FUNCTIONAL-ANALYSIS OF MET4, A YEAST TRANSCRIPTIONAL ACTIVATOR RESPONSIVE TO S-ADENOSYLMETHIONINE

Transcription of the genes necessary for sulfur amino acid biosynthesi s in Saccharomyces cerevisiae is dependent on Met4, a transcriptional activator that belongs to the basic region-leucine zipper protein fami ly. In this report, we show that one mechanism permitting the repressi on of the sulfur network by S-adenosylmethionine (AdoMet) involves inh ibition of the transcriptional activation function of Met4. Using a wi de array of deleted LexA-Met4 fusion proteins as well as various Gal4- Met4 hybrids, we identify the functional domains of Met4 and character ize their relationship. Met4 appears to contain only one activation do main, located in its N-terminal part. We demonstrate that this activat ion domain functions in a constitutive manner and that AdoMet responsi veness requires a distinct region of Met4. Furthermore, we show that w hen fused to a heterologous activation domain, this inhibitory region confers inhibition by AdoMet. Met4 contains another distinct functiona l domain that appears to function as an antagonist of the inhibitory r egion when intracellular AdoMet is low. On the basis of the presented results, a model for intramolecular regulation of Met4 is proposed.