The genes encoding the 17-kilodalton genus-common antigen have been cloned and sequenced from Rickettsia conorii, Rickettsia prowazekii, and Rickettsia typhi. Compared with the R. rickettsii sequence, this sequence had a high degree of homology within the coding and control regions (R. conorii, 99.8%; R. prowazekii, 88.1%; R. typhi, 88.7%). The 5' flanking regions, including the promoter and the transcription initiation sites, were extremely well conserved for all four species, suggesting that control and expression of this locus are important to the survival of the rickettsiae.The 17,000-molecular-weight-antigen (17K antigen) gene of Rickettsia rickettsii is one of the most thoroughly characterized loci from any rickettsial genome (2, 3; B. E. Anderson, Ph.D. dissertation, Georgia State University, Atlanta, 1988). Nucleotide sequences for transcription initiation and putative translation initiation, signal peptide, and lipid modification sites have been identified (2). To determine the degree of genetic conservation among these regions, as well as other areas within the coding region for the mature protein, we cloned and sequenced this genus-common antigen from three additional pathogenic species of rickettsiae: Rickettsia conorii, Rickettsia prowazekii, and Rickettsia typhi. In addition, nucleotide sequences were examined for regions of variability that may serve as group-or speciesspecific priming sites for polymerase chain reaction amplification of rickettsial DNA from clinical samples.To determine the level of conservation of the 17K antigen throughout the genus Rickettsia, immunoblot analysis was performed. R. rickettsii Sheila Smith, R. conorii Moroccan, R. prowazekii Breinl, R. typhi Wilmington, Rickettsia bellii RML 369-C, Rickettsia akari Hartford, and Rickettsia canada McKiel were grown in Vero cells as previously described (3) and purified by Renografin density gradient centrifugation (14). Purified rickettsiae were solubilized at 100°C and subjected to electrophoresis by the method of Laemmli (8). Resolved proteins in the gel were electroblotted to nitrocellulose filters by the method of Towbin et al. (11). The resulting filters were reacted with rabbit antiserum raised to the synthetic peptide NH2-Asp-Asn-Gly-Asn-TyrGly-Tyr-Val-Thr-Pro-Asn-Lys-Thr-Tyr-Arg-COOH, which is derived from residues 106 to 120 of the deduced amino acid sequence of the 17K antigen of R. rickettsii (3; Anderson, Ph.D. dissertation). When the filters were reacted with a horseradish peroxidase-conjugated anti-rabbit serum, all species of rickettsiae tested exhibited an antigenic protein with an approximate molecular weight of 17,000 (Fig. 1). This antigenic protein has not been observed in members of other genera, including Bacillus, Proteus, Neisseria, Escherichia, and Chlamydia (data not shown). Thus, the 17K antigen described for R. rickettsii appears to be a genuscommon antigen found in members of both the spotted fever and the typhus group rickettsiae.DNA from R. rickettsii, R. conorii, R. typhi, and R. (5), and the DNA...