGULATED DEGRADATION OF THE TRANSCRIPTION FACTOR GCN4

We report that Gcn4, a yeast transcriptional activator of the bZIP fam ily involved in the regulation of the biosynthesis of amino acids and purines, is rapidly turned over. This degradation is inhibited under c onditions of starvation for amino acids. Degradation is also inhibited by single amino acid alterations in a region adjacent to the Gcn4 act ivation domain. Furthermore, we show that degradation of Gcn4 proceeds through the ubiquitin pathway, a major proteolytic system for cytopla smic proteins, and is dependent on two specific ubiquitin conjugating enzymes, Cdc34 (Ubc3) and Rad6 (Ubc2). As a first step towards reconst ituting the Gcn4 degradation pathway lit vitro, we show that purified Cdc34 and Rad6 proteins are able to direct the specific ubiquitination of Gcn4.