Phylogenetic study of Rickettsia species using sequences of the autotransporter protein-encoding gene sca2.

The analyses of genome sequences from Rickettsia conorii and R. prowazekii have allowed the identification of five genes encoding autotransporter proteins, including ompA and four genes annotated in the R. prowazekii genome as "surface cell antigen" (sca) genes. Of these, ompA and sca5 (ompB) are known to encode membrane-exposed antigenic proteins playing a major role in the host's immune response, and sca4 encodes a truncated autotransporter protein. In order to study further the phylogeny of the genus Rickettsia, we attempted amplification and sequencing of the sca2 genes from the 20 currently validated Rickettsia species. Sixteen species exhibited a complete sca2 gene, ranging from 2,727 bp in R. bellii to 5,580 bp in R. rhipicephali. R. helvetica and R. canadensis had a split gene, and R. prowazekii and R. typhi had remnant fragments of sca2. We also identified in R. akari, R. prowazekii, and R. typhi a duplication of the sca2 gene. The phylogenetic trees inferred from both the nucleotide and protein sequences of sca2 showed four clusters of rickettsiae, that is, the R. rickettsii, R. massiliae, R. akari, and typhus groups, which were supported by significant bootstrap values.