Resistance of reaction centers from Rhodobacter sphaeroides against UV-B radiation. Effects on protein structure and electron transport.

Inhibition of electron transport and damage to the protein subunits by ultraviolet-B (UV-B, 280-320 nm) radiation have been studied in isolated reaction centers of the non-sulfur purple bacterium Rhodobacter sphaeroides R26. UV-B irradiation results in the inhibition of charge separation as detected by the loss of the initial amplitude of absorbance change at 430 nm reflecting the formation of the P(+)(QAQB)(-) state. In addition to this effect, the charge recombination accelerates and the damping of the semiquinone oscillation increases in the UV-B irradiated reaction centers. A further effect of UV-B is a 2 fold increase in the half- inhibitory concentration of o-phenanthroline. Some damage to the protein subunits of the RC is also observed as a consequence of UV-B irradiation. This effect is manifested as loss of the L, M and H subunits on Coomassie stained gels, but not accompanied with specific degradation products. The damaging effects of UV-B radiation enhanced in reaction centers where the quinone was semireduced (QB (-)) during UV-B irradiation, but decreased in reaction centers which lacked quinone at the QB binding site. In comparison with Photosystem II of green plant photosynthesis, the bacterial reaction center shows about 40 times lower sensitivity to UV-B radiation concerning the activity loss and 10 times lower sensitivity concerning the extent of reaction center protein damage. It is concluded that the main effect of UV-B radiation in the purple bacterial reaction center occurs at the QAQB quinone acceptor complex by decreasing the binding affinity of QB and shifting the electron equilibration from QAQB (-) to QA (-)QB. The inhibitory effect is likely to be caused by modification of the protein environment around the QB binding pocket and mediated by the semiquinone form of QB. The UV-resistance of the bacterial reaction center compared to Photosystem II indicates that either the QAQB acceptor complex, which is present in both types of reaction centers with similar structure and function, is much less susceptible to UV damage in purple bacteria, or, more likely, that Photosystem II contains UV-B targets which are more sensitive than its quinone complex.