E. coli and M. luteus DNA topoisomerase I can catalyze catenation of decatenation of double-stranded DNA rings.

Escherichia coli and Micrococcus luteus DNA topoisomerase I are found to promote catenation of double-stranded DNA rings. At low DNA concentration dimeric catenanes are the major catenated products; at high DNA concentration or when spermidine is present, catenanes containing more than two rings are formed. There is no requirement of extensive sequence homology between the conponent rings forming a catenane; dimeric catenanes between Pseudomonas phage PM2 DNA and E. coli plasmid pBR322 are readily formed. The formation of a dimeric catenane by these type I topoisomerases, however, requires the presence of at least one preexisting single-chain scission in one of the two component rings. This is in contrast to the cases with the type II DNA topoisomerases which can form catenanes made of covalently closed rings only. The catenanes formed by the type I enzymes can be unlinked by the same enzymes, or by DNA gyrase, a type II enzyme, upon dilution of the isolated catenanes. The catenation and decatenation of duplex DNA rings adds a fourth type of reaction promoted by these type I DNA topoisomerases to the three reported previously: relaxation of superhelical DNA, interconversion between single-stranded DNA rings with and without knots and the intertwining of single-stranded DNA rings of complementary sequences into a covalently closed duplex ring with a high linking number. All four topoisomerization reactions involve the crossing of one DNA strand through a transient break of another DNA strand. The new reaction reported here suggests that such a crossover event might not require pairing of complementary nucleotide sequences.