Abstract
We examined the effects of replacement of D1-Glu189 with Gln in Synechocystis PCC 6803 on the properties of photosynthetic oxygen evolution. Isolated photosystem II core particles from E189Q mutant evolved oxygen at 70% rate of control wild-type particles. The mutant cores showed normal S2-state EPR multiline signals. However, the redox potential of mutant Mn-cluster was lower than that of the control one as revealed by thermoluminescence study. Studies by S2/S1 FTIR difference spectroscopy revealed that the mutation resulted in the specific changes in IR bands of carboxylate ligands for Mn-cluster and a few carbonyl bands at mid-frequency (1800-1100 cm-1), and the unique changes in bands ascribed to Mn-cluster core vibrations at low-frequency (650-350 cm-1). These results indicated that the carboxylate side group of Glu189 ligates a Mn ion. Probably, the carbonyl group of Gln alternatively coordinates the Mn ion to maintain the nearly normal function of Mn-cluster in the mutant.
