Ribosomal DNA Gene Structure

40 kb in humans. A large number of biochemical studies have established that the proximal promoter is the most important element controlling RNA Pol I transcription. 

In addition to the proximal promoter, distal enhancer-like elements have been identified in organisms such as mouse, Drosophila, and Xenopus (Reeder, 1984). These sequences consist of multiple repeats of either 61 or 81 bp in length and function in either orientation, either upstream or downstream of the promoter. In Xenopus, these repeated sequences share regions of homology with the rDNA promoter and can compete for basal 

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