Asymmetry in the primary structure of undegraded phytochrome

Abstract

PHYTOCHROME is the photoreceptor for many light-mediated developmental responses in plants¹. This chromoprotein, considered a dimer of apparently identical 120,000-dalton subunits (undegraded phytochrome)^2,3, has often been purified from oats as a monomeric 60,000-dalton degradation product (degraded phytochrome)^2,3 because of its susceptibility to proteases present in crude extracts³. Degraded phytochrome is relatively resistant to further proteolysis and has almost the same photochemical properties as undegraded phytochrome². Immunochemical evidence⁴ indicates that undegraded phytochrome is asymmetric, possessing extensive primary structure not found in degraded phytochrome. In contrast, recent peptide-map data, obtained by a fluorescence procedure, have led to the conclusion that undegraded phytochrome is symmetric, being composed of two nearly identical halves, each half being equivalent to the 60,000-dalton, photoreversible degradation product⁵. We present here autoradiographic tryptic digest maps of iodinated degraded and undegraded phytochrome from etiolated oat (Avena sativa L., cv. Garry) seedlings that offer additional evidence concerning asymmetry in the primary structure of undegraded phytochrome.