A requirement for Ca2+ in the extraction of O2-evolving Photosystem 2 preparations from the cyanobacterium Anacystis nidulans.

Ca2+ has been shown to be essential for the retention of maximal O2-evolving activity in Photosystem 2 particles extracted by using dodecyldimethylamine oxide from Anacystis nidulans thylakoids. The effect cannot entirely be mimicked by using Mg2+. Ca2+ stimulates electron transport from diphenylcarbazide to 2,6-dichloroindophenol catalysed by lead-inhibited cation-free preparations, showing the presence of two cation-binding sites in these particles. Photosystem 2 preparations extracted in Ca2+-containing buffer show the presence of three polypeptides at mol. wt. 30000, 33000 and 36000, which are absent or much decreased in preparations extracted in Mg2+-containing buffer. The calmodulin antagonist chlorpromazine inhibits activity of the Photosystem 2 preparation, suggesting the presence of a Ca2+-binding protein.