MBE Advance Access published online on July 17, 2008
Molecular Biology and Evolution, doi:10.1093/molbev/msn153
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Research Article |
The evolution of the flagellar assembly pathway in endosymbiotic bacterial genomes
Department of Genetics, Smurfit Institute of Genetics, University of Dublin, Trinity College, Dublin 2, Ireland
* Corresponding Author: Mario A. Fares Department of Genetics, Smurfit Institute of Genetics, University of Dublin, Trinity College, Dublin 2, Ireland Email: faresm{at}tcd.ie TEL: +353 01 8963521 FAX: +353 01 6798558
Received for publication April 1, 2008. Revision received June 27, 2008. Accepted for publication July 7, 2008.
Genome shrinkage is a common feature of most intra-cellular pathogens and symbionts. Reduction of genome sizes is among the best-characterised natural strategies adopted by intra-cellular organisms to save and avoid maintaining expensive redundant biological processes. Endosymbiotic bacteria of insects are examples of biological economy taken to completion because their genomes are dramatically reduced. These bacteria are non-motile and their biochemical processes are intimately related to those of their host. Because of this relationship, many of the processes in these bacteria have been either lost or have suffered massive re-modelling to adapt to the intra-cellular symbiotic lifestyle. An example of such changes is the flagellum structure that is essential for bacterial motility and infectivity. Our analysis indicates that genes responsible for flagellar assembly have been partially or totally lost in most intra-cellular symbionts of gamma-Proteobacteria. Comparative genomic analyses show that flagellar genes have been differentially lost in endosymbiotic bacteria of insects. Only proteins involved in protein export within the flagella assembly pathway (type III secretion system and the basal-body) have been kept in most of the endosymbionts whereas those involved in building the filament and hook of flagella have only in few instances been kept, indicating a change in the functional purpose of this pathway. In some endosymbionts, genes controlling protein-export switch and hook length have undergone functional divergence as shown through an analysis of their evolutionary dynamics. Based on our results we suggest that genes of flagellum have diverged functionally as to specialise in the export of proteins from the bacterium to the host.
Key Words: endosymbionts • flagellar pathway • Buchnera aphidicola • Functional Divergence • Protein Export
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