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MBE Advance Access published online on June 22, 2005
Molecular Biology and Evolution, doi:10.1093/molbev/msi199
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Published by Oxford University Press 2005.
Accepted June 14, 2005

Research Article

Molecular Evolution of Rickettsia Surface Antigens: Evidence of Positive Selection

Guillaume Blanc 1*, Maxime Ngwamidiba 2, Hiroyuki Ogata 1, Pierre-Edouard Fournier 3, Jean-Michel Claverie 1, and Didier Raoult 2

1 Information Génomique et Structurale, UPR 2589, 31, Chemin Joseph Aiguier, 13402 Marseille Cedex 20, France
2 Unité des rickettsies, IFR 48 CNRS UMR 6020, Faculté de médecine, Université de la Méditerranée, 27 Boulevard Jean Moulin, 13385 Marseille cedex 05, France
3 Information Génomique et Structurale, UPR 2589, 31, Chemin Joseph Aiguier, 13402 Marseille Cedex 20, France; Unité des rickettsies, IFR 48 CNRS UMR 6020, Faculté de médecine, Université de la Méditerranée, 27 Boulevard Jean Moulin, 13385 Marseille cedex 05, France

* To whom correspondence should be addressed.
Guillaume Blanc, E-mail: guillaume.blanc{at}igs.cnrs-mrs.fr


  Abstract

The Rickettsia genus is a group of obligate intracellular parasitic {alpha}-proteobacteria that includes human pathogens responsible for the typhus disease and various types of spotted fevers. rOmpA and rOmpB are two members of the "surface cell antigen" (Sca) autotransporter protein family that may play key roles in the adhesion of the Rickettsia cells to the host tissue. These molecules are likely determinant for the pathogenicity of the Rickettsia and represent good candidates for vaccine development. We identified the seventeen members of this family of outer membrane proteins in nine fully sequenced Rickettsia genomes. The typical architecture of the Sca proteins is composed of a N-terminal signal peptide and a C-terminal autotransporter (AT) domain that promote the export of the central passenger domain to the outside of the bacteria. A characteristic of this family is the frequent degradation of the genes, which results in different subsets of sca genes being expressed among Rickettsia species. Here, we present a detailed analysis of their phylogenetic relationships and evolution. We provide strong evidence that rOmpA and rOmpB as well as three other members of the Sca protein family -Sca1, Sca2 and Sca4 -have evolved under positive selection. The exclusive distribution of the predicted positively selected sites within the passenger domains of these proteins argues that these regions are involved in the interaction with the host and may be locked in "arms race" coevolutionary conflicts.

Keywords: adaptive evolution; Rickettsia; autotransporter; adhesin; antigen.
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