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

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.

Enrichment of a 50-kilodalton polypeptide in a photosystem II-phycobilisome particle from Porphyridium cruentum

A 50-kDa polypeptide was obtained from photosynthetically active phycobilisome-photosystem II preparations from the red alga Porphyridium cruentum after removal of phycobiliproteins. Removal of phycobiliproteins caused destabilization of the structure of the phycobilisome-photosystem II preparations and was accompanied by a decline in photosystem II activity (oxygen-evolution and dichlorophenol-indophenol (DPIP) reduction). The treatments in increasing relative effectiveness were: addition of EDTA (10 mM), lowering the pH (6.8----4.4), and lowering the ionic strength (to ca. 1 mM phosphate). The lowering of the ionic strength by dialysis resulted in a preparation highly enriched in a 50-kDa polypeptide (apparent molecular mass on SDS-PAGE). This preparation retained photosystem II activity as evidenced by the photoreduction of DPIP in the presence of diphenylcarbazide (222 mumol DPIP/mg chlorophyll/h). Also it had a 698-nm (77K) fluorescence emission maximum, as compared to a 668-nm emission in the unfractionated preparation, which indicates enrichment of the photosystem II reaction center. Comparing our results with those obtained from green plants and a cyanobacterium leads us to suggest that the reaction center II polypeptides are highly similar in all chlorophyll alpha-containing plants.

Calcium depletion alters energy transfer and prevents state changes in intact Anacystis

A time-dependent loss of Photosystem II (PS II) activity seen in Anacystis nidulans grown without Ca(2+) was paralleled by a loss in chlorophyll (Chl) a fluorescence of variable yield which reflects inhibition of 'Q' reduction and of state changes. Both inhibitions were fully reversed by the addition of Ca(2+) to the growth medium. The lack of state changes in Ca(2+)-depleted cells was confirmed in 77 K fluorescence difference spectra of light versus dark-adapted cells.Absorption spectra of control and of Ca(2+)-depleted cells were identical whether measured at room temperature or at 77 K. Fluorescence emission spectra measured at 39°C (cell growth temperature) demonstrated higher yields in Ca(2+)-depleted cells compared to controls. Fluorescence emission spectra at 77 K also produced higher yields in Ca(2+)-depleted cells but the increased fluorescence at this temperature occurred principally at 683 nm. The increased relative fluorescence yield in Ca(2+)-depleted samples results from light absorbed by phycocyanin (PC), but not from light absorbed almost exclusively by Chl. The 683 run fluorescence peak probably represents increased allophycocyanin (APC) emission as intact phycobilisomes become energetically disassociated from the photosynthetic apparatus. This inferred disassociation occurred only after PSII activity was mostly inhibited in Ca(2+)-depleted cells, and was not fully reversible.

Anacystis nidulans Demonstrates a Photosystem II Cation Requirement Satisfied Only by Ca or Na

Anacystis nidulans exhibits a total loss of photosystem II (PSII) activity upon incubation in a nutrient medium deficient in Ca(2+) and Na(+) and containing a divalent cation chelator. This loss of activity is light-dependent, which corresponds to an energy requirement. Likewise, Ca(2+) efflux takes place only in cells incubated in light. The loss of PSII activity is reversible by addition of submillimolar amounts of either Ca(2+) or Na(+) to the external medium but not by the addition of any other cation. Restoration of lost PSII activity also requires light. Light saturation curves for partially depleted cells demonstrate both lower maximum O(2) evolution rates and decreased relative quantum yields when compared to control cells. Partial electron transport reactions isolate the site of the Ca(2+)/Na(+) effect to the reaction center itself or immediately on its oxidizing side and exclude the water-splitting complex. O(2) flash yields decline during cation depletion, indicating a decrease in the number of functional PSII reaction centers, but the maximum turnover rate for still functional reaction centers does not decline. Thus, PSII of A. nidulans exhibits an all-or-none cation requirement, satisfied only by Ca(2+) or Na(+).

