Differential reactivity of the red-and far-red-absorbing forms of phytochrome to [(14)C] N-ethyl maleimide.

The red-absorbing form (P r ) and the far-red absorbing form (P fr ) of undergraded, high-molecular-weight phytochrome from rye (Secale cereale L.) seedlings were examined for their reactivity toward N-ethyl-[(14)C]maleimide ([(14)C]-NEM). After pre-treatment of P r with cold NEM and extensive dialysis, photoconversion to P fr and treatment with [(14)C]NEM resulted in an approximately 70% increase in incorporation of radioactivity over the dark control. These results are discussed in relation to the view that phytochrome undergoes a protein conformational change upon phototransformation.