Nucleotide sequence of the psaE gene of cyanobacterium Synechococcus sp. PCC 6301.

The cyanobacterial PSI complex is a membrane-bound, photooxidoreductase that catalyzes the light-driven transport of electrons from reduced plastocyanin (or Cyt cSs3) to oxidized, soluble Fd or flavodoxin (for a review, see Golbeck and Bryant, 1991). The complex probably comprises single copies of 11 or 12 different polypeptides, about 100 Chl a molecules, 10 to 15 p-carotene molecules, 2 molecules of phylloquinone (vitamin K1), and 3 4Fe-4S centers denoted Fi, FA, and FB. It is presently believed that a11 of these cofactors except for the FA and FB centers, which are bound to a membrane-extrinsic, 9-kD protein denoted PsaC (Oh-oka et al., 1988), are associated with a heterodimer of the PsaA and PsaB subunits. The PSI complex of the cyanobacterium Synechococcus sp. PCC 6301 has been extensively characterized by Golbeck and coworkers, who have shown that it is possible to remove the extrinsic PsaC, PsaD, and PsaE proteins from the Synechococcus sp. PCC 6301 PSI complex by treatment with chaotropic agents such as 6.8 M urea to produce a so-called "PSI core protein" (Parrett et al., 1990; Li et al., 1991a). After the chaotropic agent is removed and under conditions in which the Fe-S centers can be reinserted into the denatured PsaC protein, the same three proteins, identified by their Nterminal amino acid sequences, can be quantitatively rebound to the PSI complex with accompanying restoration of electron transport to the terminal acceptor FA and FB (Li et al., 1991a). Findings from cross-linking (Oh-oka et al., 1989) and EM studies (Bottcher et al., 1992) suggest that these three proteins form a protruding cap on the stromal surface of the PSI complex. The PsaD protein can be cross-linked to Fd, suggesting that one role of this protein is to dock this soluble electron acceptor onto the stromal surface of PSI (Golbeck and Bryant, 1991). Reconstitution studies with recombinant PsaC and PsaD and the PSI core protein have shown that the PsaD protein is also required for proper and stable association of PsaC with the PSI core protein (Li et al., 1991b). Chitnis et al. (1989) reported the isolation of the psaE gene of Synechocystis sp. PCC 6803 and the construction of an interposon mutant. The mutation did not affect photoauto-