Topology-dependent transcription in linear and circular plasmids of the segmented genome of Borrelia burgdorferi

The segmented genome of Borrelia burgdorferi, a causative agent of Lyme disease, contains a mixture of over 20 linear and circular plasmids. Genes encoding five paralogous families of plasmid replication proteins are located on both circular and linear DNA molecules. The effect of DNA topology on the transcription of replication proteins from two B. burgdorferi plasmids, cp9 and lp17, was examined using quantitative reverse transcription polymerase chain reaction. Circular to linear conversion of a cp9-derived plasmid resulted in a 160-fold decrease in transcript levels of bbc01, believed to encode the replication initiator. A 14.9-fold reduction in plasmid copy number was also observed, resulting in a net 10.7-fold lower transcription level per gene copy on a linear versus a circular plasmid. In contrast, expression of the bbd14 replication initiator for the linear plasmid lp17 was 7.2 times higher per gene copy on a linear versus a circular plasmid. Topology-dependent transcription of these genes may help to block topological interconversions during genome evolution, offers a new avenue for global gene regulation and also has important implications for the design of genetic complementation experiments in B. burgdorferi.