Mapping of essential replication functions of the linear plasmid lp17 of B-burgdorferi by targeted deletion walking

The genome of the Lyme disease pathogen Borrelia burgdorferi strain B31 MI includes one linear chromosome, 10 circular and 12 linear plasmids. Members of four paralogous gene families, revealed by genome sequencing, have been suggested as replication/ partition functions for both the linear and circular plasmids. Some of these genes have been experimentally shown to be essential for the replication of the B. burgdorferi replicons that encode them. In this study, we located the region essential for replication of Ip17, the second smallest linear plasmid in B. burgdorferi. We used a novel in vivo method, targeted deletion walking, to systematically delete DNA from either the left or right end of Ip17. We report that the region essential for replication of Ip17 is 1.8 kb ( bp 7946 9766) and contains only one intact open reading frame (BBD14). Expression of BBD14 is required for the replication, suggesting that it is the replication initiator for Ip17. The BBD14 protein is a member of paralogous family (PF) 62 and we present the first experimental evidence for the role of a PF 62 member. Adjacent non-coding sequences are also required, suggesting that the origin lies at least partially outside the coding region. Surprisingly, deletion of BBD21, the ParA orthologue (PF 32), had little effect upon plasmid stability or incompatibility. Finally, data are presented suggesting that Ip17 replication occurs preferentially on a linear rather than a circular DNA molecule.