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Zusammenfassung:
How complex, multi-component macromolecular machines evolved remains
poorly understood. Here we reveal the evolutionary origins of the
chemosensory machinery that controls flagellar motility in Escherichia
coli. We first identify ancestral forms still present in Vibrio
cholerae, Pseudomonas aeruginosa, Shewanella oneidensis and
Methylomicrobium alcaliphilum, characterizing their structures by
electron cryotomography and finding evidence that they function in a
stress response pathway. Using bioinformatics, we trace the evolution of
the system through gamma -Proteobacteria, pinpointing key evolutionary
events that led to the machine now seen in E. coli. Our results suggest
that two ancient chemosensory systems with different inputs and outputs
(F6 and F7) existed contemporaneously, with one (F7) ultimately taking
over the inputs and outputs of the other (F6), which was subsequently
lost. Bacterial chemosensory systems are grouped into 17 flagellar
classes (F1-17). Here the authors employ electron cryotomography and
comparative genomics to characterise the chemosensory arrays in gamma
-proteobacteria and identify a structural distinct form of F7 that was
repurposed to a different biological role over the course of its
evolution.
