A 55-kilodalton antigen encoded by a gene on a Borrelia burgdorferi 49-kilobase plasmid is recognized by antibodies in sera from patients with Lyme disease.

We have identified a 55-kDa antigen encoded by a gene on a 49-kb plasmid of Borrelia burgdorferi. The screening of a B. burgdorferi DNA expression library (N40 strain) with rabbit anti-B. burgdorferi serum and then with serum from a patient with Lyme disease arthritis revealed a clone that synthesized an antigen that was reactive with both sera. DNA sequence analysis identified an operon with two genes, s1 and s2 (1,254 and 780 nucleotides), that expressed antigens with the predicted molecular masses of 55 and 29 kDa, respectively. Pulsed-field gel electrophoresis showed that the s1-s2 operon was located on the 49-kb plasmid. Recombinant S1 was synthesized as a glutathione S-transferase fusion protein in Escherichia coli. Antibodies to recombinant S1 bound to a 55-kDa protein in lysates of B. burgdorferi, indicating that cultured spirochetes synthesized S1. Thirty-one of 100 Lyme disease patients had immunoglobulin G (IgG) and/or IgM antibodies to S1. IgG antibodies to S1 were detected by enzyme-linked immunosorbent assay and immunoblots in the sera of 21 (21%) of 100 patients with Lyme disease; 11 (27.5%) of the S1-positive samples were from patients (40) with early-stage Lyme disease, and 10 (16.7%) were from patients (60) with late-stage Lyme disease. Fifteen (38.5%) of 40 serum samples from patients with early-stage Lyme disease had IgM antibodies to S1. These data suggest that the S1 antigen encoded by a gene on the 49-kb plasmid is recognized serologically by a subset of patients with early- or late-stage Lyme disease.