  Some species of purple bacteria as, e.g., Rhodobacter sphaeroides contain the
protein PufX. Concurrently, the light harvesting complexes 1 (LH1) form dimers
of open rings. In mutants without PufX, the LH1s are closed rings and
photosynthesis breaks down, because the ubiquinone exchange at the reaction
center is blocked. Thus, PufX is regarded essential for quinone exchange.
  In contrast to this view, which implicitly treats the LH1s as obstacles to
photosynthesis, we propose that the primary purpose of PufX is to improve the
efficiency of light harvesting by inducing the LH1 dimerization. Calculations
with a dipole model, which compare the photosynthetic efficiency of various
configurations of monomeric and dimeric core complexes, show that the dimer can
absorb photons directly into the RC about 30% more efficient, when related to
the number of bacteriochlorophylls, but that the performance of the more
sophisticated dimeric LH1 antenna degrades faster with structural
perturbations. The calculations predict an optimal orientation of the reaction
centers relative to the LH1 dimer, which agrees well with the experimentally
found configuration.
  For the increased required rigidity of the dimer additional modifications of
the LH1 subunits are necessary, which would lead to the observed ubiquinone
blockage, when PufX is missing.
