Both DNA topoisomerases I and II relax 2 micron plasmid DNA in living yeast cells.

Measurements at various temperatures of the linking number of yeast 2 microns plasmid DNA in wild-type cells and in cells bearing mutations in the DNA topoisomerase I and II genes show that bulk 2 microns plasmid minichromosome are maintained in a relaxed state by the combined action of topoisomerases I and II. Bulk 2 microns minichromosomes are not under torsional stress in vivo and are not substrates for a putative gyrase-like topoisomerase.