Living in a Foster Home: The Single Subpolar Flagellum Fla1 of Rhodobacter sphaeroides.

Laura Camarena,Georges Dreyfus

doi:10.3390/biom10050774

Abstract

Rhodobacter sphaeroides is an α-proteobacterium that has the particularity of having two functional flagellar systems used for swimming. Under the growth conditions commonly used in the laboratory, a single subpolar flagellum that traverses the cell membrane, is assembled on the surface. This flagellum has been named Fla1. Phylogenetic analyses have suggested that this flagellar genetic system was acquired from an ancient γ-proteobacterium. It has been shown that this flagellum has components homologous to those present in other γ-proteobacteria such as the H-ring characteristic of the Vibrio species. Other features of this flagellum such as a straight hook, and a prominent HAP region have been studied and the molecular basis underlying these features has been revealed. It has also been shown that FliL, and the protein MotF, mainly found in several species of the family Rhodobacteraceae, contribute to remodel the amphipathic region of MotB, known as the plug, in order to allow flagellar rotation. In the absence of the plug region of MotB, FliL and MotF are dispensable. In this review we have covered the most relevant aspects of the Fla1 flagellum of this remarkable photosynthetic bacterium.

Highlights

The bacterial flagellum is driven by a complex molecular motor
The flagellar basal body contains the rotor and the export apparatus, and is composed of numerous proteins arranged in several rings and a central rod
The physical properties of these two axial structures are different given that the filament is a long rigid helix and the hook is a short flexible structure that acts as a universal joint [14,15,16]

Summary

The Flagellar Structure

The bacterial flagellum is driven by a complex molecular motor. The flagellar basal body contains the rotor and the export apparatus, and is composed of numerous proteins arranged in several rings and a central rod (reviewed recently in [1]). FleQ forms a heterodimeric complex with FleT and activates the expression of the class III flagellar genes In this gene class the sigma factor σ28, called FliA, and its anti-sigma protein FlgM are expressed, as well as the genes encoding the components required to complete the basal body, the hook, and the stator proteins MotA and MotB. When the hook is completed, FlgM is exported out of the cell and FliA directs the RNA polymerase to express the class IV flagellar genes such as fliC and fliD encoding flagellin and the filament cap protein, respectively [55,72] (Figure 2). Other chemotactic components that control rotation of Fla are encoded in the chemotactic operon cheOp2 that includes cheY3, cheA2, cheW2 and cheW3, cheR2, cheB1 and tlpC This operon is expressed by the housekeeping σ70 factor and from a promoter dependent on σ28 [73,75]. It has been shown that all these components control the chemosensory response of the Fla flagella [64]; CtrA activates other chemotactic receptors [77]; it remains to be tested if these receptors affect the chemotactic response mediated by this flagellum

The Hook and Basal Body

Future Directions