The long-range supraorganization of the bacterial photosynthetic unit: A key role for PufX

Bacterial photosynthesis relies on the interplay between light harvesting a nd electron transfer complexes, all of which are located within the intracy toplasmic membrane. These complexes capture and transfer solar energy, whic h is used to generate a proton gradient. In this study, we identify one of the factors that determines the organization of these complexes. We underto ok a comparison of the organization of the light-harvesting complex 1 (LH1) /reaction center (RC) cores in the LH2(-) mutant of Rhodobacter sphaeroides in the presence or absence of the PufX protein. From polarized absorption spectra on oriented membranes, we conclude that PufX induces a specific ori entation of the reaction center in the LH1 ring, as well as the formation o f a long-range regular array of LH1-RC cores in the photosynthetic membrane . From our data, we have constructed a precise model of how the RC is posit ioned within the LH1 ring relative to the long (orientation) axis of the ph otosynthetic membrane.