On the effects of PufX on the absorption properties of the light-harvesting complexes of Rhodobacter sphaeroides.

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. However, the main purpose of the LH1 is light harvesting. We therefore investigate the effects that the PufX-induced dimerization has on the absorption properties of the core complexes. 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 reaction centers more efficiently, 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. Based on experimental observations indicating that the dimeric core complexes are indeed rather rigid, we hypothesize that in PufX(+) species the association between the LH1 and the reaction centers is enhanced. This mechanical stabilization of the core complexes would lead to the observed quinone blockage, when PufX is missing.