The relationship between step-up and step-down photophobic responses in Euglena gracilis

On the basis of studies of step-up and step-down photophobic responses in Euglena gracilis , a new hypothesis concerning the mechanisms and molecular processes involved in photoreception and photosensory transduction is presented. It is postulated that the threshold value for the step-up response is determined by: (1) plasma membrane conduction-resistance properties for phototransduction processes; (2) active chromophore molecules of the a) plasma membrane photosensitive system and b) of the flagellar swelling; and (3) photoprotective chromophores of the stigma. It is further proposed that in the flagellar swelling flavoproteins exist in two distinct signaling forms: "compact" (folded) and "unfolded". After illumination, a fraction of the flavoproteins, excited by photon absorption, undergoes a conformational change to the compact form. The strength of the step-down response is proportional to the conversion rate from the compact forms to the unfolded forms.