Photoreversible association of phytochrome with membranes. I. Distinguishing between two light-induced binding responses.

Two types of association between phytochrome and crude membrane fractions from oat (Avena sativa L.) are distinguished and compared, and that which comprises only a small fraction of the total phytochrome in extracts prepared in the absence of added divalent cations (Watson & Smith, 1982b) has been studied in detail. Extraction in the presence of phenylmethylsulphonly fluoride shows that proteolysis of Pr (the red-light absorbing form) probably does not account for the lower levels of membrane-associated phytochrome measured after far-red light than after red light. Difference spectra of soluble and membrane-associated phytochrome indicate that the latter is much less susceptible to spectral degradation in vitro than is the soluble pool. The stoichiometry of association with the membranes is such that for each phytochrome molecule associated after far-red light there are three associated after red light and it is argued that this stoichiometry is maintained independent of the extraction pH. The characteristics of this photreversible association of phytochrome with membranes are compared to the characteristics of the widely studied light-induced enhancement of phytochrome pelletability that is dependent on electrostatic interaction of phytochrome and membranes. [ABSTRACT FROM AUTHOR]