The Photoreceptor for Phototaxis in the Photosynthetic Flagellate Euglena gracilis

The unicellular flagellate Euglena gracilis shows positive phototaxis at low fluence rates (≤10 W m 2) and negative phototaxis at high fluence rates (≥100 W m 2). Currently, retinal or flavins/pterins are discussed as chromo‐phores of the photoreceptor. When grown in the presence of 4 mM nicotine, a retinal inhibitor, for several generations, the cells still showed both responses, indicating that retinal is unlikely to be the chromophoric group of the photoreceptor responsible for phototaxis. The native flavin(s) can be substituted by growing the cells in roseo‐flavin dissolved in the medium. The absorption spectrum of roseoflavin extends well beyond the action spectrum for phototaxis (up to 600 nm). Excitation at wavelengths >550 nm does not cause phototactic orientation in control cells but causes both positive and negative phototaxis in roseoflavin‐grown cells, indicating an uptake and assembly of the chromophore in the photoreceptor complex. The white mutant strain 1224‐5/1f, induced by streptomycin treatment, lacks flavins as indicated by fluorescence spectroscopy. The phototaxis‐deficient pheno‐type cannot be complemented by the addition of external riboflavin. Fluorescence spectra of intact paraxonemal bodies (PAB) indicate that both pterins and flavins are involved in photoperception and that the excitation energy is efficiently funneled from the pterins to the flavins. This energy transfer is disrupted by solubilization of the PAB. In intact PAB flavins are not accessible to reducing or oxidizing substances, indicating that they are located inside the structure, while pterins are accessible, so that their localization can be assumed to be on the surface. The results described above are discussed with regard to the potential involvement of flavins and pterins as well as retinal in photoperception.