Thermal stability of electron transport in PS II membranes and particles from the thermophilic cyanobacteria

Thermal stability of the ferricyanide (FC) and dichlorophenolindophenol (DCIP) reducing reactions was investigated in isolated membrane preparations and PS II particles with active water splitting system from the cyanobacterium Synechococcus elongatus. In a hypotonic medium, the thermostability was seen to be much higher for the DCIP than for the FC reduction reaction. After the addition of high concentrations of polyethylene glycol (Mwt = 4000) or sodium citrate to the medium, the FC reduction reaction appeared to be more temperature resistant. Data on the effects of temperature, DCMU and detergents on the electron transfer rate in PS II provide evidence suggesting that the different thermal stabilities of the two reactions are due to different physico-chemical properties of the electron donor sites to FC and DCIP. The data suggest that regions of contact between individual macromolecular complexes of the electron transport chain are the most labile sites of the photosynthetic apparatus. The role of the composition and properties of the intracellular medium on thermostability are emphasized.