Flagella of Halobacterium salinarum and their biogenesis

Like many unicellular organisms the halophilic archaeon Halobacterium salinarum posseses flagella, which - in combination with a sophisticated sensory system (see transducer proteins and CheAY-two-component systems) - enable this organism to swim and find places with optimal growth conditions. The target of the sensory system is the switch of the flagellar motor.

The knowledge about eubacterial flagella and flagellar motors is very detailed. Of the archaeal flagellar apparatus however, only a few poorly defined structures have been described so far. All presently available data clearly indicate that the archaeal flagellar apparatus is completely different from the eubacterial one in many respects (Thomas et al., 2001), although both motility systems are based on the principle of rotating flagella. We are interested to find out more about the flagellar apparatus of H. salinarum which might represent a model system for other archaea.

Depending on the growth phase the majority of H. salinarum cells are either monopolarly or bipolarly flagellated (Alam and Oesterhelt, 1984). A flagellar bundle consists of 5 - 10 filaments forming a right handed helix. The flagella are inserted into a distinct polar cap structure (Kupper et al., 1994) and it is likely that every filament posesses its own flagellar motor.

Fig. 1: Flagellated polar caps. Taken from Kupper et al., 2001

Halobacterial cells are pushed forward by clockwise and pulled backward by counterclockwise rotation of the right handed flagellar bundle. Rotation and switching of the flagellar motor assembly have been studied extensively (Marwan et al., 1991).

Flagella of H. salinarum consist of 5 different flagellins FlgA1-A2 and FlgB1-B3 (Alam and Oesterhelt, 1987; Gerl et al., 1989), which are encoded by two separate transcriptional units. The flagellins show no homology to known eubacterial flagellins and are posttranscriptionally glycosylated (Wieland et al., 1985). Further genes involved in the biogenesis of halobacterial flagella are found in the fla gene cluster. Deletion of flaI showed that this gene is essential for the biogenesis of flagella (Patenge et al., 2001).

Fig. 2: The Halobacterium salinarum fla gene cluster. Adapted from Patenge et al., 2001.
Fig. 3: Electron micrographs of uranyl acetate negatively stained H. salinarum cells. ΔflaI, flaI deletion strain; ΔflaI/flaI+ and ΔflaI/XflaI+ are complemented strains carrying flaI under the control of different promoters. ΔflaI cells lack flagella. The degree of complementation depends on the promoter used for complementation, reflecting different gene doses of flaI. Complementation under control of the bop promotor is almost complete. Taken from Patenge et al., 2001.

The fact that immature flagellins contain a cleavable signal peptide and the homology of halobacterial FlaI to P. aeruginosa PilB revealed that biogenesis and assembly of archaeal flagella are rather related to the eubacterial type IV pili secretion system than to the type III secretion system involved in the assembly of eubacterial flagella.

Fig. 4: Motility / chemotaxis assay using semi-solid swarm plates containing 0.25% agar in complex medium. S9, H. salinarum wild-type cells, motility positive control; ΔΔche, H. salinarum, chemotaxis-negative strain, motility negative control; ΔcheB, H. salinarum methylesterase CheB negative strain, fast switching phenotype. The genotypes of ΔflaI, ΔflaI/flaI+ and ΔflaI/XflaI are described in Fig. 3. Taken from Patenge et al., 2001.

Unlike in Eubacteria, where the flagellins are transported through the hollow core of the filament, archaeal flagellins are supposed to be assembled at the base of the flagellum. Halobacterial flagellar filaments are only about half as thick as their eubacterial counterparts and in a recent model of the halobacterial flagellum the filament lacks a central channel (Cohen-Krausz and Trachtenberg, 2002).

We are attempting to identify yet unknown components of the flagellar motor and the assembly-machinery of H. salinarum flagella by means of molecular biology, proteomics, bioinformatics and transcriptomics.


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    Morphology, function and isolation of halobacterial flagella.
    J. Mol. Biol., 176, 459-475.

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    The fla gene cluster is involved in the biogenesis of flagella in Halobacterium salinarum.
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