Dokument

Summary:
Bacterial cells are highly organized with respect to their shape, structure or function. In particular flagellar motility and chemotaxis of many bacteria require a precise spatiotemporal regulation of the corresponding components to avoid wasting energy. Despite the tight regulation, flagellar motility and chemotaxis are also targets of adaptation in response to extra- and intracellular cues. The balance between tight regulation and flexible adaptation allows bacteria to efficiently thrive in changing and potentially nutrient limiting environments. This thesis focuses on the adaptation of the flagella-mediated motility of the γ-proteobacterium Shewanella oneidensis MR 1 by dynamically exchanging one of its motor components and a system in Shewanella putrefaciens CN-32 that ensures proper polar localization of several proteins, among them the chemotaxis system. S. oneidensis MR-1 possesses a single polar flagellar system but harbors two types of ion-channels, the so-called stators, that power flagellar rotation. The second chapter demonstrates that both stators, the native Na+-dependent PomAB and putatively acquired H+-dependent MotAB complex, are solely sufficient to drive motility in liquid environments and may interact with the flagellar rotor in varying configurations depending on sodium-ion concentrations, likely forming a hybrid motor. The principal environmental cue that can be integrated and reacted to by PomAB/MotAB stator swapping is the external Na+ concentration. Functionality of MotAB on the other hand seems to be tied to the membrane potential and load on the flagellum. Some limitations of MotAB can be overcome by small point mutations in the plug domain of MotB, likely by changing the MotAB channel properties and/or its mechanosensing capability. The second system studied was a landmark protein that serves as an organizational platform involved in different cellular processes including chemotaxis. This transmembrane protein was identified as the functional orthologue of Vibrio cholerae HubP. In S. putrefaciens CN-32 it is required for polar localization and possibly the correct function of the chemotaxis components, but not for placement of the flagellum which depends on the GTPase FlhF. Localization of HubP itself may be dependent on its LysM peptidoglycan-binding domain. Since the swimming speed was decreased when hubP was deleted, a so far unidentified modulator of flagellar motility might require HubP for proper function. In addition, deletion of hubP caused an impairment in twitching motility and affected proper localization of the chromosome partitioning system. Due to its structural similarity to Pseudomonas aeruginosa FimV and partially matching phenotypes upon deletion, the group of HubP/FimV homologs, characterized by a rather conserved N-terminal periplasmic section and a highly variable acidic cytoplasmic part, may serve as polar markers in various bacterial species with respect to different cellular functions. Thus, two separate systems target the flagellum and chemotaxis system to the cell pole.

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