Virtually the entire Xenopus laevis rDNA multikilobase intergenic spacer serves to stimulate polymerase I transcription.
The promoter-distal half of the spacer separating the tandem Xenopus laevis rRNA genes consists of "0" and "1" repetitive elements that have been considered unimportant in polymerase I transcriptional activation. Utilizing oocyte microinjection, we now demonstrate that the 0/1 region, as well as its component 0 and 1 repeats, substantially stimulate transcription from a ribosomal promoter in cis and inhibit transcription when located in trans. Both the cis and trans responses increase linearly with increasing numbers of 0 or 1 repeats until saturation is approached. The 0/1 block and its component elements stimulate transcription in both orientations, over distances, and when placed downstream of the initiation site, properties for which the 60/81-base pair (bp) repeats have been defined as polymerase I enhancers. In their natural promoter-distal rDNA location, the 0/1 repeats can stimulate transcription from the rRNA gene promoter, above the level afforded by the intervening 60/81-bp repeats and spacer promoter. In addition, as with the 60/81-bp repeats, the 0/1 repeats bind a factor in common with the rDNA promoter. Thus, the entire X. laevis rDNA intergenic spacer (the 0 repeats, 1 repeats, spacer promoter repeats, and 60/81-bp repeats) acts together to enhance ribosomal transcription.
Mougey
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Department of Biological Chemistry, Johns Hopkins University, School of Medicine, Baltimore, Maryland 21205, USA.
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Sollner-Webb
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D000818:Animals; D004256:DNA Polymerase I / Q000378:metabolism*; D004275:DNA, Ribosomal / Q000235:genetics* / Q000378:metabolism; D004742:Enhancer Elements, Genetic; D011401:Promoter Regions, Genetic; D012091:Repetitive Sequences, Nucleic Acid; D014158:Transcription, Genetic; D014982:Xenopus laevis
D004275:DNA, Ribosomal; D004256:DNA Polymerase I