Sites of topoisomerase I action on X. laevis ribosomal chromatin: transcriptionally active rDNA has an approximately 200 bp repeating structure.
The sites at which topoisomerase I interacts with the transcriptionally active ribosomal chromatin of Xenopus oocytes were mapped by treating oocyte nuclei first with camptothecin to stabilize topoisomerase I-rDNA reaction intermediates, and then with SDS to resolve these adducts as protein-linked nicks in the DNA. Strikingly, the topoisomerase I sites are concentrated in the region encoding the 18S, 5.8S, and 28S rRNAs, where they are spaced with a periodicity of approximately 200 nucleotides. Plasmid rDNA that has assembled into a nucleoprotein structure in an oocyte nuclear extract yields this same restricted pattern of sites, while rDNA that is not in a chromatin structure exhibits a continuum of topoisomerase I cleavages. Thus, the approximately 200 bp spacing of sites of topoisomerase I interaction evidently reflects rDNA chromatin structure. These results indicate that the active rRNA genes are organized into nucleosome-like arrays, with topoisomerase I located in the linker regions.
Culotta
V
V
Department of Biological Chemistry, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205.
Sollner-Webb
B
B
D000818:Animals; D001483:Base Sequence; D001665:Binding Sites; D002166:Camptothecin; D002843:Chromatin / Q000648:ultrastructure*; D002874:Chromosome Mapping; D003001:Cloning, Molecular; D004264:DNA Topoisomerases, Type I; D004275:DNA, Ribosomal / Q000235:genetics*; D010957:Plasmids; D012335:RNA, Ribosomal / Q000235:genetics*; D014158:Transcription, Genetic; D014982:Xenopus laevis / Q000235:genetics*
D002843:Chromatin; D004275:DNA, Ribosomal; D012335:RNA, Ribosomal; D004264:DNA Topoisomerases, Type I; D002166:Camptothecin