Oxygen evolution loss and structural transitions in photosystem II induced by low intensity UV-B radiation of 280 nm wavelength.

UV-B radiation of 280 nm wavelength (UV280) and low intensity (2.0 W/m2) gives rise to an important oxygen evolution (OE) loss in photosystem II (PSII) particles isolated from the thylakoid membrane of plant chloroplasts on the one hand, and to structural changes, or transitions, in the proteins of the PSII complex on the other hand. The latter UV280 effect was studied in this work by Fourier transform infrared (FT-IR) spectroscopy. First, irradiation of the PSII particles with UV280 for about 40 min causes an almost complete loss of OE activity. The remaining OE after 15, 20, 30 and 40 min is respectively 52, 44, 27 and 12% of the OE activity in control PSH particles kept in darkness. Secondly, difference FT-IR spectra of PSII particles irradiated for 30 min, i.e., [PSII irradiated with UV280]-minus-[PSII non-irradiated], show that the UV280 light is at the origin of significant IR absorbance changes in several spectral regions: (i) amide I (1696-1620 cm(-1)) and amide II (1580-1520 cm(-1)), (ii) tyrosine side chain (1620-1580 cm(-1) and 1520-1500 cm(-1), i.e., the v8a, v8b and v19a vibrational modes, respectively), and (iii) chlorophylls (1750-1696 cm(-1)). Thirdly, comparison of the UV-B effect reported here with structural changes induced by heat-stress in PSII proteins [M. Joshi, M. Fragata, Z. Naturforsch. 54c (1999) 35-43] clearly indicates that the stability of the functional centers in the PSII complex is dependent on a dynamic equilibrium between a-helix conformers and extended chain (beta-strand) structures. In this framework, transient 'alpha-helix-to-beta-strand transitions' are susceptible of giving rise in vivo to recurrent changes in the activity of the PSII complex, and as such act as a control mechanism of the photosynthetic function in the thylakoid membrane.