Basal components of the transcription apparatus (RNA polymerase II, TATA-binding protein) contain activation domains: is the repetitive C-terminal domain (CTD) of RNA polymerase II a "portable enhancer domain"?

Regions rich in serine, threonine, and proline residues can be found in transcriptional activation domains, as well as in the N-terminal parts of mammalian TATA-binding proteins, where they are interrupted by polyglutamine stretches. Likewise, the C-terminal domain of the largest subunit of RNA polymerase II contains multiple repeats of the consensus heptapeptide sequence YSPTSPS. To test directly for possible activation functions, we fused the GAL4 DNA-binding domain to the N-terminal domain of human TBP or subdomains of it, and to the C-terminal domain (CTD) of mouse RNA polymerase II or synthetic polymers of a CTD consensus repeat. We found that these chimeric proteins were able to activate transcription when bound to a GAL4 site in front of the TATA box, a function characteristic of transcription factors. However, while subdomains of TBP functioned only from a position close to the TATA box ("promoter" position), multiple repeats of the CTD consensus sequence were also able to mediate transcriptional activation from a remote ("enhancer") position. Our findings suggest that a region of TBP that is unique to mammals functionally cooperates with "proximal" activation domains of promoter-bound transcription factors. They also imply that the C-terminal domain of RNA polymerase II includes a function that is otherwise confined to remote activation domains of enhancer-bound transcription factors. We suggest that the CTD of RNA polymerase II contains a "portable" remote activation domain that may also facilitate chromatin opening within the transcription unit.