MBE Advance Access published online on December 7, 2007
Molecular Biology and Evolution, doi:10.1093/molbev/msm252
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Research Article |
Diversifying Selection and Concerted Evolution of a Type IV Secretion System in Bartonella
1 Department of Molecular Evolution, Evolutionary Biology Centre, Uppsala University, Uppsala, S-752 36 Sweden
2 Current address: Smurfit Institute of Genetics, University of Dublin, Trinity College Dublin, Ireland
Address correspondence to: Siv G. E. Andersson, Department of Molecular Evolution, Evolutionary Biology Centre, Norbyvägen 18C, 752 36 Uppsala, Sweden Tel: +46-18-471 43 79; Fax: +46-18-471 64 04; e-mail: Siv.Andersson{at}ebc.uu.se
Received for publication July 20, 2007. Revision received November 9, 2007. Accepted for publication November 11, 2007.
We have studied the evolution of a Type IV Secretion System (T4SS), in Bartonella, which is thought to have changed function from conjugation to erythrocyte adherence following a recent horizontal gene transfer event. The system, called Trw, is unique among T4SSs in that genes encoding both exo- and intracellular components are located within the same duplicated fragment. This provides an opportunity to study the influence of selection on proteins involved in host-pathogen interactions. We sequenced the trw locus from several strains of Bartonella henselae and investigated its evolutionary history by comparisons to other Bartonella species. Several instances of recombination and gene conversion events where detected in the 2-5 fold duplicated gene fragments encompassing trwJIH, explaining the homogenization of the anchoring protein TrwI and the divergence of the minor pilus protein TrwJ. A phylogenetic analysis of the 7-8 fold duplicated gene coding for the major pilus protein TrwL displayed two distinct clades, likely representing a sub-functionalization event. The analyses of the B. henselae strains also identified a recent horizontal transfer event of almost the complete trwL region. We suggest that the switch in function of the T4SS was mediated by the duplication of the genes encoding pilus components and their diversification by combinatorial sequence shuffling within and among genomes. We suggest that the pilus proteins have evolved by diversifying selection to match a divergent set of erythrocyte surface structures, consistent with the trench warfare co-evolutionary model.
Key Words: type IV secretion system • Bartonella • duplication • recombination • gene conversion • host-pathogen interaction
* These authors contributed equally to this work
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