Abstract
It has been generally accepted that PsbU in cyanobacterial photosystem II (PS II) is a functional equivalent of PsbP or PsbQ in plant chloroplasts. After the recent discovery of PsbP and PsbQ homologues in cyanobacterial PS II [Thornton et al. (2004) Plant Cell 16, 2164], we investigated the function of PsbU using a psbU-deletion mutant (ΔPsbU) of Synechocystis 6803 focusing on the functional relation to Ca2+ and Cl-. The growth rate was highly suppressed when Cl- was eliminated, while ΔPsbU grew well when only Ca2+ was eliminated. Remarkable suppression in the oxygen-evolving activity in ΔPsbU thylakoids in the absence of Cl-, which was restored by 60% with NO3- and not by SO42-. Based on our results, we concluded that PsbU stabilizes only Ca2+, and not Cl-, through its structural nature. This work was supported by funding from NSF (MCB0215359, HBP), and from Hyogo Prefecture (YK), COE (IT and YK).
