Phytochrome Structure/Function Relationships as Probed by Monoclonal Antibodies

Monoclonal antibodies (MAbs) are ideal for elucidation of the structure/function relationships of low abundance, relatively labile macromolecules such as phytochrome, in part because of their precise epitope specificity. MAbs that react differentially with the inactive, red-absorbing (Pr) and active far-red-absorbing (Pfr) forms of phytochrome have been obtained, as have been MAbs that recognize highly conserved domains. Similarly, MAbs that can discriminate among different pools of phytochrome from the same plant have been characterized. The locations of many of the epitopes recognized by these MAbs have now been mapped, permitting functional domains on this chromoprotein to be correlated with its primary structure. In particular, the epitope recognized by MAb Pea-25, which detects a highly conserved domain found on phytochrome from angiosperms to algae, is resolved by analysis of fusion proteins to a sequence of seven amino acids (-pro-ile-phe-gly-ala-asp-glu-, residues 765–771) on the carboxy-terminal domain of this chromoprotein.