CHARACTERIZATION OF BACTERIAL REACTION CENTERS HAVING MUTATIONS OF AROMATIC RESIDUES IN THE BINDING-SITE OF THE BACTERIOPHEOPHYTIN INTERMEDIARY ELECTRON CARRIER

We report the initial characterization of a series of reaction centers (RCs) from the photosynthetic bacterium Rhodobacter capsulatus having single or double mutations of phenylalanines 97 and 121 on the L poly peptide. Substitution of these aromatic amino acids, which may interac t with the photoactive bacteriopheophytin associated with the L polype ptide (BPh(L)), was carried out to examine their possible roles in ele ctron transfer, charge stabilization, and/or BPh(L) binding. In some m utant RCs, the wild-type pigment content is obtained while in certain others a bacteriochlorophyll (BChl) replaces BPh(L). The mutant RCs wi th wild-type pigment content are found to have overall photochemistry effectively identical to that of wild-type RCs. This indicates that ar omatic residues at L97 and L121 are not critical factors in the charge separation process, although an approximate 2-fold increase in the ra te of electron transfer from BPh(L)(-) to QA is observed in two mutant s where residue L121 is leucine. In two double mutants where L121 is h istidine and L97 is either valine or cysteine, BPh(L) is replaced with a BChl (denoted beta). This pigment content is surprising since in th e native RC structure amino acid L121 is not in optimum geometry for c oordination to the Mg in the center of the pigment macrocycle. Charge separation takes place in the beta-containing mutants with an similar to 70% yield of P(+)Q(A)(-) at 285 K compared to similar to 100% for w ild-type. The photochemistry of these new beta-type RCs is very simila r to that reported previously for the beta RC from Rhodobacter sphaero ides wherein the same pigment change was induced by a mutation in the M polypeptide.