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The spacer region of the Xenopus laevis ribosomal gene contains blocks of repetitive sequence elements that are 60 or 81 bp long. These 60/81 bp elements function as enhancer elements for the RNA polymerase I promoter at the 5' end of the gene. An RNA polymerase I promoter adjacent to a block of 60/81 bp elements is always dominant over a promoter on a second plasmid when both are coinjected into oocyte nuclei. If two promoters are placed on the same plasmid containing enhancers, both promoters come under their influence and are codominant. The influence of the enhancers can be transmitted through several kilobases of plasmid sequence, through a potentially active promoter, and is independent of the orientation of the enhancers. The enhancers appear to compete with promoters for the same transcription factor(s); however, the enhancers can only compete when they are on a circular plasmid.

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.

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We show that three protein fractions are required for accurate transcription initiation at a Xenopus laevis ribosomal gene promoter in vitro: RNA polymerase I, Rib1 and xUBF. The Rib1 and xUBF fractions are both necessary and sufficient for formation of a stable initiation complex. The xUBF fraction can be completely replaced by recombinant xUBF. We also report the sequence of a cDNA clone for xUBF. xUBF is 701 amino acids in length, contains domain which are related to a domain found in chromosomal proteins HMG 1 and 2, and has an acidic carboxy terminus of 87 amino acids. xUBF is closely similar in amino acid sequence to its previously reported human homolog, hUBF, except that xUBF has only three of the HMG‐related domains while hUBF has four and therefore is 63 amino acids longer than xUBF.