A Photoreversible 39 kDalton Fragment from the Pfr Form of 124 kDalton Oat Phytochrome

Limited proteolysis of 124 kdalton oat phytochrome in the P fr form is described which leads to a photoreversible 39 kdalton fragment. Whereas the absorption maximum of the P r form is only slightly shifted (from 667 to 660 nm) no long-wavelength band is observed in the P fr form. A “bleached” (i.e. low absorbing) form appears instead with a broad absorption maximum at 640 nm. This property corresponds with that of a 40.3 kdalton fragment obtained from pea phytochrome by limited proteolysis (Yamamoto and Furuya, Plant and Cell Physiol. 24, 713, 1983). Dark reversion of the bleached form to the P r form is faster (t 1/2 - 101 min) than dark reversion of 118 kdalton or 60 kdalton P fr . Low temperature spectroscopy of the 39 kdalton fragment showed that the intermediates lumi-R and meta-Ra are formed like in intact phytochrome or 60 kdalton or 114/118 kdalton fragments. It is discussed that limited proteolysis removed that part of the peptide chain which is responsible for the interaction with the P fr chromophore but that the site for interaction with the chromophore of P r and the intermediates lumi-R and meta-Ra is still intact.