Functional analysis of Met4, a yeast transcriptional activator responsive to S-adenosylmethionine.

Transcription of the genes necessary for sulfur amino acid biosynthesis in Saccharomyces cerevisiae is dependent on Met4, a transcriptional activator that belongs to the basic region-leucine zipper protein family. In this report, we show that one mechanism permitting the repression of the sulfur network by S-adenosylmethionine (AdoMet) involves inhibition of the transcriptional activation function of Met4. Using a wide array of deleted LexA-Met4 fusion proteins as well as various Gal4-Met4 hybrids, we identify the functional domains of Met4 and characterize their relationship. Met4 appears to contain only one activation domain, located in its N-terminal part. We demonstrate that this activation domain functions in a constitutive manner and that AdoMet responsiveness requires a distinct region of Met4. Furthermore, we show that when fused to a heterologous activation domain, this inhibitory region confers inhibition by AdoMet. Met4 contains another distinct functional domain that appears to function as an antagonist of the inhibitory region when intracellular AdoMet is low. On the basis of the presented results, a model for intramolecular regulation of Met4 is proposed.