The Xenopus ribosomal gene enhancers bind an essential polymerase I transcription factor, xUBF.

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Abstract
We purified xUBF on the basis of its ability to specifically bind the enhancer elements of the Xenopus laevis rRNA genes. xUBF also binds to both upstream and downstream regions of the X. laevis ribosomal gene promoter and is essential for polymerase I transcription. Unexpectedly, xUBF binds to both upstream and downstream regions of the human ribosomal gene promoter, producing footprints that are indistinguishable from the footprints produced by hUBF, a previously described polymerase I transcription factor isolated from human cells. Despite extensive sequence divergence of vertebrate polymerase I promoters, these data suggest an evolutionary conservation of the primary DNA-protein interaction.