In the purple phototrophic bacterium Rhodobacter sphaeroides, light harvesting LH2 complexes transfer absorbed solar energy to RC-LH1-PufX core complexes, which are mainly found in the dimeric state. Many other purple phototrophs have monomeric core complexes and the basis for requiring dimeric cores is not fully established, so we analysed strains of Rba. sphaeroides that contain either native dimeric core complexes or altered monomeric cores harbouring a deletion of the first 12 residues from the N-terminus of PufX, which retains the PufX polypeptide but removes the major determinant of core complex dimerization. Membranes were purified from strains with dimeric or monomeric cores, and with either high or low levels of the LH2 complex. Samples were interrogated with absorption, steady-state fluorescence, and picosecond time-resolved fluorescence kinetic spectroscopies to reveal their light-harvesting and energy trapping properties. We find that under saturating excitation light intensity the photosynthetic membranes containing LH2 and monomeric core complexes have fluorescence lifetimes nearly twice that of membranes with LH2 plus dimeric core complexes. This trend of increased lifetime is maintained with RCs in the open state as well, and for two different levels of LH2 content. Thus, energy trapping is more efficient when photosynthetic membranes of Rba. sphaeroides consist of RC-LH1-PufX dimers and LH2 complexes.
Bibliographical noteFunding Information:
This work was supported by the Estonian Research Council (grant IUT02-28 ), the ESF DoRa 4 program (grant NLOFY12523T ), and the H2020-MSCA-RISE-2015 program (grant 690853 ). CNH gratefully acknowledges financial support from the Biotechnology and Biological Sciences Research Council (BBSRC UK) , award number BB/G021546/1 . CNH was also supported by an Advanced Award 338895 from the European Research Council and as part of the Photosynthetic Antenna Research Center (PARC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC 0001035 . PGA was supported by a doctoral studentship and an Anniversary Future Leader Fellowship from the Biotechnology and Biological Sciences Research Council (UK) . We are grateful to Girinath G. Pillai for generating the graphical representation of RC–LH1–PufX dimer complex from Rba. sphaeroides.
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- Light harvesting
- Photosynthetic excitons