Molecular mechanisms governing chemotaxis and motility in the soil bacterium Sinorhizobium meliloti and related species differ from the well-studied taxis systems of enterobacteria by new features.
In addition to seven transmembrane receptors, S.meliloti has two cyto-plasmic receptor proteins, McpY and IcpA, that probably monitor the metabolic state of the
The tactic response is mediated by two response regulators, CheY1 and
CheY2, but no phosphatase, CheZ. The
phosphorylated CheY2 (Che2-P)) is the dominant regulator of flagellar
motor rotation by binding to the C-ring (FliM, whereas CheY1 assumes the
role of a ‘sink’ for phosphate that is retro-transferred from CheY-P
to the cognate kinase, CheA. This
phosphate shuttle from surplus CheY2-P to CheA to CheY1 is a new
reaction that replaces the CheZ phosphatase.
S.meliloti flagella have a complex structure with helical ribbons that
render the filaments rigid and unable to undergo polymorphic transitions
from right- to lefthandedness. The flagella rotate only clockwise and
their motor can increase and decrease rotary speed. Hence, swimming
cells change their direction during slow-down, when one or more flagella
rotate at different speeds and the flagellar bundle disintegrates. We
have shown that certain charged amino acid residues at the stator (MotA)
– rotor (FliG) interface are directly involved in the modulation of
rotary motor speed.
A working model of the rotary flagellar motor has
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