Ca(2+) and Mg(2+)-binding and a putative calmodulin type Ca(2+)-binding site in Synechococcus Photosystem II.

Thylakoids and Photosystem II particles prepared from the cyanobacterium Synechococcus PCC 7942 washed with a HEPES/glycerol buffer exhibited low rates of light-induced oxygen evolution. Addition of either Ca(2+) or Mg(2+) to both thylakoids and Photosystem II particles increased oxygen evolution independently, maximal rates being obtained by addition of both ions. If either preparation was washed with NaCl, light induced O2 evolution was completely inhibited, but re-activated in the same manner by Ca(2+) and Mg(2+) but to a lower level. In the presence of Mg(2+), the reactivation of O2 evolution by Ca(2+) allowed sigmoid kinetics, implying co-operative binding. The results are interpreted as indicating that not only Ca(2+), but also Mg(2+), is essential for light-induced oxygen evolution in thylakoids and Photosystem II particles from Synechococcus PC 7942. The significance of the reactivation kinetics is discussed. Reactivation by Ca(2+) was inhibited by antibodies to mammalian calmodulin, indicating that the binding site in Photosystem II may be analogous to that of this protein.