Photoreversible binding in vitro of cytosolic phytochrome to particulate fraction isolated from pea epicotyls.

If the 1000×g supernatant from a homogenate of etiolated pea (Pisum sativum L. cv. Alaska) epicotyls is briefly irradiated with red light at ca. 0° C, the increase of phytochrome content in the 1000-7000×g particulate fraction is strongly dependent upon the Ca(2+)concentration, being highest at 10 mM. This Ca(2+)-dependent binding of phytochrome to the particulate fraction in the supernatant is partially red/far-red reversible. Mg(2+) is significantly less effective than Ca(2+) while Na(+) and K(+) have no effect.The 80 000×g supernatant (S) and the 1000-7000×g particulate fraction (P) were prepared and separated from a pea homogenate. When either S or P were briefly exposed to red light and then mixed together at 0° C in darkness and the presence of 10 mM Ca(2+), the amount of phytochrome bound to the 7000×g pellet increased immediately. This increase was 30% after mixing irradiated S with dark P, 18% after mixing irradiated P with dark S and 55% after both S and P had been irradiated and mixed. In all cases, the red light effect is totally far-red reversible.