Building from the Inside Out

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Science  11 May 2007:
Vol. 316, Issue 5826, pp. 799
DOI: 10.1126/science.316.5826.799c

The evolutionary origins of complex organs, which in their current state of assembly feature many distinct components that apparently have no function in isolation, have long been debated. Liu and Ochman have unraveled the history of the origins of bacterial flagella by using a phylogenetic profiling method applied across whole genome sequences to identify a set of 24 core genes in the common ancestor of bacteria. The members of this core set were probably derived from a single gene that had undergone a combination of successive duplication, loss, transfer, and diversification events. The evolution of the flagellar components apparently followed the present-day order of assembly, with the oldest proteins (the rotary motor) being those proximal to the bacterial inner membrane and the most recent (the filament monomers) being the most distal. Hence, the flagellum probably started life as a simple proton-driven transporter that evolved into a more elaborate secretory apparatus—of a sort still found in bacteria today in the form of the type III secretion system—and finally into the self-secretory motility organelle of modern species. — CA

Proc. Natl. Acad. Sci. U.S.A. 104, 7116 (2007).

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