Connecting a promoter-bound protein to TBP bypasses the need for a transcriptional activation domain

Biochemical analyses have suggested potential targets for transcriptional activation domains, which include several components of the RNA polymerase II machinery, as well as the chromatin template. Here we examine the mechanism of transcriptional activation in yeast cells by connecting a heterologous DNA-binding domain (LexA) to the TATA-binding protein (TBP). LexA-TBP efficiently activates transcription from a promoter containing a LexA operator upstream of a TATA element. Activation is promoter-specific and is sensitive to mutations on the DNA-binding surface of TBP; hence it is not due to a fortuitous activation domain on TBP. Thus a promoter-bound protein lacking an activation domain can stimulate transcription if it is directly connected to TBP. This suggests that recruitment of TBP to the promoter can be a rate-limiting step for transcription in vivo, and that interactions between activation domains and factors that function after TBP recruitment can be bypassed for activation.