Abstract
The photoconversions of phytochrome, P r ⇌ P fr , occur both in vivo and in vitro . Structural differences between P r and P fr are discussed for chromophore and apoprotein. The chemical structure of the P r chromophore has been established. The P fr chromophore was recently demonstrated to be the 15 E isomer. The red shift of absorption to 730 nm in native P fr is discussed as interaction between chromophore and apoprotein. The nature of this interaction is still unknown. Small changes in the apoprotein surface are of particular interest because they could be part of the signal chain in photoperception.