In vivo relations between pAMbeta1-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 Topbeta encoded by the Gram-positive broad-host-range plasmid pAMbeta1. 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 pAMbeta1, 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 pAMbeta1 replication in Bacillus subtilis cells, Topbeta promotes premature arrest of DNA polymerase I, approximately 190bp downstream of the replication initiation point. We propose that Topbeta acts on the early replication intermediates of pAMbeta1, 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.