DISCOVERY OF SIGNALING EFFECT OF UV-B C LIGHT IN THE EXTENDED UV-A/BLUE-TYPE ACTION SPECTRA FOR STEP-DOWN AND STEP-UP PHOTOPHOBIC RESPONSESIN THE UNICELLULAR FLAGELLATE ALGA EUGLENA-GRACILIS/

Cultures of unicellular algal flagellate Euglena gracilis grown in dif ferent conditions were subjected to action spectroscopy for step-down and step-up photophobic responses, respectively. The spectral region w as extended into the UV-BIC as well as in the UVA and visible regions with the Okazaki Large Spectrograph as the monochromatic light source. The photophobic responses of the cells were measured with an individu al-cell assay method with the aid of a computerized video motion analy zer. In the W-A and visible regions, the shapes of the action spectra were the so-called UV-A/blue type. In the newly studied UV-BIC region, new action peaks were found at 270 nm for the step-down response and at 280 nm for the step-up one. The absorption spectrum of flavin adeni ne dinucleotide (FAD) appeared to fit the action spectrum for the step -up response, whereas the shape of the step-down action spectrum, whic h has a UV-A peak (at 370 nm) higher than the blue peak (at 450 nm), a ppeared to be mimicked by the absorption spectrum of a mixed solution of 6-biopterin and FAD. These observations might also account for the fact that the UV-B/C peak wavelength at 270 nm of the action spectrum for the step-down response is shorter by 10 nm than the action spectru m for the step-up response at 280 nm.