SPECTROSCOPIC CHARACTERIZATION OF QUINONE-SITE MUTANTS OF THE BACTERIAL PHOTOSYNTHETIC REACTION-CENTER

Site-specific mutations in the quinone binding sites of the photosynth etic reaction center (RC) protein complexes of Rhodobacter (R.) capsul atus caused pronounced effects on sequential electron transfer. Conser ved residues that break the twofold symmetry in this region of the RC - M246Ala and M247Ala in the Q(A) binding pocket, and L212Glu and L213 Asp in the Q(B) binding pocket - were targeted. We constructed a Q(B)- site mutant, L212Glu-L213Asp --> Ala-Ala, and a Q(A)-site mutant, M246 Ala-M247Ala --> Glu-Asp, to partially balance the differences in charg e distribution normally found between the two quinone binding sites. I n addition, two photocompetent revertants were isolated from the photo synthetically-incompetent M246Glu-M247Asp mutant: M246Ala-M247Asp and M246Gly-M247Asp. Sequential electron transfer was investigated by cont inuous light excitation and time-resolved electron paramagnetic resona nce (EPR), and time-resolved optical techniques. Several lines of EPR evidence suggested that the forward electron transfer rate to Q(A), k( Q), was slowed in those strains containing altered Q(A) sites. The slo wer rates of secondary electron transfer were confirmed by time-resolv ed optical results with the M246Glu-M247Asp mutations in the Q(A) site resulting in a dramatically lowered secondary electron transfer effic iency [k(Q) < (2 ns)(-1)] in comparison with either the native R. caps ulatus RC or the Q(B) site mutant [k(Q) approximate to (200 ps)(-1)]. Secondary electron transfer in the two revertants was intermediate bet ween that of the native RC and the Q(A) mutant. The P(+)Q(A) --> PQ(A) charge recombination rates were also changed in the strains that carr ied altered Q(A) sites. We show that local mutations in the Q(A) site, presumably through local electrostatic changes, significantly alter b inding and electron transfer properties of Q(A).