niiicholas
Posts: 319
Joined: May 2002
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In the "A Different Way to Control a Bacterial Flagellum" category:
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J Mol Microbiol Biotechnol 2002 May;4(3):183-6
Sensory transduction to the flagellar motor of Sinorhizobium meliloti.
Scharf B, Schmitt R.
Lehrstuhl fur Genetik, Universitat Regensburg, Germany.
Molecular mechanisms that govern chemotaxis and motility in the nitrogen-fixing soil bacterium, Sinorhizobium meliloti, are distinguished from the well-studied taxis systems of enterobacteria by new features. (i) In addition to six transmembrane chemotaxis receptors, S. meliloti has two cytoplasmic receptor proteins, McpY (methyl-accepting chemotaxis protein) and IcpA (internal chemotaxis protein). (ii) The tactic response is mediated by two response regulators, CheY1 and CheY2, but no phosphatase, CheZ. Phosphorylated CheY2 (CheY2-P) is the main regulator of motor function, whereas CheY1 assumes the role of a 'sink' for phosphate that is shuttled from CheY2-P back to CheA. This phospho-transfer from surplus CheY2-P to CheA to CheY1 replaces CheZ phosphatase. (iii) S. meliloti flagella have a complex structure with three helical ribbons that render the filaments rigid and unable to undergo polymorphic transitions from right- to left-handedness. Flagella rotate only clockwise and their motors can increase and decrease rotary speed. Hence, directional changes of a swimming cell occur during slow-down, when several flagella rotate at different speed. Two novel motility proteins, the periplasmic MotC and the cytoplasmic MotD, are essential for motility and rotary speed variation. A model consistent with these data postulates a MotC-mediated gating of the energizing MotA-MotB proton channels leading to variations in flagellar rotary speed.
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Random points:
1) Yet another non-bidirectional flagellum
2) This flagellum has required parts that other flagella don't require (MotC and MotD)
3) Other flagella have required parts (e.g. CheZ) that this flagellum doesn't require
4) I always thought it made more sense to regulate the motor rather than the C-ring (or at least it was easier to imagine); it appears that Sinorhizobium meliloti agrees.
5) Does "rigid flagella" indicate that these guys get away without even hook proteins (the more-flexible proteins at the base of the external rod structure)?
6) Chemotaxis systems in general are a huge mess in terms of IC-interpretation. In some cases similar systems are coupled to wildly different motility systems (and probably even to nonmotility systems; cells have to react in multiple ways including motion), and some motile cells appear to get by without the chemotaxis system at all; see:
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