Polyclonal antibodies raised to phycocyanins contain components specific for the red-absorbing form of phytochrome

Polyclonal antibodies raised in rabbits to a mixture of sodium-dodecyl-sulphate-denatured C- and allo-phycocyanin, isolated from Anabaena cylindrica, cross-react with 124-kilodalton (kDa) phytochrome from etiolated oats, in enzyme-linked immunosorbent assays and on Western blots. The component(s) of the anti-phycocyanin serum that cross-reacts with phytochrome appears to be specific for the red-absorbing form of phytochrome (Pr). These antibodies can be detached from Pr by irradiation with red light, and thus show photoreversible binding. This property has been used to immunopurify the anti-phytochrome component from the antiserum using red light as the eluting agent. Competition assays and epitope-mapping studies indicate that the anti-phytochrome component may bind to a site located between 6 and 10 kDa from the amino-terminus of etiolated oat phytochrome.