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 phototransduction pathway, changes in the molecular structure of light-exposed blepharismins and oxyblepharismins, were examined. When exposed to light, blepharismins (pink 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/p-hydroxybenzaldehyde. Since both blepharismins and oxyblepharismins can activate the phototransduction chain leading to the step-up photophobic response presumably through the same pathway, dissociation of p-hydroxybenzaldehyde may not be involved in signal transduction. The fact that the step-up photophobic response requires O2, and both blepharismins and oxyblepharismins produce hydroxyl (OH) radicals probably derived from O2 implies that OH radicals may activate the photosignalling pathway. The step-up photophobic response was not suppressed by a spin trapping reagent for hydroxyl radicals. Other possible primary responses leading to the step-up photophobic response are discussed.