Nested control regions promote Xenopus ribosomal RNA synthesis by RNA polymerase I.

The DNA regions required for accurate initiation by RNA polymerase 1 on the ribosomal RNA genes of Xenopus laevis consist of at least two domains within a 150 bp segment adjoining the initiation site. A region of only 13 nucleotides (from -7 to +6) appears sufficient to specify accurate and efficient initiation when various 5' and 3' deletion mutants are injected into oocyte nuclei. In contrast, a 10-fold larger region (from -142 to +6) is required for maximal synthesis when deletion mutants are assayed in oocyte nuclear homogenates; only a low level of synthesis is specified by the 13 bp promoter domain in this in vitro system. Site-specific mutants demonstrate that a sequence adjoining nucleotide -75 is also essential for efficient in vitro initiation. Under other conditions, transcription can be strongly affected by sequences that extend several kb upstream in the rDNA spacer and contain duplications of sequences found within the 150 bp promoter. We propose a model in which transcription of the X. laevis rRNA genes is regulated by interaction of at least three sequence domains that extend over a several kb region.