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The photoreceptor for phototaxis in the photosynthetic flagellate Euglena gracilis Donat-P. Häder and Michael Lebert Institut für Botanik und Pharmazeutische Biologie, Friedrich-Alexander-Universität, Staudtstr. 5, D-91058 Erlangen, Germany Correspondence to: Prof. Dr. Donat-P. Häder, Institut für Botanik und Pharmazeutische Biologie, Friedrich-Alexander Universität, Staudtstraße 5, D-91058 Erlangen, Fed. Rep. Germany. Tel.: 49 9131 858216, Telefax: 49 9131 858215 Summary The unicellular flagellate Euglena gracilis shows positive phototaxis at low fluence rates (£ 10 W m-2) and negative phototaxis at high fluence rates (³ 100 W m-2). When grown in the presence of 4 mM nicotine for several generations, the cells still showed both responses, indicating that rhodopsin is unlikely be the photoreceptor. Fluorescence emission 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 pterin to the flavin. This energy transfer is disrupted by solubilization of the PABs. In intact PABs 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 native flavin can be substituted by growing the cells in roseoflavin dissolved in the medium. Roseoflavin is incorporated into the cells, and its absorption spectrum 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. The white mutant strain 1224-5/1f, induced by streptomycin treatment, lacks flavins as indicated by fluorescence spectroscopy. It does not show positive or negative phototaxis but diaphototaxis. The switch between positive and negative phototaxis seems to be controlled by the electric membrane potential, since application of the lipophilic cation TPMP+ causes positive phototaxis to change to negative but not the inverse. Key words: Euglena gracilis, flagellate, flavin, mutant, nicotine, paraxonemal body (PAB), phototaxis, pterin, roseoflavin, stigma, TPMP+