Euglena: the photoreceptor system for phototaxis

Area of Science:

• Cell Biology
• Biophysics
• Photobiology

Background:

• Euglena exhibits phototaxis, the ability to move towards or away from light.
• The precise photoreceptor structures and molecular mechanisms underlying Euglena phototaxis have been a subject of investigation.
• Understanding these mechanisms can provide insights into sensory biology and light-responsive systems in microorganisms.

Purpose of the Study:

• To investigate the ultrastructure of the photoreceptor complex in Euglena responsible for phototaxis.
• To identify the photoreceptor molecule and elucidate the photochemical processes involved in light detection.
• To compare the Euglena phototaxis system with known sensory mechanisms.

Main Methods:

• Electron microscopy was employed to examine the photoreceptor structures (eyespot, paraflagellar body, flagellum).
• Optical diffraction and image filtering were used to analyze the paraflagellar body's structure.
• Microspectrophotometry and biochemical analysis (flavin analysis) were performed to identify the photoreceptor molecule and its spectral properties.

Main Results:

• The paraflagellar body was identified as the photoreceptor, possessing a highly ordered crystalline lamellar structure.
• Optical diffraction suggested the paraflagellar body is composed of rods arranged helically.
• Action spectra and in situ measurements indicated that a flavoprotein is the photoreceptor molecule, with a photochemical scheme involving photo-excited flavin and cytochrome proposed.

Conclusions:

• The photoreceptor structure and photochemistry in Euglena phototaxis are consistent with a flavoprotein acting as the primary photoreceptor.
• The proposed photochemical model involves flavin and cytochrome, suggesting a sophisticated light-sensing mechanism.
• The Euglena phototaxis system shares similarities with photoneuro sensory cells, highlighting convergent evolution in sensory biology.