Stereostructure of replicative DNA catenanes from eukaryotic cells

We isolated DNA catenanes that accumulate after inhibition of the major type-2 topoisomerase in eukaryotic organisms. Catenated plasmids were isolated from a Saccharomyces cerevisiae topoisomerase II temperature-sensitive mutant exposed to a nonpermissive temperature. The sterostructure of the catenanes was analyzed by electron microscopy after coating the DNA with RecA protein. All were right-handed and belonged to the torus family. Catenanes arising among nascent SV40 DNA under hypertonic growth conditions were analyzed by gel electrophoresis. Spacing of the electrophoretic ladder was consistent with a torus form. Furthermore, the supercoiling induced by catenation was positive, implying that the catenanes were right-handed