Localization of light-induced structural changes in bacterial photosynthetic reaction centers

Abstract

Light-induced structural changes in photosynthetic reaction centers from Rhodobacter sphaeroides were investigated using two approaches. Cu2+ was used as a paramagnetic structural probe. The EPR spectrum of Cu2+ incorporated into the metal-depleted reaction centers was affected by 1,10-phenanthroline, an electron transfer inhibitor substituting QB, which suggests a localization of Cu2+ in a vicinity of the QB− site. However, the spectrum was not influenced by low temperature (77 K) illumination of the sample which suggests that the copper ion position is not exactly the same as that of the iron ion. Freezing the reaction centers under illumination in the presence of potassium ferricyanide and 1,10-phenanthroline caused a change in the shape of the Cu2+ EPR spectrum in comparison to that of a sample frozen in darkness. These data indicate a change of the Cu2+ ligand symmetry owing to light-induced structural changes which are probably located near the acceptor side of the reaction center. Partial trypsinolysis of reaction centers was also used to locate the structural changes. Trypsin treatment in the dark and under illumination resulted in different peptide patterns as detected by gel electrophoresis and reverse-phase high-performance liquid chromatography. Partial amino-acid sequence analysis of a number of peptides, characteristic of either light- or dark-treated reaction centers, showed that they originated from the acceptor sides of the H and M subunits. The occurrence of light-induced structural differences in the H-subunit is consistent with the suggestion that it may be involved in regulating electron transfer in this part of the reaction center.