Primary stages in photosignal transduction leading to step-up photophobic response in the unicellular eukaryote Blepharisma japonicum

ln order to elucidate the primary stage in the blepharismin phototransduction pathway, changes in the molecular structure of light-exposed blepharismins and oxyblepharismins, were examined. When exposed to light, blepharismins (pink e form) were converted into oxyblepharismins (blue form) or dissociated into stentorins/p-hydroxybenzaldehyde with an O2-requiring process, whereas light-exposed oxyblepharismins (were not dissociated into stentorins/phydroxybenzaldehyde. Since both blepharismins and oxyblepharismins can activate the phototransduction cha in leading to the step-up photophobic response presumably through the same pathway, dissociation of p--hydroxybenzaldehyde mayénot be involved in signal transduction. The fact that thestep-up photophobic response requiresO2, and both blepharismins and oxyblepharismins produce hydroxyl (OH) radicals probably derived from, O2 implies that OH radicals may activate the photosignalling pathway. The stepeup photophobic response was not suppressed by a spin trappi ng reagent for hydroxyl radicals. Other possible primary responses leading to the step-up photophobic response are discussed.