Enhancer-like properties of the 60/81 bp elements in the ribosomal gene spacer of Xenopus laevis.

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.