Direct Energy Transfer from the Peripheral LH2 Antenna to the Reaction Center in a Mutant of Rhodobacter sphaeroides That Lacks the Core LH1 Antenna

The light-harvesting apparatus of the photosynthetic bacterium Rhodobacter sphaeroides is composed of a peripheral LH2 complex which directs excitation energy to the LH1/reaction center core. The puf BA genes encoding the LH1 polypeptides have been deleted, producing a photosynthetically-competent strain which contains LH2 and reaction centers. Time-resolved absorption and fluorescence measurements demonstrate that energy is efficiently transferred from LH2 to the reaction center, despite the absence of LH1. Energy trapping takes place in 55 ± 5 ps at room temperature, compared to the result for the wild-type strain of 60 ± 5 ps. At 77 K, the results for the mutant and wild type are 75 ± 5 and approximately 35 ps, respectively; the slower time in the mutant is attributed to the small differences in antenna/reaction center contacts and relative distances that are bound to exist as a consequence of LH1 and LH2 being assembled from different α-and β-polypeptides. Measurements with closed reaction centers provided new information on the nature of fast energy transfer within the B850 pigments of LH2. We conclude that the absorption band is inhomogeneously broadened, and the fast (∼ 10 ps) lifetime observed in the 847–857-nm region is interpreted as very rapid (1–5 ps) hopping of the excitation energy from high-energy to low-energy pigments within the B850 absorption band. Time-resolved anisotropy studies demonstrate that energy-transfer events within B850 occur on a subpicosecond to picosecond time scale. In respect of internal energy transfer, LH2 therefore behaves similarly to other light-harvesting antennas, and in a wider sense, LH2 can substitute efficiently for LH1 in delivering excitations to the reaction center. This implies some similarity in the three-dimensional arrangement of LH1 and LH2, and notwithstanding this a relatively nonspecific protein-protein interaction between the antenna and the reaction center.