RESUMO
The yeast Mcm1 protein is a member of the MADS box family of transcriptional regulatory factors, a class of DNA-binding proteins that control numerous cellular and developmental processes in yeast, Drosophila melanogaster, plants, and mammals. Although these proteins bind DNA on their own, they often combine with different cofactors to bind with increased affinity and specificity to their target sites. To understand how this class of proteins functions, we have made a series of alanine substitutions in the MADS box domain of Mcm1 and examined the effects of these mutations in combination with its cofactors that regulate mating in yeast. Our results indicate which residues of Mcm1 are essential for viability and transcriptional regulation with its cofactors in vivo. Most of the mutations in Mcm1 that are lethal affect DNA-binding affinity. Interestingly, the lethality of many of these mutations can be suppressed if the MCM1 gene is expressed from a high-copy-number plasmid. Although many of the alanine substitutions affect the ability of Mcm1 to activate transcription alone or in combination with the alpha 1 and Ste12 cofactors, most mutations have little or no effect on Mcm1-mediated repression in combination with the alpha 2 cofactor. Even nonconservative amino acid substitutions of residues in Mcm1 that directly contact alpha 2 do not significantly affect repression. These results suggest that within the same region of the Mcm1 MADS box domain, there are different requirements for interaction with alpha 2 than for interaction with either alpha1 or Ste12. Our results suggest how a small domain, the MADS box, interacts with multiple cofactors to achieve specificity in transcriptional regulation and how subtle differences in the sequences of different MADS box proteins can influence the interactions with specific cofactors while not affecting the interactions with common cofactors.