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Isolated outer membranes of Borrelia burgdorferi 297 were utilized to obtain partial amino acid sequence information for a low-molecular-weight, outer membrane-associated polypeptide. Degenerate oligonucleotide primers based upon this information were used to amplify a 100-bp probe for detection of the corresponding full-length gene within a B. burgdorferi total genomic library. The relevant open reading frame (ORF) encoded a polypeptide comprised of a 17-amino-acid putative signal peptide terminated by LFVAC, a probable consensus sequence for lipoprotein modification, and a mature protein of 51 amino acids (predicted molecular mass of 5.8 kDa). The DNA sequences of the corresponding ORFs in B. burgdorferi 297 and B31 were identical; the corresponding ORF in strain N40 differed by only one nucleotide. Assuming conventional processing and acylation, the molecular weight of the lipoprotein, designated lp6.6, is about 6,600. The lp6.6 gene, which was localized to the 49-kb linear plasmid of B. burgdorferi, subsequently was cloned and expressed in Escherichia coli as a fusion protein with glutathione S-transferase. Immunoblot analysis with monoclonal antibody 240.7 revealed that lp6.6 was identical to a low-molecular-weight, highly conserved B. burgdorferi lipoprotein reported previously (L. I. Katona, G. Beck, and G. S. Habicht, Infect. Immun. 60:4995-5003, 1992). Results of indirect immunofluorescence assays, growth inhibition assays, passive immunizations, and active immunizations indicated that this outer membrane-associated antigen is not surface exposed in B. burgdorferi. Particularly interesting was the finding that mice and rhesus monkeys chronically infected with B. burgdorferi failed to develop antibodies against this antigen. We propose that high-level expression of lp6.6 is associated with the arthropod phase of the spirochetal life cycle and that expression of the gene is downregulated during mammalian infection.