Transcriptional enhancement by upstream activators is brought about by different molecular mechanisms for class I and II RNA polymerase genes.

Ribosomal gene transcription requires the functional interplay of at least two promoter elements, the upstream control element (UCE) and the start site proximal core, which operate in concert to promote efficient and accurate transcription initiation by RNA polymerase I (pol I). Because this bipartite organization of the rDNA promoter is formally analogous to the organization of a typical pol II promoter, we have examined whether transcriptional activation by upstream activating sequences is brought about by similar molecular mechanisms for both classes of genes. We have replaced the UCE of the mouse rDNA promoter by three different pol II activating sequences (the yeast GAL4 binding sites, the target sequence of the enhancer binding protein E2 from bovine papilloma virus type 1 and the octamer motif), and measured the template activity of these chimeric promoters in the presence of the trans-activating proteins either in a cell free transcription system or in vivo after transfection into mouse cells. In the context of the pol I promoter none of these transcriptional activators enhanced rDNA transcription. The results indicate that activation by UCEs is not interchangeable between genes transcribed by RNA pol I and II, respectively, and suggest that different molecular mechanisms mediate the synergistic action of distant control sequences of different classes of genes.