ANALYSIS OF THE PROPOSED FE-S-X BINDING REGION OF PHOTOSYSTEM-1 BY SITE-DIRECTED MUTATION OF PSAA IN CHLAMYDOMONAS-REINHARDTII

The psaA and psaB genes of the chloroplast genome in oxygenic photosyn thetic organisms code for the major peptides of the Photosystem 1 reac tion center. A heterodimer of the two polypeptides PsaA and PsaB is th ought to bind the reaction center chlorophyll, P700, and the early ele ctron accepters A(0), A(1) and Fe-S-X. Fe-S-X is a 4Fe4S center requir ing 4 cysteine residues as ligands from the protein. As PsaA and PsaB have only three and two conserved cysteine residues respectively, it h as been proposed by several groups that Fe-S-X is an unusual inter-pep tide center liganded by two cysteines from each peptide. This hypothes is has been tested by site directed mutagenesis of PsaA residue C575 a nd the adjacent D576. The C575D mutant does not assemble Photosystem 1 . The C575H mutant contains a photoxidisable chlorophyll with EPR prop erties of P700, but no other Photosystem 1 function has been detected. The D576L mutant assembles a modified Photosystem 1 in which the EPR properties of the Fe-S-A/B centers are altered. The results confirm th e importance of the conserved cysteine motif region in Photosystem 1 s tructure.