Single point mutations in Met4p impair the transcriptional repression of MET genes in Saccharomyces cerevisiae.

Transcription of MET genes in Saccharomyces cerevisiae depends on a transcriptional activator, the MET4 gene product (Met4p). Using in vitro mutagenesis, we isolated two mutant MET4 alleles encoding [Pro215]Met4p and [Ser156]Met4p. These mutations impeded Met4p's responsiveness to methionine in the media, and yeast cells carrying mutant alleles exhibited enhanced transcription of MET genes under repressing conditions. The enhanced transcription was dependent on the CBF1 gene, but did not compete with an excess of wild-type Met4p, suggesting that some changes in the affinity of Met4p to other factors might be involved in S-adenosylmethionine-mediated transcriptional regulation.