The role of Ca2+ in photosystem 2 of anacystis nidulans

Abstract

It has been previously shown in various studies that Ca 2 have at least two roles in P32, that of regulation (England and
Evans, 1983) and that of stabilisation (Barber, 1987).
It was decided, during this study, to investigate both roles to a greater extent. Through the extraction of P52 from Anacystis
nidulans and its subsequent reactivation with Ca2+ , it was found that 4 Ca2 were required for the complete stimulation of PS2. Through cooperative binding, these 4 Ca 2 join to their site, the affinity of which changes towards Ca2 according to the absence or presence of Mg2t
These observed phenomena are consistent with the theory that the Ca 2k-site in P32 is calmodulin-related. This has already been shown in a previous study by Sparrow and England (1984) who extracted a calmodulin-related protein from PS2 of lettuce. However, a similar protein has never been shown to exist in P52 of cyanobacteria. During the research reported here, this was demonstrated with the use of antibody experiments
Sequential removal of the extrinsic proteins from P32 using the method of Ono and Inoue (1983) and subsequent reactivation of the washed particles by Ca2 indicated that the Ca2 site was located on one of the intrinsic proteins of P32. Furthermore, it was found using difference spectra, that on binding to this site, Ca2+ cause a conformational change in one of the chlorophyll-binding proteins. Both observations indicate that the Ca 2 site is located on one of the Dl or 02 proteins, a conclusion that would agree with McColl (1988).
Secondly, in this research, it was found that M9 stimulates P32 of A.nidulans independently of Ca 2 . This was suggested, initially by the observation that Ca2 in the extraction buffer protects the Mg2 site to a greater extent that the Ca2 site. This indicated that the Ca2 and Mg2 sites are separate and different. Secondly, PS2 particles extracted from A.nidulans grown in differing Ca 2 concentrations changed their affinity towards Ca 2 but not Mg2 suggesting two independent sites for Ca2 and Mg2 .
Finally, it was found during this research that the Ca 2 concentration in the media of A.nidulans physiologically affects the.rate at which the organism photosynthesized. This effect seemed to be controlled by the phycocyanin concentration and was thought to be due to higher Ca2+ concentrations exerting stress on the organism.