The high affinity binding site for calcium on the oxidizing side of photosystem II

Preparations of photosystem II complex from spinach chloroplasts with Triton X-100 were treated with 1 M KCl to release 17 KDa and 23 KDa polypeptides. The inhibited oxygen evolution activity could be reactivated by adding high concentration (mM) of Ca++ or by reconstituting 17 KDa and 23 KDa polypeptides which were found to promote high affinity binding of Ca++ to the reconstituted membranes (Ghanotakis et al. FEBS (1984) 170, 169-173). Inclusion of 50 mM Ca++ during KCl treatment did not prevent the release of 17 KDa and 23 KDa polypeptides but protected oxygen evolution from being inactivated. It is explained by preservation of the high affinity binding site for Ca++ if, Ca++ is present during the salt treatment even though depletion of 17 KDa and 23 KDa polypeptides usually results in replacement by a low affinity (mM) binding site for Ca++. It also implies that the high affinity binding site is not located on 17 KDa and 23 KDa polypeptides.

Evidence for direct roles of calcium in photosynthesis

Calcium may function directly in several aspects of photosynthesis. It appears to modulate activity of the phosphatase enzymes in the carbon reduction cycle and also to regulate chloroplast NAD+ kinase activity through a calmodulin-like protein. Some evidence supports a calcium function in the water-splitting complex, and other evidence indicates a reaction center function in photosystem II. Calcium in reaction center II may be tightly bound in chloroplasts and weakly bound in blue-green algal thylakoids. Free calcium concentration in stroma is probably less than 10(-6) M, although the absolute concentration is not yet known. Intrathylakoid calcium content is likely very high. Stromal calcium may regulate several enzyme activities, while intrathylakoid calcium may promote photosystem II constitutively. Results to date demonstrate the need for more attention to cation composition in studies of both light and dark reactions of photosynthesis, and the need to identify free calcium levels in chloroplasts.

Stimulatory effect of calcium on photoactivation of the water oxidation system in dark-grown spruce chloroplasts

The water-oxidation system in chloroplasts from Picea abies leaves grown without light was activated and stabilized upon exposure of the isolated chloroplasts to weak light. The rate of electron transport from diphenylcarbazide to 2,6-dichloroindophenol in the photosystem II remained constant. Ca2+ added to the chloroplast suspension was incorporated into thylakoid membranes during illumination and strongly stimulated the photoactivation of latent water-oxidation system but not electron transport in photosystem II. It is concluded that the site which requires Ca2+ and is activated by light is the water-oxidation system.

Effects of Mn2+, Ca2+ and chlorpromazine on photosystem II of Anacystis nidulans. An attempt to establish a functional relationship of amino acid oxidase to photosystem II

Washing with EDTA changes the specificity of Anacystis nidulans particles having photosystem II activities for activation by cations. A specific requirement for Mn2+ and a somewhat lower specificity for Ca2+ can be demonstrated in the EDTA-washed particles. Both ions must be added to reconstitute the system evolving O2 in the light. EDTA-washed particles retain the L-amino acid oxidase with high specificity for the basic L-amino acids [Pistorius, E. K. and Voss, H. (1980) Biochem. Biophys. Acta, 611, 227-240] as well as the ability to reduce 2,6-dichloroindophenol with diphenylcarbazide as a donor in the light. The latter reaction which does not require added cations, can be inhibited by chlorpromazine, and this inhibition can be partially relieved by Ca2+ ions. Evidence is also presented that the L-amino-acid oxidase is inhibited by chlorpromazine, and this inhibition can be relieved by L-arginine in much the same way as the inhibition of the enzyme by Ca2+ ions can be relieved by L-arginine. The data are compatible with, but do not prove, an involvement of the L-amino-acid oxidase in the redox reactions of photosystem II of A. nidulans.