Mutations in RNA polymerase II enhance or suppress mutations in GAL4

The activation domains of eukaryotic DNA-binding transcription factors, such as GAL4, may regulate transcription by contacting RNA polymerase II. One potential site on RNA polymerase II for such interactions is the C-terminal tandemly repeated heptapeptide domain in the largest subunit (RPO21). We have changed the number of heptapeptide repeats in this yeast RPO21 C-terminal domain and have expressed these mutant RNA polymerase II polypeptides in yeast cells containing either wild-type or defective GAL4 proteins. Although the number of RPO21 heptapeptide repeats had no effect on the activity of wild-type GAL4, changing the length of the C-terminal domain modified the ability of mutant GAL4 proteins to activate transcription. Shorter or longer RPO21 C-terminal domains enhanced or partially suppressed, respectively, the effects of deletions in the transcriptional-activation domains of GAL4. The same RPO21 mutations also affected transcriptional activation by a GAL4-GCN4 chimera. These data suggest that the activation domains of DNA-binding transcription factors could interact, either directly or indirectly, with the heptapeptide repeats of RNA polymerase II.