THE BORRELIA-BURGDORFERI CIRCULAR PLASMID CP26 - CONSERVATION OF PLASMID STRUCTURE AND TARGETED INACTIVATION OF THE OSPC GENE

The 26 to 28 kb circular plasmid of B. burgdorferi sensu lato (cp26) i s ubiquitous among bacteria of this group and contains loci implicated in the mouse-tick transmission cycle. Restriction mapping and Souther n hybridization indicated that the structure of cp26 is conserved amon g isolates from different origins and culture passage histories. The c p26 ospC gene encodes an outer surface protein whose synthesis within infected ticks increases when the ticks feed, and whose synthesis in c ulture increases after a temperature upshift. Previous studies of ospC coding sequences showed them to have stretches of sequence apparently derived from the ospC genes of distantly related isolates by homologo us recombination after DNA transfer. We found conservation of the prom oter regions of the ospC and guaA genes, which are divergently transcr ibed. We also demonstrated that the increase in OspC protein after a t emperature upshift parallels increases in mRNA levels, as expected if regulatory regions adjoin the conserved sequences in the promoter regi ons. Finally, we used directed insertion to inactivate the ospC gene o f a non-infectious isolate. This first example of directed gene inacti vation in B. burgdorferi shows that the OspC protein is not required f or stable maintenance of cp26 or growth in culture.