CONVERSION OF A LINEAR TO A CIRCULAR PLASMID IN THE RELAPSING FEVER AGENT BORRELIA-HERMSII

Spirochetes of the genus Borrelia have genomes composed of both linear and circular replicons. We characterized the genomic organization of B. burgdorferi, B. hermsii, B. turicatae, and B. anserina with pulsed- field gel electrophoresis. All four species contained a linear chromos ome approximately 1 Mb in size and multiple linear plasmids in the 16- to 200-kb size range. Plasmids 180 and 170 kb in size, present in the relapsing fever agents B. hermsii and B. turicatae but not in the oth er two species, behaved as linear duplex DNA molecules under different electrophoretic conditions. A variant of strain HS1 of B. hermsii had a 180-kb circular instead of linear plasmid. There were no detectable differences in the growth rates or in the expression of cellular prot eins between cells bearing linear forms and those bearing circular for ms of the plasmid. The conversion to a circular conformation of monome ric length was demonstrated by the introduction of strand breaks with irradiation, restriction endonuclease analysis, and direct observation of the DNA molecules by fluorescent microscopy. Consideration of diff erent models for the replication of linear DNA suggests that circular intermediates may be involved in the replication of linear replicons i n Borrelia spp.