The enhancement of ribosomal transcription by the recycling of RNA polymerase I.

It has been suggested that the tandemly repeated ribosomal genes of eukaryotes may be subject to a special mechanism of transcriptional enhancement, called Readthrough Enhancement, in which transcription factors are recycled. Recent experiments with the mouse ribosomal genes, although consistent with this possibility, were unable to distinguish between true Readthrough Enhancement and promoter occlusion. To test directly for Readthrough Enhancement, the pre-ribosomal RNA of Xenopus laevis was prematurely terminated within the 18S gene on a circular template. This premature termination was found to reduce the efficiency of pre-ribosomal RNA promotion in cis by 80 to 90%. Since the pre-ribosomal RNA is normally terminated only 213 base pairs upstream of its own initiation site, the results strongly suggest that the recycling of RNA polymerase, or Readthrough Enhancement, is a means by which ribosomal transcription is enhanced in Xenopus laevis.