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 isola ted reaction centers of the non-sulfur purple bacterium Rhodobacter sp haeroides R26. UV-B irradiation results in the inhibition of charge se paration as detected by the loss of the initial amplitude of absorbanc e change at 430 nm reflecting the formation of the P+(Q(A)Q(B))(-) sta te. In addition to this effect, the charge recombination accelerates a nd the damping of the semiquinone oscillation increases in the UV-B ir radiated reaction centers. A further effect of W-B is a 2 fold increas e in the half- inhibitory concentration of o-phenanthroline. Some dama ge 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 an d H subunits on Coomassie stained gels, but not accompanied with speci fic degradation products. The damaging effects of W-B radiation enhanc ed in reaction centers where the quinone was semireduced (Q(B)(-)) dur ing UV-B irradiation, but decreased in reaction centers which lacked q uinone at the Q(B) 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 l oss and 10 times lower sensitivity concerning the extent of reaction c enter protein damage. It is concluded that the main effect of UV-B rad iation in the purple bacterial reaction center occurs at the Q(A)Q(B) quinone acceptor complex by decreasing the binding affinity of Q(B) an d shifting the electron equilibration from Q(A)Q(B)(-) to Q(A)(-)Q(B). The inhibitory effect is likely to be caused by modification of the p rotein environment around the Q(B) binding pocket and mediated by the semiquinone form of Q(B). The UV-resistance of the bacterial reaction center compared to Photosystem II indicates that either the Q(A)Q(B) a cceptor complex, which is present in both types of reaction centers wi th similar structure and function, is much less susceptible to UV dama ge in purple bacteria, or, more likely, that Photosystem II contains U V-B targets which are more sensitive than its quinone complex.