EVIDENCE THAT THE F-X DOMAIN IN PHOTOSYSTEM-I INTERACTS WITH THE SUBUNIT PSAC - SITE-DIRECTED CHANGES IN PSAB DESTABILIZE THE SUBUNIT INTERACTION IN CHLAMYDOMONAS-REINHARDTII

The highly conserved amino acid sequence PCDGPGRGGTC in both photosyst em I reaction center core proteins PsaA and PsaB has been predicted to contribute the four cysteine ligands for coordination of the 4Fe-4S i ron-sulfur cluster F-X, and we have proposed a working model for the b inding of PsaC to this domain of the reaction center core heterodimer [Rodday et al. (1993) Photosynth. Res. 36, 1-9]. We have investigated structure-function relationships between this domain and the PsaC subu nit by site-directed mutagenesis of the conserved prolines P560 and P5 64, and the charged residues D562 and R566 in the eucaryotic alga Chla mydomonas reinhardtii. The D562N and R566E mutants did not accumulate the PsaA and PsaB reaction center proteins, indicating that these resi dues are essential for the stable assembly of photosystem I. The P560A , P560L, and P564L mutants accumulated functional reaction centers but showed an impaired interaction between the reaction center core compl ex and the PsaC subunit. We observed that the reaction centers of the proline mutants dissociated more readily in urea, and reconstitution o f the mutant core preparations using PsaC and Fe-S cluster insertion p rotocols in vitro were incomplete. We suggest that P560 and D562 contr ibute to the stability of the F-X cluster, most likely by providing es sential hydrogen bonding to the C561 ligand. The data obtained from th e P564 and R566 replacements provide direct evidence that the intercys teinyl region in PsaB is a domain involved in the interaction between PsaC and the reaction center core.