RHODOPSIN - A PHOTOPIGMENT FOR PHOTOTAXIS IN EUGLENA-GRACILIS

For over 100 years, a major focus of photobiological studies has been the unicellular flagellate, Euglena gracilis, an organism well suited for such investigations by its special complement of organelles that m ay be considered an ancient, yet complete ''visual'' system. The possi ble photoreceptive roles of the cytoplasmic stigma and the photorecept or (paraflagellar swelling) of E. gracilis are still under debate, bec ause of conflicting interpretations of the results produced so far by the different research groups working on this microorganism. This arti cle deals with our hypothesis, first put forward in the late 1980s, th at rhodopsin-like proteins are responsible for photo-detection and tha t the paraxial rod is involved in the control of flagellar movements. This hypothesis uses oriented dipole and electroconformational couplin g mechanisms as the physical phenomena that produce signal transductio n. A model for phototaxis is presented.