INTERCHANGEABLE RNA-POLYMERASE-I AND POLYMERASE-II ENHANCERS

The RNA polymerase I (pol I) enhancer of Saccharomyces cerevisiae contains at least three elements commonly associated with RNA polymerase II (pol II) enhancers, binding sites for the transcriptional activators general regulatory factor 2 and autonomously replicating sequence-binding factor I, and a thymidine-rich element. When the particular form of the thymidine-rich element found in the pol I enhancer was placed in front of a pol II promoter, transcription was stimulated 43-fold, comparable to the effect of a powerful pol II activator such as Gal4. Conversely, when two copies of a thymidine-rich element from a pol II enhancer were placed upstream of a pol I promoter, transcription was stimulated 38-fold. This functional reciprocity of pol I and II enhancers may reflect similarities in the mechanisms of transcriptional activation. The pol I enhancer also contains an element that appears to be pol I-specific and prevent the activation of pol II.