Abstract
We report results of experiments in which we demonstrated the existence of a polymerase I promoter within the ribosomal DNA spacer upstream from the rRNA initiation site in Chinese hamsters and mice. Transcription of the CHO spacer promoter was achieved by the same protein factors, C and D, that catalyzed transcription of the gene promoter, and these factors bound stably to the CHO spacer promoter in a preinitiation complex, just as they did to the gene promoter. In contrast to the CHO spacer promoter, which was transcribed in vitro nearly as efficiently as the gene promoter, the mouse spacer promoter was far less active; this low activity was attributable to the fact that the mouse spacer promoter bound factor D inefficiently. It is striking that the active CHO spacer promoter violated the otherwise universal rule that metazoan RNA polymerase I promoters all have a G residue at position -16. Sequence comparisons also revealed a great similarity between the CHO and mouse spacer promoter regions, yet there was much less similarity between the flanking sequences. There was also only limited homology between the spacer and gene promoter regions, but despite this the two kinds of initiation regions were organized similarly, both consisting of an essential core promoter domain and a stimulatory domain that extended upstream to approximately residue -135. Evolutionary considerations argue strongly that the presence of ribosomal DNA spacer promoters offers a significant selective advantage.