Mutations that modify or exclude binding of the QA ubiquinone and carotenoid in the reaction center from Rhodobacter sphaeroides

Three single-site mutations have been introduced at positions close to the Q(A) ubiquinone in the reaction centre from Rhodobacter sphaeroides. Two of these mutations, Ala M260 to Trp and Ala M248 to Trp, result in a reaction centre that does not support photosynthetic growth of the bacterium, and i n which electron transfer to the Q(A) ubiquinone is abolished. In the react ion centre with an Afa to Trp mutation at the M260 residue, electron transf er from the primary donor to the acceptor bacteriopheophytin is not affecte d by the mutation, but electron transfer from the acceptor bacteriopheophyt in to Q(A) is not observed. The most Likely basis for these effects is that the mutation produces a structural change that excludes binding of the Q(A ) ubiquinone. A third mutation, Leu M215 to Trp, produces a reaction centre that has an impaired capacity for supporting photosynthetic growth. The mu tation changes the nature of ubiquinone binding at the Q(A) site, and rende rs the site sensitive to quinone site inhibitors such as o-phenanthroline. Adopting a similar approach, in which a small residue located close to a co factor is changed to a more bulky residue, we show that the reaction centre can be rendered carotenoid-less by the mutation Gly M71 to Leu.