Site-directed mutations were introduced into PsbO protein of photosystem 2 to study the role of two lysine residues, 223 and 226 (LGAKPPK), in the green alga Chlamydomonas reinhardtii. Lysines 223 and 226 homologous to His228 and His231 from cyanobacteria are located on the protein side facing the lumen and can participate in formation of a channel connecting the Mn cluster with the intrathylakoid space. The K223E and K226E mutants were generated on the basis of the ΔpsbO strain of C. reinhardtii with the substitution of glutamic acid for the lysine residues. The K226E mutation leads to a decrease in stability of the protein and development of the ΔpsbO phenotype (the absence of both photosynthetic activity of photosystem 2 and photoautotrophic growth), with substantially decreased PsbO content in the cells. In the case of K223E, the mutant strain accumulated the normal level of PsbO protein and was able to grow photoautotrophically and to evolve oxygen. However, the rate of oxygen evolution and the F(v)/F(m) ratio were reduced by 15-20% compared to the control. Also, the time of the dark decay of F(v) in the presence of DCMU in the cells of the K223E mutant was increased, indicating impairment in the water-oxidizing complex. In general, our study shows the importance of amino acids K223 and K226 located at the lumenal surface of PsbO protein for the activity of the water-oxidizing complex.