In vivo relations between pAMβ1-encoded type I topoisomerase and plasmid replication

A number of large extrachromosomal elements encode prokaryotic type I topoisomerases of unknown functions. Here, we analysed the topoisomerase Top beta encoded by the Gram-positive broad-host-range plasmid pAM beta 1. We show that this enzyme possesses the DNA relaxation activity of type I topoisomerases. Interestingly, it is active only on plasmids that use DNA polymerase I to initiate replication, such as pAM beta 1, and depends on the activity of this polymerase. This is the first example, to our knowledge, of prokaryotic type I topoisomerase that is specific for a given type of replicon. During pAM beta 1 replication in Bacillus subtilis cells, Top beta promotes premature arrest of DNA polymerase I, approximate to 190 bp downstream of the replication initiation point. We propose that Top beta acts on the early replication intermediates of pAM beta 1, which contain D-loops formed by DNA polymerase I-mediated strand displacement. The possible role of the resulting DNA pol I arrest in plasmid replication is discussed.