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.