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Gravitaxis in Euglena.

Donat-P Häder1, Ruth Hemmersbach2

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Summary

Euglena exhibits active gravitaxis, using mechano-sensitive calcium channels as gravireceptors. This process involves a signal transduction chain leading to flagellar reorientation for optimal positioning in water.

Keywords:
Adenylyl cyclaseCalmodulinEuglenaGravireceptorGravitaxisHypergravityMicrogravityProtein kinaseSensory transductionSpace flightcAMP

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Area of Science:

  • Microbiology
  • Cell Biology
  • Biophysics

Background:

  • Motile microorganisms respond to various environmental stimuli, including gravity, to determine niche selection.
  • In Euglena, gravitaxis precision is modulated by an internal rhythm synchronized with the light-dark cycle, cooperating with phototaxis for optimal positioning.
  • Previous theories of passive orientation in Euglena have been superseded by evidence of a physiological gravireceptor and active orientation.

Purpose of the Study:

  • To investigate the mechanisms underlying gravitaxis in Euglena.
  • To identify the physiological, biochemical, and molecular components of the gravitaxis signal transduction pathway.
  • To characterize the gravireceptors and their role in cellular reorientation.

Main Methods:

  • Experiments were conducted in space (satellites, rockets, shuttles) and simulated microgravity (clinostats).
  • Determined the threshold for gravity perception in Euglena.
  • Utilized inhibitors and RNA interference (RNAi) to identify components of the signal transduction chain.

Main Results:

  • Euglena possesses active gravitaxis, lacking sedimenting statoliths found in other organisms.
  • Gravireceptors identified as mechano-sensitive Ca2+-conducting ion channels at the cell's anterior.
  • A signal transduction pathway involving calcium influx, calmodulin (CaM.2), adenylyl cyclase, and protein kinase A (PK.4) was elucidated, leading to flagellar reorientation.

Conclusions:

  • Euglena actively senses and responds to gravity via a novel gravireceptor system.
  • The identified signal transduction pathway provides a molecular basis for gravitaxis in Euglena.
  • This research clarifies the active orientation mechanism in Euglena, distinct from passive models.