On the nature of the fluorescence decrease due to phosphorylation of chloroplast membrane proteins

1. Phosphorylation of chloroplast membranes by illumination in the presence of ATP results in a 15–20% increase in the rate of Photosystem I electron transfer at low light intensity. 2. Phosphorylated membranes when depleted of Mg²⁺ and resuspended in a low salt medium still show a 17% lower yield of Photosystem II fluorescence than do unphosphorylated membranes. A 31% difference is seen after restoration of the maximal yield by addition of Mg²⁺. 3. The concentration of Mg²⁺ required to induce a half-maximal increase in fluorescence is 0.9 mM for control and 1.8 mM for phosphorylated chloroplasts. Phosphorylation at 1 mM Mg²⁺ can therefore cause more than double the amount of decrease in fluorescence yield from Photosystem II compared to phosphorylation at 5 mM. 4. The above results are discussed in terms of the mechanism of the ATP-induced fluorescence changes and a suggestion is made that the apparent interaction between phosphorylation and Mg²⁺ concentration may be a physiologically important phenomenon.