On the role of the light-harvesting B880 in the correct insertion of the reaction center of Rhodobacter capsulatus and Rhodobacter sphaeroides.

The purple, non-sulfur photosynthetic bacteria Rhodobacter capsulatus and Rhodobacter sphaeroides have two types of pigment-protein complexes that absorb incident light and funnel it to the photochemical reaction center. One of these, B880, is present at an essentially constant ratio to the reaction center, while the abundance of the other, B800-850, varies with growth conditions. Independent work in our two laboratories has indicated that while the absence of B800-850 permits photosynthetic growth in both organisms, the lack of B880 produces a different phenotype in the two species. Thus R. sphaeroides is still photosynthetically competent when it lacks this complex, while R. capsulatus is not. This unanticipated difference in what appear to be very closely related organisms has caused us to reexamine the properties of the two mutants simultaneously, under identical conditions. We find that the original descriptions are indeed correct; the reaction center of R. capsulatus is not correctly inserted into the photosynthetic membrane in the absence of B880, while that of R. sphaeroides is.