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

In order to elucidate the primary stage in the blepharismin phototransducti on pathway, changes in the molecular structure of light-exposed blepharismi ns and oxyblepharismins, were examined. When exposed to light, blepharismin s (pink form) were converted into oxyblepharismins (blue form) or dissociat ed into stentorins/p-hydroxybenzaidehyde with an O-2-requiring process, whe reas light-exposed oxyblepharismins were not dissociated into stentorins/p- hydroxy-benzaldehyde. Since both blepharismins and oxyblepharismins can act ivate the phototransduction chain leading to the step-up photophobic respon se presumably through the same pathway, dissociation of p-hydroxybenzaldehy de may not be involved in signal transduction. The fact that the step-up ph otophobic response requires O-2, and both blepharismins and oxyblepharismin s produce hydroxyl (OH) radicals probably derived from O-2 implies that OH radicals may activate the photosignalling pathway. The step-up photophobic response was not suppressed by a spin trapping reagent for hydroxyl radical s. Other possible primary responses leading to the step-up photophobic resp onse are discussed.