Molecular and evolutionary characterization of the cp32/18 family of supercoiled plasmids in Borrelia burgdorferi 297

In this study, we characterized seven members of the cp32/18 family of supe rcoiled plasmids in Borrelia burgdorferi 297. Complete sequence analysis of a 21-kb plasmid (cp18-2) confirmed that the strain 297 plasmids are simila r in overall content and organization to their B31 counterparts. Of the 31 open reading frames (ORFs) in cp18-2, only three showed sequence relatednes s to proteins with known functions, and only one, a ParA/SopA ortholog, was related to nonborrelial polypeptides. Besides the lipoproteins, none of th e ORFs appeared likely to encode a surface-exposed protein. Comparison with the B31 genomic sequence indicated that paralogs for most of the ORFs in c p18-2 can be identified on other genetic elements. cp18-2 was found to lack a 9- to 10-kb fragment present in the 32-kb homologs which, by extrapolati on from the B31 cp32 sequences, contains at least 15 genes presumed to be u nnecessary for plasmid maintenance. Sequence analysis of the lipoprotein-en coding variable loci provided evidence that recombinatorial processes withi n these regions may result in the acquisition of exogenous DNA. Pairwise an alysis with random shuffling revealed that the multiple lipoproteins (Mlp; formerly designated 2.9 LPs) fall into two distinct homology groups which a ppear to have arisen by gene fusion events similar to those recently propos ed to have generated the three OspE, OspF, and Elp lipoprotein families (D. R. Akins, M. J. Caimano, X. Yang, F. Cerna, M. V. Norgard, and J. D. Radol f, Infect. Immun. 67:1526-1532, 1999). Comparative analysis of the variable regions also indicated that recombination within the loci of each plasmid may occur independently. Last, comparison of variable loci revealed that th e cp32/18 plasmid complements of the B31 and 297 isolates differ substantia lly, indicating that the two strains have been subject to divergent adaptiv e pressures. In addition to providing evidence for two different types of r ecombinatorial events involving cp32/18 plasmids, these findings underscore the need for genetic analysis of diverse borrelial isolates in order to el ucidate the Lyme disease spirochete's complex parasitic strategies.