Photoaffinity labeling of the azidoatrazine receptor site in reaction centers of Rhodopseudomonas sphaeroides

Photoaffinity labeling of photosynthetic reaction centers of Rhodopseudomonas sphaeroides by the herbicide inhibitor, azido[ 14 C]atrazine, occurs principally on the L‐subunit. The specificity of labeling is greater at 77 than at 295 K. Kinetic studies of charge recombination in reaction centers indicate competition between azidoatrazine and ubiquinone‐1 (Q‐1) for binding to the reaction center. This competition occurs through the L‐subunit binding site, as increasing concentrations of Q‐1 decrease azido[ 14 C]atrazine labeling of this site. It is proposed that the inhibitor binding site, predominantly on the L‐subunit, and the secondary quinone binding site on the M‐subunit, are adjacent so that there is partial overlap by one molecule of the domain occupied by the other.