Euglena: the photoreceptor system for phototaxis.

The photoreceptor structures (eyespot-paraflagellar body-flagellum) for Euglena phototaxis were investigated by electron microscopy. The paraflagellar body--the photoreceptor--is a highly ordered crystalline lamellar structure. Optical diffraction of the electron micrographs and resulting filtered images of the paraflagellar body suggest that it is formed of rods in a helical arrangement. The action spectra for phototaxis, the in situ spectrum by microspectrophotometry of the paraflagellar body, and flavin analysis of the organism indicate that the photoreceptor molecule is a flavoprotein. The phototaxis action spectrum is similar to the spectrum for O2 evolution and implies that similar molecules participate in the photo processes. As a result, a photochemical scheme is suggested in which a photo-excited flavin and a cytochrome participate in the photoprocess. The photochemistry and photoreceptor structures for Euglena phototaxis are likened to a photoneuro sensory cell.