Genetic evidence for transcriptional activation by the yeast IME1 gene product.

IME1 is required in yeast for meiosis and for expression of IME2 and other early meiotic genes. IME1 is a 360-amino acid polypeptide with central and C-terminal tyrosine-rich regions. We report here that a fusion protein composed of the lexA DNA-binding domain and IME1 activates transcription in vivo of a reporter gene containing upstream lexA binding sites. Activation by the fusion protein shares several features with natural IME1 activity: both are dependent on the RIM11 gene product; both are impaired by the same ime1 missense mutations; both are restored by intragenic suppressors. The central tyrosine-rich region is sufficient to activate transcription when fused to lexA. Deletion of this putative activation domain results in a defective IME1 derivative. Function of the deletion derivative is restored by fusion to the acidic Herpesvirus VP16 activation domain. The C-terminal tyrosine-rich region is dispensable for transcriptional activation; rather it renders activation dependent upon starvation and RIM11. Immunofluorescence studies indicate that an IME1-lacZ fusion protein is concentrated in the nucleus. These observations are consistent with a model in which IME1 normally stimulates IME2 expression by providing a transcriptional activation domain at the IME2 5' regulatory region.