Binding properties in vitro of phytochrome to a membrane fraction.

Brief irradiation of a buffer extract of dark-grown zucchini squash seedlings with red light results in the binding of the far-red-absorbing form of phytochrome to a particulate fraction. A low concentration of magnesium (0.1 mM) permits partial far-red reversal of the binding. A higher concentration (10 mM) yields actually enhanced binding after the far-red treatment. Both magnesium and calcium have a strong effect on the vesicularization of the phytochrome-binding particles and on their aggregation into readily sedimentable complexes. At concentrations above 10 mM, binding of the far-red-absorbing form of phytochrome is inhibited. These effects were not observed with sodium or potassium. Increasing the H(+) concentration led to increased binding of the far-red-absorbing form. This form of phytochrome bound at pH 6.5 and 10 mM magnesium is released if either the pH is raised to 8.0 or the magnesium concentration is raised to 50 mM. These properties suggest a new method for phytochrome purification.