A comparison between cation and protein phosphorylation effects on the fluorescence induction curve in chloroplasts treated with 3-(3,4-dichlorophenyl)-1,1-dimethylurea

Fluorescence induction curves in chloroplasts phosphorylated by the thylakoid protein kinase activated at low light intensity and high chlorophyll concentration have been measured. At 5 mM Mg²⁺, phosphorylation did not preferentially quench variable fluorescence. At 1 mM, preferential quenching of variable fluorescence was observed, indicating a second effect of phosphorylation at low Mg²⁺ (Horton, P. and Black, M.T. (1982) Biochim. Biophys. Acta 680, 22–27). Comparison of the extent of fluorescence decrease and the resulting ratio of variable to maximum fluorescence after phosphorylation and after lowering Mg²⁺ concentration demonstrated a difference between these two mechanisms of lowering of fluorescence. The significance of these results in terms of how phosphorylation may alter membrane organization is discussed.