A subthreshold level of DNA topoisomerases leads to the excision of yeast rDNA as extrachromosomal rings.
In a yeast DNA topoisomerase double mutant TG205 (delta top1 top2-4), over half of the rDNA is present as extrachromosomal rings containing one 9 kb unit of the rDNA gene or tandem repeats of it. Expression of a plasmid-borne TOP1 or TOP2 gene in the strain leads to the integation of the extrachromosomal rDNA rings back into the chromosomal rDNA cluster. When the plasmid-borne topoisomerase gene is expressed from an inducible promoter of the GAL1 gene, repression of the gene by dextrose leads to reappearance of the extrachromosomal rDNA rings. The DNA topoisomerase-dependent excision/integration of rDNA is discussed in terms of the possibility of rDNA supercoiling by transcription and the effects of DNA topology on intra- and interchromosomal recombination.
Kim
R A
RA
Department of Biochemistry and Molecular Biology, Harvard University, Cambridge, Massachusetts 02138.
Wang
J C
JC
D004264:DNA Topoisomerases, Type I / Q000235:genetics*; D004270:DNA, Circular / Q000235:genetics; D004271:DNA, Fungal / Q000235:genetics / Q000648:ultrastructure; D004275:DNA, Ribosomal / Q000235:genetics*; D004278:DNA, Superhelical / Q000502:physiology; D005111:Extrachromosomal Inheritance; D005786:Gene Expression Regulation; D009154:Mutation; D011995:Recombination, Genetic; D012441:Saccharomyces cerevisiae / Q000235:genetics*; D014158:Transcription, Genetic
D004270:DNA, Circular; D004271:DNA, Fungal; D004275:DNA, Ribosomal; D004278:DNA, Superhelical; D004264:DNA Topoisomerases, Type I