l h e wild-type, PsaD-less, and PsaL-less strains of the cyanobacterium Synechocystis sp. PCC 6803 were used to study subunit interactions in photosystem I (PSI). When the membranes of a PsaD-less strain were solubilized with Triton X-100 and PSI was purified using ion-exchange chromatography and sucrose-gradient ultracentrifugation, the PsaL subunit was substantially removed from the core of PSI, whereas other subunits, such as PsaE and PsaF, were quantitatively retained during purification. When the wild-type PSI was exposed to increasing concentrations of Nal, the PsaE, PsaD, and PsaC subunits were gradually removed, whereas PsaF, PsaL, PsaK, and PsaJ resisted removal by up to 3 M Nal. PSI in cyanobacteria and chloroplasts is a multisubunit membrane-protein complex that catalyzes electron transfer from reduced plastocyanin (or Cyt c6) to oxidized Fd (or flavodoxin) (Chitnis and Nelson, 1991; Bryant, 1992; Golbeck, 1993). The PsaA and PsaB subunits of PSI form a heterodimeric core that harbors approximately 100 antenna Chl a molecules, the primary electron donor, P700, and a chain of electron acceptors, Ao, A,, and Fx. The PsaC subunit binds the terminal electron acceptors, FA and FB, each a [4Fe-451 cluster. PsaD provides an essential Fd-docking site on the reducing side of PSI (Zanetti and Merati, 1987;Wynn et al., 1989;Xu et al., 1994a) and is also required for in vitro assembly of PsaC and PsaE into the PSI complex (Li et al., 1991b; Chitnis and Nelson, 1992). PsaE may be involved in Fd reduction (Sonoike et al., 1993;Strotmann and Weber,