Monoclonal antibody from rat hybridoma cell line ARC MAC 50.1 binds more strongly to Pfr than to Pr in a sandwich ELISA of phytochrome in crude Avena sativa L. extracts or highly purified Avena phytochrome. Discrimination is not a consequence of differential modification of Pr and Pfr during the assay. Loss of a 6 kDa peptide from the N-terminal end of the phytochrome apoprotein during purification does not prevent discrimination. Neither is preferential binding to Pfr a consequence of a steric interaction between ARC MAC 50.1 and the antibody used with it in the sandwich ELISA. It is concluded that the binding site for ARC MAC 50.1 undergoes a reversible conformational change upon photoconversion and may thus represent a functional region of the phytochrome molecule.
