Abstract
Movement of the DNA replication machinery through the double helix induces acute positive supercoiling ahead of the fork and precatenanes behind it. Because topoisomerase I and II create transient single- and double-stranded DNA breaks, respectively, it has been assumed that type I enzymes relax the positive supercoils that precede the replication fork. Conversely, type II enzymes primarily resolve the precatenanes and untangle catenated daughter chromosomes. However, studies on yeast and bacteria suggest that type II topoisomerases may also function ahead of the replication machinery. If this is the case, then positive DNA supercoils should be the preferred relaxation substrate for topoisomerase IIalpha, the enzyme isoform involved in replicative processes in humans. Results indicate that human topoisomerase IIalpha relaxes positively supercoiled plasmids >10-fold faster than negatively supercoiled molecules. In contrast, topoisomerase IIbeta, which is not required for DNA replication, displays no such preference. In addition to its high rates of relaxation, topoisomerase IIalpha maintains lower levels of DNA cleavage complexes with positively supercoiled molecules. These properties suggest that human topoisomerase IIalpha has the potential to alleviate torsional stress ahead of replication forks in an efficient and safe manner.
Highlights
One of the most striking features of DNA is the intertwining of the two complementary strands of the double helix [1]
If the replisome rotates around the helical axis of the DNA, this underwinding allows some of the torsional stress in the prereplicated DNA to be translated to the newly replicated daughter molecules in the form of precatenanes (Fig. 1C) [6, 7, 11]
In most cases, positive superhelical twists were introduced in the DNA by the addition of intercalating agents such as ethidium bromide or chloroquine [58, 61,62,63]
Summary
Studies on yeast and bacteria suggest that type II topoisomerases may function ahead of the replication machinery If this is the case, positive DNA supercoils should be the preferred relaxation substrate for topoisomerase II␣, the enzyme isoform involved in replicative processes in humans. If type II topoisomerases play a role ahead of replication forks in vertebrate species, positively supercoiled DNA should be the preferred relaxation substrate for topoisomerase II␣, the isoform that is involved in replicative processes. Topoisomerase II␣ maintains lower levels of DNA cleavage complexes with positively supercoiled molecules These enzymological properties are consistent with those of a topoisomerase that functions ahead of the replication machinery
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