Site-directed mutants of the D1 protein generated in Chlamydomonas reinhardtii have been characterized to determine whether specific lumenal side histidine residues participate in or directly influence electron transfer. Histidine 195 (H195), a conserved residue located near the amino-terminal end of the D1 transmembrane alpha-helix containing the putative P680 chlorophyll ligand H198, was changed to asparagine (H195N), aspartic acid (H195D), and tyrosine (H195Y). These H195 mutants displayed essentially wild-type rates of electron transfer from the water-oxidizing complex to 2,6-dichlorophenolindophenol. Flash-induced chlorophyll a (Chl a) fluorescence yield rise and decay measurements for Mn-depleted membranes of the H195Y and H195D mutants, however, revealed modified YZ to P680+ electron transfer kinetics. The rate of the variable Chl a fluorescence rise was reduced approximately 10-fold in H195Y and H195D relative to the wild type. In addition, the rate of Chl a fluorescence decay in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea was approximately 50-fold more rapid in H195D than in the wild type. These results can be accommodated by a change in the midpoint potential of YZ+/YZ which is apparent only upon the removal of the Mn cluster. In addition, we have generated a histidine to phenylalanine substitution at histidine 190 (H190), a conserved residue located near the lumenal thylakoid surface of D1 in close proximity to the secondary donor YZ. The H190F mutant is characterized by an inability to oxidize water associated with the loss of the Mn cluster and severely altered donor side kinetics. These and other results suggest that H190 may participate in redox reactions leading to the assembly of the Mn cluster.