Figure 1.

(A) Model of the core complex in which each RC (Rasmol structure of the Rb. sphaeroides RC, Protein Data Bank (RCSB) accession no. 1aij) is surrounded by a closed ring of LH1 (pink) interrupted by PufX (black) and interacting with the RC at the Q_B site (orange space-fill tail of Q_B). The bc1 complex (gray) is elsewhere in the membrane. In the Rasmol presentation, the RC is viewed from the extracellular side of the membrane, the H-polypeptide is yellow, the L-polypeptide is cyan, the M-polypeptide is green, bacteriochlorophyll are blue, bacteriopheophytin are magenta, and Q_A and Q_B are orange. (B) Model of an intimate RC dimer (light blue) in which PufX and another possible component (red) link LH1 with the RC and cytochrome b (28, 29). Other color assignments are as for A. (C) Model of an RC-cytochrome b-RC complex in which two RC are intimately associated with a bc1 complex linked by PufX (23). Colors are as in A. (D) Model of a core complex-cytochrome b-core complex in which the linkage to cytochrome b is through PufX (22). Colors are as in A.